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Extreme spread / Standard Deviation

 
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Old 02-11-2002, 01:00 AM
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Extreme spread / Standard Deviation

Hello, all.

Well, on the topic of Ballistics, here's a question for those of you who specialize in Long Range: How important is minimizing Extreme Spread and Standard Deviation to long range accuracy? What is a good thumbrule, per se, for a max acceptable value of either or both?

A little background to the question: I have a Remington 40XB-KS in .220 Swift, w/ an Answer Rifle brake, Leupold 8.5-25x50mm LR scope. Everything else is pretty much 'out-of-the-box', w/ the small exception of the folks at Answer Rifles deemed it necessary to lop off an inch of my barrel whilst installing the brake (grrrr...), so I have 26 1/4" vs the normal factory 27 1/4", FWIW. I got this gun for long(er) range prairie dog and rock chuck hunting. I'd been working up loads using 50 Hornady Frontier brass that had been selected from the 80 rds of factory ammo that went down the pipe prior to me getting dies for it. The brass was neck-turned to clean up ~75-85% of the circumference, primer pockets uniformed, trimmed to length w/ a Wilson trimmer, chamfered, deburred, etc., so on and so forth. Loading was done on a Redding turret press using Redding Competition Neck and Seating dies. Total Indicated Runout per round was usually under a thousandth.

Some loads would shoot fairly small little holes @ 100 yds (0.2 is pretty small by my standards for a factory Swift), but the 'average' for most loads was ~0.4". But when I would try these loads (tried multiple powders, bullets, primers) @ 200yds, the groups would open up to somewhere between 1.5-3.0" Some would actually be say, 4 into 0.6-0.7", but never 5 shots into less than 1.5" I realize some of this is probably due to the nut behind the trigger, but it struck me as kind of odd, even so. The tests were done over multiple days, w/ pretty consistent (sadly) results, so I would guess that weather/wind probably wasn't a major factor. Oh, I forgot to mention: my group(pattern!) was more or less vertical in shape. I was always taught that that meant one of two things (usually) : a) shooter error, or b) muzzle velocity variations. So I unpacked the chronograph on the next trip to the range, and lo and behold, I'm getting extreme spreads of _over_ 100fps, and standard deviations of 50, 60, 70, and more. Yikes!!!

After some conferring w/ the local gunshop, I chunked that lot of brass (fired 6-7 times, I think) and am starting over w/ 200 new Winchester brass. I neck turned the entire circumference (on most; on some the setting only did 90-95%), removed the ejector from the bolt and am fire-forming the cases to the chamber and the bolt face, to eliminate any questions of crooked or otherwise 'weird' brass. Once I have all the brass fire-formed, I planned on sorting by case weight, but when I originally posted this on the Long-Range-Rifle mailing list, at least one fellow suggested chrono'ing each round and segregating any that shot abnormally faster or slower than the rest as a better method of culling out the inconsistencies ( a 'performance' based sort, if you will).

In the end, I hope to get things stabilized to where I can get a load that consistently performs out to (at least) 500 yds. Again, any comments or suggestions are welcome.

Thanks in advance,

Monte
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Old 02-11-2002, 09:53 AM
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Re: Extreme spread / Standard Deviation

Monte

Normally vertical stringing suggests that you have too much powder OR not enough. There are other reasons also but, lets start there.

Your variation might suggest uneven powder weight charges.

Are you weighing each powder charge seperatly? Some people count on the powder measurer to drop out the SAME charge EACH time. I don't know of any that will unless you have the newer one from RCBS that has a digital scale and a powder feed that drops the charge slowly until it reaches the powder setting you programed into it. It will then shut off the powder feed at that point. Some of the benchrest folks are using these.

The extreme spread we try to maintain is always under 20 FPS. Usually around 10 FPS to 12 FPS with matched cases. 5 fps variation is even much better and can be obtained sometimes.

Try this test.
Weigh your brass and seperate it. Make sure you have prepared the brass with all the procedures you must do. Weigh your bullets and seperate them. Use a good match bullet for the tests. Weigh out EACH powder charge seperatly with a good digital or cross beam scale.
Make sure you have removed ALL copper from your barrel and it is clean with NO build up of carbon or copper in the throat or muzzle area.

Try the above over your chronagraph and see what you get. What brand of Chronagraph do you have? If it's the older chrony, that could be your problem also. They have had a lot of problems with these. The groups have shown you that the extreme spread is very extreme, so the chronagaph is probably not the problem.

What speed are you running the Swift with 50 gr bullets? Excess velocity will KILL good accuracy most times.

Interested to hear your results.

You would be surprised to see what we have done with the factory Ruger Varmint in the 220 Swift. Using 52 Gr match bullets and when the wind was not blowing or gusting, it will repeat very nicly at 1000 yards.
Did this one time at a yearly shoot out we have and a friend of mine bought the rifle the next day.

Darryl Cassel
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Old 02-11-2002, 10:13 AM
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Re: Extreme spread / Standard Deviation

A few other thoughts:
Have you tried weighing your cases? With your new batch of cases the variance between lots of your once fired brass should be minimized, but it's worth a try to find the best matched cases and see if the inconsistancy continues.
When you say "deburring" I hope you mean the flash holes...
Are you using Bench Primers? The differance with RL-22 and Fed 215, CCI 350s, CCI 250s and CCI BR-2s has made a major differance in group size and standard deviation in my experience. With the BR-2s at the top of the heap with that particular powder as well as with Varget. Gold Medal match primers may be another viable option depending on your favorite powders.
Your barrel is free floated isn't it? Action bedded?
Good shooting, Coyoter

[ 02-11-2002: Message edited by: Coyoter ]
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Old 02-11-2002, 11:12 AM
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Re: Extreme spread / Standard Deviation

Thanks for the responses, guys.

For further details, when I was talking about uniforming/deburring, I meant the following: primer pockts uniformed, flash holes deburred, and case mouths chamferred/deburred w/ Sinclair tools.

As far as the powder scale vs. measure thing, I have a Redding BR-30 w/ a Sinclair accessory kit (stand, drop tube, pill bottle) which is pretty darn consistent w/ stuff like H335 and Xterminator for my .223, but for load work up, or stuff w/ stick powders like I've been mostly using so far in the Swift, I have been using my Pact electronic scale and electronic powder measure. AFAIK, the RCBS one is a green copy of this model.

The chronograph in question is a Chrony Beta. The thing is starting to get aggravating, as it doesn't take much overcast or shadow (like from a covered bench area) to render it inoperable.

The components I've been using are the following: Sierra MatchKing 53gr HP, 52gr HPBT, moly 50gr BlitKing, Berger moly MEF 52gr, 55gr, 64gr, CCI BR2 primers (mostly), Fed. Gold Medal primers, Win 760, H4831, IMR4064, Varget. I have cans of H380 & IMR4895 waiting; I was planning trying H380 & AA2700 next.

The scary thing is that the accuracy was actually best w/ the 64gr Bergers (0.18); and also, this gun so far (@ 100 yds) seems to like HOT loads the best. Like 1/2 gr over listed max in the books I have (Nosler, Hornady, Sierra). I'm not sure how much of this is the gun's characteristics, and how much is that the book loads are for non-coated bullets and most of my shooting has been w/ the moly Berger MEFs and Sierra BlitzKings. Ideally, I'd really like to get this thing shooting w/ the moly BlitzKings, as I already keep a buttload of them on hand for my .223's.

The gun barrel is completely free floated, but as far as I can tell, it is not pillar bedded in any way, but it is about the tightest fit I have ever seen for an stock to an action. Removing the stock from the gun is an experience, to say the least. VERY snug fit.

I'm off to fire-form another 50 rds this morning, when I get done w/ the last 50 tomorrow or the day after, I'll start the sorting of cases.

Thanks,

Monte
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Old 02-11-2002, 11:48 AM
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Re: Extreme spread / Standard Deviation

Milanuk,
I agree with Darryl above in the 20fps or less 100%. That's what the competition guys strive for. But also don't get caught up in the actual SD and ES numbers. 5, 10, or even 20 shots strings aren't enough of a sampling size to get good true numbers. I'm not a statistion so I can't say how many it would take either. So then the question is what numbers do you pay attention too? Your min/max spread. Shoot 10 shots and take the highest and lowest and find the difference. Keep good records and patterns wil start to emerge. This is what most shooters refer to when talking about ES. But it's not true ES in the statisical world of numbers. Just thought I would throw that out there for you to think about. My PACT chrono along with the other chrono out there automatically calculate ES, SD, and a host of other numbers but I don't pay attention to them. Just give me actual velocity numbers and go from there. Maybe Warren can keep me honest with the above statement??

Also you say your 200yd groups open up to 1.5" for 5 shots when your 100yd accuracy is .4" on the average. To save you some wear and tear on your rifle, I think your barking up the wrong tree by trying to minimize your min/max velocities with sorting cases and such right now, and here's why. Later on, yes you will want to sort cases, change primers, etc ,etc, but not right now.
If it was velocity change alone then you would need a velocity drop of approx. 270fps to make a bullet drop 1.1" (1.5"-.4"=1.1") more than another at 200yds. Your seeing 100fps spread and that is only worth .3" of drop at 200yds. So your seeing something else besides velocity variation to get a minimum change of 1.1" @ 200yds. FYI: I based my above numbers on a Nosler 50gr BT at 3600fps and adjusted the velocity down from there in my ballistics program. Also you mentioned you did this on different days so you don't think weather conitions played much of a factor. Don't underestimate the power of changing conditions at any range. Obviosuly I wasn't at your range when you shot so I can't say they did or didn't play into the equation. Just remember even if you can't feel it or see it the air is moving around out there and it is a factor.

What I think your seeing is the barrel harmonics causing the up/down in your groups. I've been down this road with rifles before and you need to adjust your load up or down until you start shooting round and smaller groups at whatever distance your shooting. Then start playing with cases, primers, and such to really fine tune it.

This harmonics thing is hard to explain typing here, but I've been there done that with my 338 long range gun. I've got one particular powder that gave me the less than 10-12fps extreme spread velocity mentioned above, but I could adjust that load (up and down) and shoot almost perfectly horizontal or vertical strings for groups and tuning in between wouldn't produce round groups in that barrel. Bottom line is that powder, bullet, barrel, etc, etc, etc combo didn't agree with each other. And all of the fine tuning I wanted to try wasn't going to tune that out. Those "stringing" groups were at 100yds and measured in the .3" for 5 and 10 shots, but at 1000yd in competition..... it was embarrassing. 16 and 18" groups going up and down but only 5-6" wide. Classic sign of harmonics/load tuning issue. In this particular case I ran into pressure but I could tune out the stringing. So I switched powders.

Play with your powder and loads first to get smaller and round groups. then fine tune.

Good luck with your Swift!
Steve
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Old 02-11-2002, 11:59 AM
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Re: Extreme spread / Standard Deviation

Monte,
It took me so long to type up that response, you had posted in the meantime. One thing in that second post really jumped out at me and I think Darryl's eyes will probably open up when he reads it...... moly.

How are you cleaning and how many shot to settle the barrel down, etc. I still believe in my above statements, but moly does play into this especially when dealing with up/down variations. Most long Range BR guys have gone away from moly for this same reason. You were always chasing the up/down and trying to keep an even coat of moly in the bore.
Also moly could be the explaination of why your loads are over the book maximum also without pressure signs. The reduction of friction in the bore lets most rifles shoot more powder to get back to the previous load pressure when shooting bare bullets. That's not unusual. Just be carefull if you ever go back to bare bullets. Pressure will increase.

Let us know,
Steve
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Old 02-11-2002, 03:10 PM
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Re: Extreme spread / Standard Deviation

Steve

I agree with you especially on the "MOLY" bullet problems some of the 1000 yard benchrest shooters have had in the past with the stuff..

I think he also has a problem with the chronagraph not reading consistant.

Just a thought.
Darryl Cassel
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