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#5561450 - 01/23/15 11:12 PM Why are my load Standard Deviation numbers bad?
RickC Offline
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Registered: 08/25/10
Posts: 802
Loc: Fort Worth
I have a pet load for 2 of my .223 Rem rifles that shoot true .75MOA all day, every day. I'm happy with it but....

The SD numbers are 27.4 over 12 shots, and 26.2 over 20 rounds. 2 different rifles. One rifle a 24" and the other a 20", both 1:9 twist. I'm happy with the load, it seems to work well but WHY are these numbers so bad? What can I do to lower them and get more consistent velocities? I use the same equipment and techniques that produce a SD of 9 in my favorite 270 load.

I'm using a RCBS Chargemaster to drop charges, and have check weights and use them. Scale seems accurate enough to a grain or 2 of powder per .1g.

Different powder? I'm using 24.5 Varget in front of a 69 Sierra. If I understand SD, this means my velocities are not consistent at all.

Any ideas?
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#5561496 - 01/23/15 11:49 PM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
Cleric Online   content
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Registered: 03/11/12
Posts: 2579
have you tried moving a bit up or down to see if you find a load that your gun likes more...

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#5561498 - 01/23/15 11:55 PM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
RickC Offline
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Registered: 08/25/10
Posts: 802
Loc: Fort Worth
Yes, I originally found this load by doing.3 grain increments with 5 shot groups, selecting for accuracy and group size.
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#5561572 - 01/24/15 05:18 AM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
postoak Online   content
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Registered: 08/19/09
Posts: 2557
Loc: The Woodlands, Tx
Is the .223 bullet not crimped and the .270 crimped?
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#5561606 - 01/24/15 06:58 AM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
DStroud Online   content
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Registered: 11/02/11
Posts: 1158
Loc: Waco
Neck tension is probably the biggest factor which ties to consistent brass. All the brass the same? Trimmed the same.
I would prep 20 pieces as close as possible also using a busshing or a Lee collet die can help maintain good neck but not totally nessasary if your expander is smooth.

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#5561611 - 01/24/15 07:03 AM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
GasGuzzler Offline
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Registered: 12/15/13
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Loc: Cooke County, Texas
If it shoots that good I wouldn't care. The paper doesn't lie.
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#5561683 - 01/24/15 08:06 AM Re: Why are my load Standard Deviation numbers bad? [Re: GasGuzzler]
decook Online   content
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Registered: 08/26/09
Posts: 1218
Loc: Montgomery, TX USA
Originally Posted By: GasGuzzler
If it shoots that good I wouldn't care. The paper doesn't lie.


And that is what really matters. I can understand solving this puzzle because it gives you confidence in your next shot. But in the end if you are getting the accuracy you want and expect at the ranges you require then you can relegate the SD spreads to a lesser priority in your load development.

I went down this same rabbit hole back in my old competition days. I would take a perfectly fine shooting load and totally screw it up by constantly changing darned near everything I could to get the SD into low single digits. I finally learned that single digit SD is not necessarily a requirement to accuracy shooting - but trigger time is. It took a lot of lead and powder for me to learn it though. Check out this wikipedia link so you are sure of what you are trying to solve, http://en.wikipedia.org/wiki/Standard_deviation

So then, the list of things to try is long, and in your case some are limiting (such as COAL), but here are a few things you can try to tighten that SD.
1. Cartridge OAL
2. Primer
3. De-burring the primer flash hole
4. Case neck brass thickness uniforming
5. Uniform brass neck hardness (annealing)
6. Indexing brass, but may be somewhat impractical in a semi-auto
7. Weighing brass
8. Only shooting development loads on same temp/density altitude days
9. Changing lots of powders
10. Changing powder
11. Add more to the list that I can't think of right off the top of my head

I didn't go back to my notes, and these were not in any particular order, but you can start to see the level of absurdity I was going towards that never materialized in a better score or more silhouettes hitting the dirt. I reasoned that the proper way of going about this is to completely eliminate every variable YOU can eliminate, and then only modify those things that you cannot. I even went as far as developing loads through one single brass case. And in the end, all I did was get tired of it. I got my SD into single digits, but the accuracy wasn't really any (or much) better than it already was. I chased my tail right into the hole of diminishing returns.
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#5561736 - 01/24/15 08:47 AM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
jdk1985 Online   content
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Registered: 08/15/11
Posts: 11706
Loc: Waco, TX
Originally Posted By: RickC

I'm using 24.5 Varget in front of a 69 Sierra.

Any ideas?


My first suggestion is to put the bullet in front of the powder... that might make a world of difference! Problem solved banana

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#5561754 - 01/24/15 08:57 AM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
RickC Offline
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Registered: 08/25/10
Posts: 802
Loc: Fort Worth
Thanks for the comments and knowledge. Solving these kind of puzzles is fun to me, although I do understand the load does work, and works well across several rifles! not just mine.

From the Wiki link: a high standard deviation indicates that the data points are spread out over a large range of values. Yep, I got that in this load. I see total velocity spreads of 96 FPS across 20 rounds. There is a problem, and I think it will shoot more consistently if I can find the cause. And as we all know, consistency is accuracy.

I'm using Lapua brass, trimmed using a Redding trim die, one set of brass had just been trimmed, and the other set due for a trim this time across the bench. I did notice while trimming that this Lapua brass has some variation in neck thickness, but not much. CCI BR4 primer.

I neck size using a Lee Collet die, set where the press just breaks over for consistency in this step. I'll admit, I think this is where I will find my solution. Neck Tension. maybe need to modify the mandrel slightly to increase neck tension. The 270 rounds did offer more resistance during the seating operation. Would consistent light seating tension cause this? The bullets are not loose.

And I seat with a Forster Seater die, no crimp. I'm doing everything I know to do to produce consistent ammo. BTW, these are Rem 700 bolt rifles.

I do understand this is a rabbit hole, and this is my first attempt to fix this. I've got a couple new ideas, thanks to you guys.
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#5561759 - 01/24/15 09:01 AM Re: Why are my load Standard Deviation numbers bad? [Re: jdk1985]
RickC Offline
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Registered: 08/25/10
Posts: 802
Loc: Fort Worth
Originally Posted By: jdk1985
Originally Posted By: RickC

I'm using 24.5 Varget in front of a 69 Sierra.

Any ideas?


My first suggestion is to put the bullet in front of the powder... that might make a world of difference! Problem solved banana


sick problem solved!
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#5561840 - 01/24/15 10:03 AM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
DStroud Online   content
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Registered: 11/02/11
Posts: 1158
Loc: Waco
I would try the collet die setup the way the instructions show (not camming over) and see if that helps

Annealing if you have not would make a difference as well


Edited by DStroud (01/24/15 10:05 AM)

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#5561887 - 01/24/15 10:42 AM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
Rocklock Offline
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Registered: 08/09/10
Posts: 395
Loc: Cedar Valley, Travis Co., TX
Inconsistent neck tension seems most likely.
TC
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#5562030 - 01/24/15 12:50 PM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
kmon1 Online   content
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Registered: 09/27/06
Posts: 20297
Loc: Texas
Rick you are using good equipment, what are the inside of the neck measurements on sized brass? Are they consistent? If not we have a sizing issue.

96 fps is a big swing and a SD of 24 like you and others have seen might cause some accuracy loss at 100 yards it will be worse the further out you get.

Are you just dropping to your charge weight with the chargemaster or drop a few tenths low then trickle in the remaining? The smaller the case volume of a cartridge the more critical this becomes.

I know it can be frustrating but try eliminating one thing at a time.
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#5562113 - 01/24/15 01:51 PM Re: Why are my load Standard Deviation numbers bad? [Re: RickC]
RickC Offline
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Registered: 08/25/10
Posts: 802
Loc: Fort Worth
I think we do have a sizing issue. The mandrel in the Lee Collet die is .220, and the case mouths range from.220 to .224. Hard to measure accurately with a caliper, but I can see that there are inconsistencies in diameter within individual cases by .001, and a few .002.

Maybe I should stop camming it over lightly, as it doesn't seem to be putting enough pressure on the sizing collet. It's tempting to blame Richard Lee, but I won't. I did pop the alum retaining cap out early on putting too much pressure on the handle. Had to replace the cap.
I wish a Lee collet would size without decapping. :-(

I let the Chargemaster drop the load, and then wait for the weight after the count before I touch the scale. I do not tricle the last few grains.

I will modify my neck sizing technique and see if my numbers change. Should I just go to a Redding S type die?
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#5562140 - 01/24/15 02:16 PM Re: Why are my load Standard Deviation numbers bad? [Re: kmon1]
decook Online   content
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Registered: 08/26/09
Posts: 1218
Loc: Montgomery, TX USA
96 FPS is a pretty big swing in 20 shots. I think you're already on the best thing to try first, re-adjusting the collet die like you said earlier.

Just a thought, have you shot across a different chrono or longer ranges by chance to verify that is what you are really getting? I know that large of a swing would also show up in an unknown hit from a called shot break. JBM sez that 96 fps is only .1MOA elevation difference at 100 yds, so that's hardle noticible but like kmon1 wrote it sure would at longer distances.

PS - good equipment.
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