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Why Knowledge of External Ballistics?

 
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  #1  
Old 09-06-2002, 02:55 PM
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Join Date: Sep 2001
Location: San Jose, CA
Posts: 66
Why Knowledge of External Ballistics?

Some time ago, I was involved in an argument about the actual path a bullet takes versus the theoretical trajectory; I had one view, the other guy disagreed with me. After a number of postings back and forth, someone posted the question, "What difference does it make whose right?" In response, I agreed that it made little difference in terms of what actually happened - the bullet was going to do it own thing irrespective of what I or anyone else thought.

However, there is a practical reason for knowing something about external ballistics as a recent experience of mine shows.

As I contemplated what to load for my .308 (1-in-12 twist), I was concerned that 168 gr. SMK was not sufficient for 1000 yds shooting - the general consensus is that beyond 800 yds or so, the round is 'iffy.' My rifle didn't like the 175 SMK, so I went to the 178 A-MAX which did O.K. out to 1000 yds. However, as I was preparing for a long-range class, the 178's went squirrelly on me (for reasons I still don't know), so I decided to move to the 185 gr Lapua Scenar if I could get the velocity up to 2700fps. In subsequent testing, I was successful using VV550 - but it certainly was a max load, and I mean very max. It seemed to work well at 100 yds and 300yds and according to my computer ballistics program, out at 1000yds, the bullet would still be traveling 1400fps and use only 31 MOA to get there. So, I loaded up 500 rounds and headed to the class.

Things went O.K. out to 600 yds. At 800 yds., my ability to hit steel deteriorated and at 1000 yds. I couldn't hit at all. Further, to reach 1000 yds, I was cranking in 39 MOA and even then, sometimes the bullet would hit low (so I'd crank in another MOA or so) and the next round would sail over the target by the same amount I had just added. It went on like this all morning.

When I got home, I hit the books and this is what I found out. The bullet was undoubtedly stabilizing. Although none of my groups were anything to write home about, I was hitting metal consistently at 600 yds and on paper, the bullet holes were round. What I learned, however, is that while the bullet eventually became stable (i.e., to "go to sleep" as they say), it was taking a long time to do so - possibly 200 yds or so. And during this 200 yds, while the bullet wobbled, the BC fell dramatically. The 185 gr. Scenar has an advertised BC of .521, but it was probably in the .3's during this wobbling phase - I simply did not have enough muzzle velocity to get its RPM's sufficiently high for quick dampening. Because of the high drag for the first couple of hundred yards, velocity was significantly impaired causing me to have to add MOA's to get the bullet out to 1000 yds and once there, I doubt it was supersonic. Accuracy at that distance was non-existent.

Back at the workbench, I went back to the 178's and loaded up some to get 2750 fps. I used Varget and I don't even want to tell you how much. Suffice it to say that when I did get the velocity to 2750, the 100 yd group could comfortably be covered by a dime. I may try to get that velocity with another powder, because using Varget is beyond "hot" - its downright dangerous.

I just ordered some handbuilt bullets from Bob Cauterucio - 173 grs. They look good. Even though I would like to shoot a heavier, longer bullet, the fact is that doing so is counter-productive for the reasons outlined here. Which brings me back to the point I'm trying to make - a good working knowledge of external ballistics is a good thing to have. Otherwise, explainable phenomena is relegated to the "mystery" category or results in wrongly blaming equipment or components. I wonder how many people have bashed the Hornady 178 A-MAX because it was the wrong round for their barrel.

Anyway, when I did figure out what was happening, I felt a lot better about the class. My shooting had been poor overall and I couldn't explain why. When the explanation was discovered, my damaged self-confidence was repaired. And this, as everyone knows, is a Good Thing.
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Old 09-06-2002, 05:11 PM
*WyoWhisper*
 
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Re: Why Knowledge of External Ballistics?

I too have tried alot of things with the .308 it was my favorite until I got the 6.5/284...

I have some questions for ya...

why 2750? If you plan on hunting with it don't use the a-maxs!!!
If deer are your quarry then 800 ft lbs will kill any deer.. accuracy is the key..

My 308 ( Nor-cal Nighthawk ) 23.5 " barrel loves the Varget and I get 2700+ from 45 grns. I have deadly accuarcy to 1000 and beyond. I have a self imposed range of 650 for deer.
Your 12 twist is the reason the bullets won't "go to sleep" ya ain't got enough to stabilize the 178's properly

I tried and tried to get the 175's to shoot in mine and they do shoot but not as well as the 168's so I just accepted the fact the 168 is the one I gotta go with.
I may rebarrel to about 26" and try a faster twist for 175's. They say the 175's need 1:11 to do the job well...

I feel your pain but don't look for your gun to do what it can't do well....
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Old 09-08-2002, 10:19 PM
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Join Date: Dec 2001
Posts: 469
Re: Why Knowledge of External Ballistics?

I'm sorry to hear that you had ammo problems at Mike's training class. Seems like everything goes wrong when you don't want it to.

I am shooting 175 grain matchkings with my 1 in 12 twist schneider barrel @ 2635 fps with no problems at all. I used to shoot the 168's and they perform well enough to shoot to 1000 yards. I would go with the 168's or rebarrel to a 1 in 11 twist.

Other than the ammo problem how did the class go? I wish I could have gone with you guys.


WyoWhisper

What kind of groups were you getting with the 175's compared to the 168's?

Mine are shooting consistantly under .5 moa over 43 grains of Varget in Lapua brass and fed 210M primers.

[ 09-08-2002: Message edited by: sr90 ]

[ 09-08-2002: Message edited by: sr90 ]
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