Acceptable variance in velocity?

[Jeffjoiner5]

Hey guys, trying to work up a load to achieve the ultimate accuracy from my rifle and have a question.

What is considered an acceptable spread in muzzle velocity from round to round? I recently shot eight rounds that had identical charge weights and these are my chrono readings:

2569
2510
2554
2583
2539
2594
2530
2594

That is the order that they were shot, each shot was about 30 seconds apart. Seems to me if you throw out the fastest and slowest you still have a difference of 64fps, which seems extremely too large to me.

Thanks fellas.


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[918v]

You just learned that ES has nothing to do with charge weight precision. It has to do with voodoo.

 

[Jeffjoiner5]

Please forgive my ignorance but what is ES?


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[captcoop]

Extreme Spread .

 

[High Binder]

An "acceptable" variance in speed would be based on several factors only one of which would be speed. If you were shooting an 800gr bullet and you have a spread of 100FPS, that spread would matter far less than if the same spread were applied to a 30gr bullet...

So the answer to your question is: It depends ('cus physics)...

 

[918v]

My best 308 load has an ES of 13.

Jeff,

What are the details of your load and rifle?

 

[Jeffjoiner5]

Hornady 168gr bthp
42.3 gr TAC
POF P308 20"
Borrowed the chrono and didn't do any other sort of test, just quick trip to the woods with some paper. Based on research I was expecting speeds more in the 2650 range.

Thanks


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[918v]

What was your OAL?

 

[918v]

What brass are you using?

 

[918v]

Are you using WLR primers?

Cuz they tend to increase ES.

Your load seems light. Maybe you need to increase the pressure a bit to get the powder combusting more efficiently.

 

[Jeffjoiner5]

Acceptable variance in velocity?

CCI #34 primers
OAL is 2.800
Brass is once or twice fired LC

As far as the load goes, I can't find any book listing the hornady 168 hpbt so I use the Sierra 168 hpbt info. In two different books my load is over the max suggested load and in the ramshot book the max load is 43.7gr. The factory hornady load using that bullet says it's loaded to 2700fps in a 24" test barrel IIRC.


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[918v]

Try FC210 or CCI200 primers. Try reducing the load a little. LC cases have less capacity than commercial so you're prolly at max or over. LC cases require about a grain and a half powder than Winchester, which Ramshot used to develop its data.

 

[Jeffjoiner5]

What's the difference with the primers? I have 1000 of these, is something wrong with them? Shouldn't I be able to get a solid 2675fps out of this brass/bullet/powder combo without being close to over pressuring?


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[mudpig]

For accuracy work and especially long range I would try to keep your Standard Deviation under 12. Single digits would be better. Having your Extreme Spread at these numbers would be that much better. I've got a 338 LM that shoots extreme spread of 8fps normally, but there is a ton that goes into that:
1) All brass is Lapua, annealed every 3 loadings, neck turned for uniform bullet pull, primer pockets uniformed, and flash holes deburred, and brass is within 1gr in weight to each other so they have uniform load capacity.
2) All charges of powder are weighed and then swirl charged into each case for uniform powder density within the case.
3) Powder charge is 100% of case capacity with bullet of choice (94gr Retumbo and Hornady 285gr HPBT at COAL of 3.709 (.010 off lands)).
4) Primers Federal's Gold Medal Match Magnum for reliable ignition of large charges and uniform ignition shot to shot.
5) Brass that is resized properly to the rifle's chamber so you don't have a lot of sloppiness or excess Head Space.
6) A rifle with proper firing pin protrusion, travel, and a firing pin spring in good condition for shot to shot uniform impact pressure.

Any of these things can affect velocity to some extent. Good luck and keep us posted.
Chris

 

[918v]

The difference is they're different. I'm not trying to sound stupid but that's the way it is. In my experience FC210 give the lowest ES, WLR the highest, but it's not set in stone. I see a 33% reduction in ES just by switching from WLR to FC210. Now, CCI 34's are magnum strength primers which means they are hotter. Hotter means they're bad for low ES.

So, combine the fact you're using heavy low capacity brass, a magnum primer, and a max charge of powder, no wonder your ES is high.

Try switching primers. If that don't work, lower the powder charge. Don't worry about the velocity. It doesn't matter.

 

[Jeffjoiner5]

10-4. Thank you for dumbing it on down for me, lol. I'll get back with you on results.


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[ColoYooper]

That's a Standard Deviation of 31.15 fps. That's pretty common for non-premium factory loads. For my Benchrest loads I expect a Standard Deviation of less than 10....commonly around 5. For my hand loaded hunting loads I expect a Standard Deviation in the teens.

 

[Onewolf42]

#1) 20" Barrel is not helping your velocity
#2) Your ES is 84 which is POOR for long range precision rounds. My 260 Rem load normally has an ES of about 20fps for 10 rounds.
#3) Your SD is 28.6 which is POOR for long range precision rounds. My 260 Rem load normally has an SD less then 8 for 10 rounds.

I think you need to do more load development (powder weight, bullet seating depth, etc) and pay close attention to consistent brass preparation.

 

[Jeffjoiner5]

I'm surprised that 42.3gr of powder is too much in LC brass considering it still quite a bit below where the bullet is seated to achieve the 2.800 OAL. I assumed all charges were closer to optimal when the powder was closer to the bullet, am I mistaken? I certainly appreciate all of your time answering my questions guys, I'm trying to learn all there is to know about this stuff.

Another quick question, do you guys wash your cases after you lube them and resize?


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[918v]

I wipe the Imperial sizing wax off the case after resizing. I don't lube the inside of the neck cuz I use a collet neck die. But if you have lube in the neck on a live round, it will hinder bullet retention and increase ES. So yes you should tumble your brass after sizing.

You will never get 100% load density with TAC.

If 43.7grs of TAC is max in a Winchester commercial case, then 42.2 is going to be max in a LC case. That's just the case volume difference. Winchester cases hold over 2grs of water more than LC.

 

[Jeffjoiner5]

#1) 20" Barrel is not helping your velocity
#2) Your ES is 84 which is POOR for long range precision rounds. My 260 Rem load normally has an ES of about 20fps for 10 rounds.
#3) Your SD is 28.6 which is POOR for long range precision rounds. My 260 Rem load normally has an SD less then 8 for 10 rounds.

I think you need to do more load development (powder weight, bullet seating depth, etc) and pay close attention to consistent brass preparation.

I knew that had to be a horrible SD. I was going to test some factory 147gr loads just to see what sort of SD I got out of them just for giggles.

So seating the bullet to make the OAL 2.800 isn't necessarily what I should be doing? Do I have to trim brass after each time it's fired from the same rifle? Should i or shouldn't I be crimping? Rifle is semi auto.



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[918v]

You could try seating them a little shorter in .010" increments.