Cowboy, I don't believe Bryan reached the same interpretations you reached and expressed about barrel wear affect on bullet BC. I understood Bryan to say he didn't know with the class of cartridges often used by many posting on this Forum, and that it could be an issue worthy of further study. Regardless, your focus on the point about barrel wear and resulting affects on BC seems to ignore all the other common factors that affect the accuracy of a BDC turret built for one specific load/bullet under one set of environmental conditions, when one takes that rifle scope to another location for use under differing environmental conditions. I think GG identified any number of additional factors/points and the resulting concerns.
The big ones that affect every hunter, even if barrel wear over time causes absolutely no change on the bullet's BC;
1)Shooting/hunting at significantly differing elevations.
3)Shooting/hunting at vastly different temperature due to the affect on air density and MV.
Trying to fudge the settings on a BDC turret sounds a lot like guessing to me. A substantial difference in any of these three affecting factors can cause a complete miss on large game animals. The further the shot, the greater the error created by a BDC turret tuned for a different set of conditions.
My conclusions and preferences are similar to GGs. The longer the shot, the greater any change to these three environmental factors will cause to accurate predicted drops. Len says he's comfortable out to ~800 yds with the BDC turrets. That's similar to where John Burns is coming from also, to my understanding. To emply them successfully at longer yardage without a significant knowledge of exterior ballistics and significant experience with the BDC turrets just gets harder and harder. The fudging gets bigger and bigger. My own opinion based on the differing environmental conditions I experience on my hunts is ~650 yds, and the advantage then swings heavily to the mil or moa turret scopes. How much you'll need to fudge on BDC turret settings will depend on the trajectory of your cartridge/bullet combination, and how different any of the above three environment factors are from the conditions for which your BDC turret was built.
So why buy a dragster just to put a throttle restrictor on the engine? If you're going to invest in a rifle with accuracy capable of taking shots out to 1000 or more yards, why place BDC turrets on your scope that will handicap engagement on game or targets at substantially lesser ranges?
I would only purchase a BDC turreted scope for hunting in one location/elevation, and provided the terrain was flat. This fall we took a Mt. Goat at 730 yds up a 30-degree slope. With a BDC turreted scope and no PDA/PPC with field ballistic software, I'd have been just as well off throwing the scope at the goat as taking a shot. In fact, that would probably have been the more appropriate action, unless one had a BDC turret designed for 30-degree sloped shots.
Safety back on.
I completly accept those challenges to a BDC knob that you and GG have pointed out. I would like you guys to understand that those same challenges are compensated for by the Shooter weather he uses a BDC or a 1/4 moa turret;
Im presented with a 30 degree shot; I take the angle and convert it to the (factor X the yardage), I now have my actual distance to dial the BDC turret. I don't use a pda, with my 1/4 moa turreted guns or mil-dot set-ups. I calculate the shot yardage the same way, you guys use a pda, every way you look at it, it is still a neccesary compensation.
I agree if I live at 1000 feet and I take a trip to CO and hunt at 9000 feet and I plan on shooting long range I need 2 turrets. expected temp. can be put on the turrets to within + or - 15 degrees of what I will actually see on a normal trip. Yes this may be seen as a slight handicap, but then again no one at the BOTW or myself has suggested shots over 1000 yards, where the error increases significantly.
I may be a bit in the stone age myself, with my 300 rum and NXS I still print out charts for my gun based on actual drops and then change the conditions for the envirnment I will be hunting in. I then shoot to confirm that chart when I reach my destination. This has been the way it was done for many years, with success. My hope is to be brought up to speed on the PDA and eventually use one.
I have never stated the 1/4 moa click is a disadvantage, I know they are the preference for extreme long range work. With that said I don't discount the ability of a PROPERLY set-up BDC to be used to humainly harvest game with-in its designed parameters. The Huskemaw was the first BDC I have used and I happen to use it well and like the additional features of the scope.
I don't own stock in the scope and if you like Johns scope buy his. Hell I sell Leupold and NF scopes. I don't feel that I have ever pushed anyone into buying a Husky but when a customer calls and is aprehensive about the moa system and wants a mid LR set-up, I have complete confidence in suggesting a PROPERLY set-up BDC, Because I HAVE USED IT, I have practiced with it, and I understand how it works.
With that I politly bow out of this discussion, I feel I have nothing further of value to contribute in this thread. I do thank you fellows for the brisk debate.
I think if you watched all the video links and the shooting tips videos, you would see that they are definatly getting the point acrost that practice, wind reading, ect are all necessary skills. I too would not want newbies ordering a set-up and throwing pot-shots all over the country side.
I just took a look at what you said and you are dead on. There is lots of that up there on their site.they seem to be working hard at saying you have to practice and that even with their BDC system you have to make adjustments. Kind of like you did in your recent 826 yard uphill shot.
This was the first animal I harvested with the Huskemaw scope. The range finder gave a reading of 826 yards. I used the angle conversions (taped on scope bell)to find my multiplier and came up with the equivlant of a 800 yard dope. So that is what I dialed the scope to.
But here is the video I mentioned. I took it off their site.
In one clip they talk about inclines, another reading the wind, and I noted that one of their guns even has on its scope both a scope level and an angle cosine indicator. They seem to be pretty serious about the need for adjustments. I imagine that it's pretty hard to get all that across in a single tv segment but they seem to be providing quite an important service to the long range shooting public. I shoot a Nightforce scope but I personally plan to give them some slack in this argument.
First to the statement Aaron made much earlier that my system was diagramed in several books before I started building guns is not true.
I was the first person to publish, in either print or video, the process of using muzzle velocity, actual drop data, and the atmospheric conditions of the test site to obtain the real BC of the bullet being used and then develop trajectories for use under different conditions. (Who Cares??)
I do believe I was the first person to take a big game animal with a professionally made custom turret set up for that exact rifle and load under a set of atmospheric conditions. I know I was the first gunsmith to offer a complete system including custom rifle, custom ammo, and custom optic with custom DCT setup for that rifle and load under a set of atmospheric conditions. (Whoppee)
I built all of my first turrets myself on CNC machines using a 4th axis machining center to engrave the numbers on turrets I made on CNC turning centers from barstock. I drew the prints, programmed the machines, and ran the parts myself. (Big Deal)
I remember the day I showed up at the range with my custom turrets and John Porter told me it was a dumb idea and he was going to stick to using the old MOA charts and the factory dial.
I later went to Mark Kentonís turrets because he could produce them cheaper than I could and did and still does a good job. I still did all the ballistic calculations and sent Mark a spread sheet with the location of the graduations.
We now have a dedicated laser in-house here at Greybull Precision and have our turrets blanked in a local machine shop and engrave them in-house. We are building the best DCTs (Drop Compensating Turret) on the market and I am really happy with the setup.
At the end of the day all of the above just does not matter much at all to anyone but a very, very few people and is very open to a long boring argument for which I donít have time or the energy.
The real question is: What is the best system for your use.
Maybe we can categorize us into 2 different groups. Long Range Hunters and Trophy Hunters that sometimes shoot at long range
1. Long Range Hunters and Long Range Hunting.
a. I think a Long Range Hunter is going afield with the plan of making a long range shot. The size or trophy quality of the animal and even the taking of an animal is much less important than the distance of the shot.
b. This mind set is very similar to the bowhunter who chooses to use equipment that is a handicap under most hunting conditions. Only in the less than 50yds range does a bow even compare to the efficiency of the modern hunting rifle and this is the draw to bowhunting.
c. In the same way a dedicated pure long range rig is a huge handicap under most conditions that the majority of Trophy Game is shot.
2. Trophy (Meat) Hunters who sometimes shoot at longer than normal ranges
a. This mindset is one who wants the best hunting tool for use under the widest set of conditions and will maximize the chances of success in the field
b. The goal is to come home with the animal and if the shot is short that works but the hunter also wants to be prepared for any reasonable opportunity.
If your goal is to shoot something beyond 1000yds you are Long Range Hunting and you can certainly use a factory MOA dial and you should be carrying a PDA and or very detailed charts to generate the trajectory for each shot as the solution will vary considerably as the day changes. You can also use a custom DCT in combination with the PDA but you will need a chart to convert your DCT marking to MOA. This adds one step but Long Range Hunting lends itself to a lot of time to consider the shot.
The rifle should also but quite a bit heavier than we would consider normal and 15-20lbs is a good place to start. Really the bullet of choice for this game is the 300gr SMK until Berger gets their 338 VLD going. Muzzle velocities between 2800fps and 3000fps. At least 60MOA of adjustment left in the optic after zero.
This is about the only situation where muzzle breaks might be useful because everyone can use hearing protection and position themselves to mitigate the damage breaks cause.
Even the best at this game miss quite often and if this is not acceptable then Long Range Hunting is not your game. A good spotter is a definite requirement and is really the most difficult part of the puzzle. Very few people can read trace at extended ranges and accurately call the shot at 750yds much less at 1960yds.
The number of trophy animals taken beyond 1000yds is very small and very few hunters would use this technique if trophy quality was important in the same manner that few hunters would choose to use a bow on a hunt of a lifetime unless the regs required it.
Trophy (meat) hunting means using the best equipment available for successfully taking the animal you are after under the most likely conditions you will encounter. This means 10lb rifles and custom optics with custom DCTs setup for the appropriate conditions.
The idea of getting out a PDA for a normal 600yd shot at the sheep, elk, or deer of a lifetime is very questionable in the mind of any serious hunter I know. Of all the shots I have taken in the field I can only think of one where I missed because the data on my DCT was not right for the conditions. I can think of many shots that I made but that there would not have been time to calculate the shot with a PDA. Even a feeding animal may require multiple rangings as he feeds and moves until the shot presents itself and trying to calculate each range will overwhelm the hunter and make breaking a good shot much more difficult. Can it be done? Of course but why make thing harder when the buck of a lifetime is out there?
I would be interested in seeing the best trophy anyone has taken beyond 1000yds using the PDA method.
I have taken a 356 inch bull elk at 1102yds but I used my DCT to do the compensating. I did dial down 3 clicks to compensate for the high altitude and warm temp but gosh darn sure got it done and on video to boot. (2 Shots 2 hits) The 10 mile ride back to camp with 2 mules loaded with meat and horns was one of the most satisfying rides I have ever taken and I never missed the PDA sitting in the truck at the trailhead.
Thanks for the comments but I do have to point out you are also involved in commercializing Long Range Hunting. You are teaching classes, you are setting up rifles for clients and you intend to make your own LR video. I say great and more power to you but it does put you in the class of a competitor to both Aaron and me.
I think you have a dog in this hunt and an axe to grind.
You think your method is better and want to convince people to take your class, let you setup their rifle and at some point buy a video from you. Nothing wrong with that in the least, in fact you can put me down as your first preorder on the video because I am interested in your techniques and ideas. I try and buy all the available videos to try and stay as current as possible.
Here is my take on your questions and some of what I write is tongue in cheek and not meant to piss anybody off, just trying to have some fun with this:
With the complete (rifle/optic/ammo) Greybull Precision systems I sell (shameless plug) the BC will not change over the useful life of the barrel (to core stripping). This is guaranteed and if it would actually happen we will replace either the barrel or the DCT free of charge. I have tested this many times to the destruction of the barrel and am very confident in this position.
I donít understand how you could generate an accurate trajectory solution by any method (PDA, MOA Charts, or DCT) if you have a system in which the BC is constantly changing due to use of the system??
How do you know when the next change is coming?? I would feel very nervous taking such equipment in the field.
DCT made with wrong BC:
We recommend the customer use the bullets he intends to hunt with to gather the data we will use to make his custom DCT. Berger bullets donít change BC from one box to another nor from 1 lot to another. I do recommend that you purchase enough bullets to get most of the expected life out of your barrel at 1 time.
Hunting at 5000ft with 7000ft DCT
All of my personal system carry 7000ft 30deg DCTs on them. I am very comfortable hunting with these at 5000ft. I will not be shooting beyond 1000yds most likely but if for some reason I would want to I have several PDAs (Palm, Pocket PC, and Ipod) and charts to show me the required modification I will need for any shot at which my load stays above the transonic range. All of my personal systems will dial on to 1 mile and I regularly shoot them at that range for fun (steel and rocks).
If I slipped and fell on my Husky/Gunwerks and broke the scope in two I would actually be kinda happy and might drop it again and see if I could break the rifle in to 3 pieces, but that is really a different story. I did hear a rumor that actually owning or even shooting such a setup might affect your sexual orientation but this is unverified and must just be internet scuttlebutt. I have shot them but other than the scope hitting me in the head because it could not be mounted far enough forward and the bolt hitting my trigger finger I appeared to be none the worse for wear. Others might disagree but the girls still look good to me and I fail to see the attraction to Tom Cruise. I am going to quit tempting fate though and stick to my wonderful Greybull Precision system. (Shameless plug)
If I slipped and fell on my Greybull Precision optic (shameless plug) and it broke in two I would be very sad and would have to go and get the other system that is with me anytime I have my truck. This is much faster than going to a store and mounting another optic. I have never hunted with a guide that had an extra scope with him nor would I be comfortable trusting a different brand of scope than my Greybull Precision (shameless plug).
Really if you destroy your optic on a hunt this is going to be a bad thing and it seems to me to have no good solution other than avoiding the situation. I go to great lengths to set up the mounting system to keep things like this from happening and I have never had my personal system damaged in the field to the point of changing zero much less breaking into pieces. I have never had a mule be able to change my zero even though they have given it a try once or twice when I was not paying attention.
The real answer is use a system that is extremely durable, treat it like an egg, donít load it on mules or horses that will roll or smash it into trees, practice carrying it without dropping it, if you tend to fall often (I do) I would invest in the Eberlestock pack (I did), and if space permits take a backup.
If you are flying it is a simple matter to ship 2 boxes of ammo to your destination prior to the hunt in addition to the ammo you will carry with you. This ensures you will have your ammo one way or another.
If you choose to shoot a wildcat you wonít be able to buy ammo anywhere. Seems this applies to many of the .338 cartridges of those that use the PDA dialing system. I would not choose to hunt with a wildcat that offered no advantage over existing factory chambering simply because the name was edgy, cool or reminds you of the sound Thorís hammer makes. (Just having fun here guys)
My 7mm systems will shoot multiple factory loads (Rem 150gr Swifts, Fed 150 Nos BT, Hornady 154 SST and 154 Interbond) to the DCT out to your 500yd mark very well. We addressed this many years ago and have had a solution to this situation for a long time.
The idea that each gun requires a bunch of load tweaking seems to me to indicate a system of manufacture that is less than robust. If each rifle is identical to the next it will shoot the same as the next.
Many gun makers can not pull this off. Greybull Precision can (ridiculously shameless plug).
Each Greybull Precision 7mm will shoot the same load under Ĺ MOA. Again this is a guarantee. The idea that some minor change in load will transform a mediocre performer into a star long range rig simply does not wash. If your equipment is finicky it will let you down at the worst time.
When the rubber meets the road the custom DCT will provide the most efficient and effective method of compensating for variations under real hunting conditions for the Trophy (meat) hunter.
None of this is meant as anything but my opinion based on my experiences with some kidding around included.
Brian Litz had some interesting comments concerning the reported loss of BC as a barrel wore.
I donít believe barrel wear can affect the surface finish on a bullet enough to effect the BC in any manner that would concern the Hunter, either Trophy or Long Range. The laminar flow in the in the boundary layer (skin friction) has less than a 5% total effect on BC and the surface finish of the bullet is dominated by the engraving of the rifling. The area of the bore that gets rough is the throat after which the bullet picks up speed and continues to the relatively smooth and polished rest of the bore. Recovered bullets from very rough throats show essentially the same surface finish as those from a new barrel with a very smooth throat and no one could make a determination of the amount of throat wear of a particular barrel based on the surface finish of recovered bullets. (CSI TV shows to the contrary)
A rough or powder fouled or off center or improperly designed throat can allow the bullet to enter the barrel out of alignment with the bore centerline. This will cause the bullet to start the flight with an initial angle of attack relative to the airflow. This will dampen quickly and might affect the deceleration during the first 200yds or so. This phenomenon is the reason some rifles will seem to shoot VLDs better at longer ranges than at 100yds.
We can demonstrate this by using VLDs at high velocities and allowing the throat to powder foul. Accuracy will deteriorate and at some point the initial angle of attack will overcome the gyroscopic stability and we will have bullets tumbling willy nilly about the range.
My experiments have shown me that accuracy is gone before I could find a change in BC.
The method of determining BC by drop is in fact much more reliable that the use of 2 consumer grade chronos under the varying conditions outside. Consumer grade chronos are not able to resolve the velocity of an individual shot (a 1% error is plus or minus 30fps on a 3000fps cartridge) with the degree of accuracy required to get a BC that is valid for any true long range shooting.
Another problem to getting truly accurate BCs from the 2 chrono method is most proponents of the method use a single G1 BC for the entire flight of the bullet. This works fine for most shooting but will begin to show errors the closer the bullet gets to the transonic range (long range). Sierra uses the G1 drag model and the 2 chrono method (indoor range with professional grade chrono) but tries to mitigate the errors by supplying 3 different BC #s to get the G1 to accurately predict actual deceleration over the entire velocity range with the type of bullets suitable to Long Range Hunting.
Bill Davis developed the G7 drag model to work with his VLD designs and it is slightly better than the G1 for predicting drop and much better for predicting actual deceleration.
For me the only thing I need a BC for is to allow me to predict how much drop and drift I will get at various ranges under various conditions. I must be able to actually prove that data is correct. If I can prove it is correct or incorrect by shooting it then I can develop it in the first place by shooting.
Another way to look at it is if we cannot determine the actual BC from measuring our drop at long range then we cannot use the actual BC from a different process to determine our drop at long range.
As Bryan stated getting accurate data is problematic.
The process requires optics that adjust with true precision, ammo that performs consistently, rifles that are accurate in the extreme, and a shooter having a good day. Sounds just like every long range hunting shot ever made that was not lucky.
In summary if you donít have the ability and equipment get accurate data by using the drop method then you cannot use accurate data with your ability and equipment to make a long range first round hit. The egg comes first then the chicken.
Bryonís method would appear to be very accurate and the new BCs on the Berger products are very, very close to the numbers we use for those bullets. One can assume based on his shooting accomplishments that he has the ability and equipment to also test using the drop method but if he is testing many different bullets one can see the undeniable advantages to his sophisticated setup. Only thing that would seem better is radar and that would only be better if you didnít have to pay for it (military).
Sorry for such a ridiculously long post but I wanted to clear up some things regarding the use of the custom DCT.
First off I want to thank you for getting me into the long range shooting/hunting. If it wasn't for your show some 4-5years back catching my eye I probably wouldn't even be writing this now.
John you must have started your reply around lunch time:-) Very intersting and I agree with most of your statements. Definitely liked the subtle pokes and jokes. Here is one for you. How much trade in will you give for a brand new gunwerks lrh1000??? J/J John. Really did like ur last post. In fact I better read it again to sink in. Also need to read Bryon's again too, as well as his book one more time.
A quick question to GG, John, Bryon... or anyone else: Has anyone altered some of the surface area on a bullet just upstream of the surface bearing area of the bullet perhaps a small ring/groove around the bullet just prior to where the rifling will contact the bullet, (just above where a cannelure ring would be). If so how much does the BC change? Does cannelure in general have an impact on BC? Or doesn't it get degraded/rubbed away by the rifling and not make any difference. I know that the tip or meplat has a significant impact for a bench rest shooter. Basically does cannelure have an impact for us LRH's? (I've never use a cannelure bullet except in a 223 load for an ar-15 that didn't get shot at long range or scrutenized much on accuracy or long range performance...high BC bullet it was not.)
newby and layman at this game. Had a coach in highschool smallbore, peaked my interest in shooting. Got a little oldr asked about a elk gun, he sent me on my way. First year shot a 6x6 at 500+, it was old school, put daylight over the back w/300 zero. Asked more questions, he was a gun nut, had a shooting lab and computer 20 yrs ago,made me drop charts then, picked up a leupy target scope. Marked dropes on my one piece mnt, engraved. Put stickers and tape on the turret, like I BET MANY HER HAVE BEFORE, BOW. Just trying to hunt better. Dont know John, BUT I think he put LRH on the map, for GQ public to see, think the system has many merits,as do the other methods. it is personal choice, and I think people will make a good choice, but the more they know the better, so educate them. Right now I have a B&C reticle, w/moa, w custom turret, so I can use a few methods if I need to. IM hear just like I was 20 yrs. ago, the same kid , just trying to learn more, to help fill my needs. Would I like a Grey bull, OR a NXS, YOU BET. I will learn the pros and cons of both.
Holy Moly, I included you by name in my post to see if I could draw you out of the brush. You've outdone yourself.
Very good post and interesting to get your perspective - first hand.
Not to start a squabble, but I do use and rely on chronographs to determine velocity loss and calculate BC due to a lack of locations to shoot long range and collect drops sufficient to generate drop calculated BCs with adequate confidence. In addition, I have the opinion that measured velocity loss would be more accurate than the measured drops system to determine confident and accurate bullet BC. I use dual chronographs set up in tandem to measure velocity. One Oehler and one PACT with two sets of skyscreens set up on a single rail. I use 4' and 4 1/2' spacings. Bogus recorded velocity from one or both chronographs can be identified with this system by comparing the differences in velocity recorded for each shot with the two chronographs. The difference in velocities appears to vary somewhat with different caliber bullets, but when the chrono's are giving me credible velocity, the range of the magnitude of the differences between recorded velocities will develop a pattern - and for a specific bullet, the range of the differences between recorded velocity will often fall within about 5 or 6fps. If I get readings from any shot that significantly exceed that 'normal' range, then I can discard those velocity readings and only use recorded velocities that fall within the 'normal' range for that specific bullet. By the time I've developed a load, I've already documented the loads MVs. Next the dual chrono system is set up and shot over at ~990 yds in order to ensure the second velocity is recorded following a large velocity drop. In this manner, any chrono reading data errors used to calculate BC are minimized (percentage error wise) due to the ~1500 fps velocity difference between the two readings. I shield my skyscreens with a piece of AR 500 steel plate - 1/2" thick.
Since an accurate BC determination is dependent upon the velocity loss over a known range, whether or not a 2975fps recorded velocity should have really been 2997fps is not critical for BC calculation. I presume any bias in recorded velocity will repeat itself down range, such that the velocity loss will in fact be pretty darn accurate. Same idea as stepping on the bathroom scales with and without a package in your hands to determine the actual weight of the package. Whether or not the scale is correctly zero calibrated isn't crucial. The difference between the two measured weights is the required information, and that's what I'm measuring with my chronographs.
Yes, my method sounds like a lot of expense, time and effort, but it's also part of a hobby so it's interesting shooting and collecting the data to determine an accurate BC. I am also collecting drop data while collecting the downrange bullet velocity. I find it valuable and reassuring to confirm that my ballistics program predicts 990 yd velocity consistent with my recorded velocity. If it doesn't, I know something is amiss before I ever begin to place confidence in my software's predicted drops. Once PDA/PPC predicted 990 yd velocity is a close match to my measured velocity, I then have some evidence, and reason, to believe the PDA/PPC predicted drops at other ranges and various environmental conditions and shot conditions will be close.
Why do I prefer this chronographed velocity method to collecting measured drop data for BC determination? Two primary reasons. 1) It's very difficult to find multiple settings to shoot and collect long range (like 1000 yd) drop data where I live. There's very few roads and the terrain isn't wide open like out west. I currently have only one location where I can shoot 1000 yds and it's a secret location of my own development and making. No way I can feasibly get drop data under various conditions and slopes to help ensure my drop generated BC data is resulting in correct PDA/PPC predicted drops at long ranges under variable conditions and settings. 2) By recording two credible velocities over such a long separation distance I remove any variety of miscellaneous error such as human error, equipment error, and environmental condition error that can complicate the collection of accurate measured drops at long - like 1000 yd - distances. Slight updrafts and downdrafts may affect measured drops but they'll not affect measured velocity.
It would be very comforting to have the detailed drops collected under numerous settings and conditions for confirmation that not only the BC is good, but all systems are functioning properly to predict and deliver at long range. I don't have realistic options to shoot and record extensive and repetitive long range drops here. Don't get me wrong. I demonstrate ability to hit at yardages before I engage game at those yardages. But I am more limited in the amount of proofing and practicing I can get. It's about impossible to set up and check 20 degree angled shots and practice shooting rocks and targets under a variety of locations and circumstances. I have climbed a few mountains to check some angled shots, but this is a very time consuming process. So I proceed the best I can given my circumstances. This inherently requires a greater reliance on the predicted drop from my ballistics software than for others who can readily practice long range shots at will, where access is easier and it's legal to shoot and target practice.
John, while I agree with much of what you said, I need to second Phorwath in disagreeing with this:
Originally Posted by John Burns
Consumer grade chronos are not able to resolve the velocity of an individual shot (a 1% error is plus or minus 30fps on a 3000fps cartridge) with the degree of accuracy required to get a BC that is valid for any true long range shooting.
I can only surmise you have never actually tested this, have only tested with cheap, lousy chronos or are simply confusing the guaranteed accuracy in the specs of the machines with their actual precision. When you actually test this with a couple good chronographs:
You find that the variation of the delta between the two when measuring the same bullet is in the low single digits. More than good enough to derive very accurate BC data when separated by a couple hundred yards.
Now I'm not vouching for GG's methods--I know he has measured some BC's in this way in the past but I don't recall any of the "lost .100 BC" issues being verified with that method, though I haven't been around all that much and may have simply missed it. Frankly I agree with you and doubt throat erosion is going to alter a bullet's BC that much. I'd need to see some really good raw data to buy that.