308 question, military vs commercial brass

[ny32182]

I'll preface this by saying that my 308 loading experience is pretty minimal, but with the 'Rona boredom setting in, well... you know.

So I have notes from years ago where I had a 147gr FMJ load with 42.6gr Varget, WCC brass (this is the middle of the data that I have) that chronoed 2542 average out of an M1A Scout Squad, a 16" rifle that I no longer own.

I wanted to baseline the same load in my Armalite DEF-10, another 16" gun. Everything else was the same except the brass was commercial Winchester rather than military WCC. Velocity yesterday was average 2380.

This seems like a pretty wild swing even between two different 16" barrels. I've just read some stuff online about how military x51 brass really does have a good bit less case volume than commercial, to the point where it would affect the load more so than in the .223 vs 5.56 world.

This still seems like quite a difference to me... is it possible that the brass is to blame for this kind of velocity difference? I probably have some WCC brass somewhere to compare with... would have to do some digging.
Thanks

 

[troy fairweather]

A common rule is to reduce 2 grains when going from commercial to military.

 

[Poper]

Military brass does indeed have a smaller internal case capacity and it does affect velocity for a given powder charge. I had similar experience 25 or so years ago with my Remington 788. I had a hundred LC match, a hundred WW, and a hundred RP cases. The LC Match were by far the heaviest and the WW the lightest. Surprisingly, the RP were the most consistent.

My velocity spread between the LC Match and the WW was not as great as yours, but still, very significant.

Be advised, there are many other things that will also cause a differential in velocity. Not the least of which is a different lot of powder or primers or bullets. Or the difference in chamber size and/or leade/freebore. If a chamber is cut with a new reamer, it will be larger than a chamber cut with a reamer that is nearly worn out and ready to be replaced.
A slightly tighter bore dimension, a better lapped bore, and other things can also affect velocity. add up a multiple variables and you can get a very significant swing in velocity using the same data.

It is ALWAYS recommended that loads be worked up from starting data in the manual for every rifle and that one should reduce powder charges by 10% when a change in any component lot occurs and work up from there, and watch closely for signs of excessive pressure.

I hope this helps.

 

[ny32182]

It does, thanks. I just noticed I have velocity data from a bunch of surplus that I still have, in the M1A, so that should give me an apples to apples comparison between the barrels at least.
Powder and primers will be the same lots at before.

 

[Charlie98]

The Scout is an 18" barrel, the Socom16 is a 16" barrel... so there is some of your difference.

 

[Charlie98]

It does, thanks. I just noticed I have velocity data from a bunch of surplus that I still have, in the M1A, so that should give me an apples to apples comparison between the barrels at least.
Powder and primers will be the same lots at before.

Any time I start chrono'ing handloads, I always start with a few factory rounds across the screens, usually from the firearm in question, or another that I have a recorded baseline velocity for, just to prove I have the chrono set up properly. I started doing this after I didn't have my Chrony folded out all the way (I must have bumped it when I was adjusting it for height or something) and I was getting either errors, or very high velocity.

 

[ny32182]

My Prochrono doesn't fold

The Scout is an 18" barrel, the Socom16 is a 16" barrel... so there is some of your difference.

Holy crap you're right. Not sure how I forgot that.

Ok, so that is some of the difference then for sure.
The chrono is a Prochrono that doesn't fold and has tested consistently against numerous other chronos so I'm confident in it being correct/consistent.

 

[Charlie98]

Now I think you are back on the right path! It's true, though... sometimes chrono data will have you scratching your head wondering if you got something mixed up.

Just FYI, in my Socom16, I've switched to IMR3031 in the 16" barrel... it works better than IMR4895 or IMR4064. Varget is about as slow as IMR4064... you might give a faster powder like IMR3031 a try...

 

[jmr40]

In my experience 2" of barrel is just not enough to matter, but the difference between 2 barrels of the same length can be significant. I have seen as much as 130 fps difference with loads out of the same box, fired within 5 minutes of each other from 2 different rifles with the same barrel length. That is uncommonly high, but 25-50 fps difference is normal. I have 2 rifles in 30-06, both with 22" barrels. The Winchester is consistently 60-90 fps faster than the Remington depending on the load. There is a bigger difference with heavier bullets than lighter bullets.

I have several 308 bolt guns with barrels ranging from 18" to 22". The three with 22" barrels are within 25-30 fps of each other. The one with a 20" barrel is the fastest of the 5 by a slight margin. The 18" gun is 50-60 fps slower.

You may well be losing some speed from the shorter barrel, plus some from the tolerances between the 2 barrels. But I'm betting the majority of it is the difference in brass.

Using a faster burning powder does not help with shorter barrels. At least not rifle barrels. All of the powder is burned in 6-8" anyway. The longer barrels just give more time for the pressure to work on the bullet before it leaves the muzzle meaning more velocity. A slower powder might need another inch or so, but will all be burned long before the bullet leaves the muzzle. The same powder that gives best speeds from a 26" barrel will also give the best speeds from a 16" barrel. When you get to shorter pistol length barrels it could make a difference.

 

[Slamfire]

Cartridge case weights vary all over the place. IMI 308 brass weighed as much as Lake City. Some of my Federal Gold Medal match had case weights as low as 155 grains. Most of my Federal Gold Medal match was around 165 grains but some recent stuff was averaging 175 grains, plus or minus a couple of grains.

Here is some data when I weighed out LC 88 NM

LC88 7.62 NM trimmed
once fired, first resizing

Average wt grains 177.0
std deviation 0.7
minimum case wt grs 175.2
maximum case wt grs 178.5
extreme spread grains 3.3
median 177.1
mode 177.4
cartridges weighed 75



R-P Nickle Unprimed, unsized
308 Win

Average wt grains 167.8
std deviation 0.8
minimum case wt grs 166.3
maximum case wt grs 169.3
extreme spread grains 3.0
median 167.7
mode 167.7
cartridges weighed 47

Ten grains difference is significant with maximum loads. A case ten grains heavier with a maximum load developed with a lighter case will blow primers. Day to day temperature differences are also significant with maximum loads, I am always discovering that I have to cut my loads the more I shoot. I regularly test, chronograph, analyze. After examining my data, I conclude what is a safe and reliable load, and later, leaky primers and blown primers show that something unexpected happened. Then it is cut, cut, and cut the charge level till the problems go away.

But, look at the LC 88 data, that is a typical case weight for military cases used in M1a in the National Matches. However, the US military and commercial firms are free to change case weights whenever they want. Therefore, do check weights, weigh a few and see if something has changed.

By the way, barrels and chambers vary a bunch, so, it might not be the ammunition.

 

[Charlie98]

Using a faster burning powder does not help with shorter barrels. At least not rifle barrels. All of the powder is burned in 6-8" anyway. The longer barrels just give more time for the pressure to work on the bullet before it leaves the muzzle meaning more velocity. A slower powder might need another inch or so, but will all be burned long before the bullet leaves the muzzle. The same powder that gives best speeds from a 26" barrel will also give the best speeds from a 16" barrel. When you get to shorter pistol length barrels it could make a difference.

You will note in my previous comments that I said nothing about velocity, if ultimate velocity is the OP's goal in a handload, then Varget is probably a good choice. I've had better results with faster powders in my rifles with shorter barrels... not only in my M1a, but, as an example, with my 20" barreled Browning 71. Quite literally, I got better, more consistent velocity with IMR3031 vs H4831, a powder even slower than Varget, and no gigantic muzzle blast. The OP is also loading a 150~ grn bullet, I think a faster powder would give him more consistency than a slower powder like Varget, so, as I suggested, it might be worth trying. The proof is in the pudding, as they say... if the OP is happy with his load combination, then he is.

The OP did have a question about a mystery loss of velocity... 2" less barrel with a slow powder like Varget DOES make a difference, as does the difference in case volume.



It's interesting... I worked up a cast load for my .30-30, 28grn IMR3031 under a .170grn cast gas checked bullet. Out of my 20" Marlin 336, I got 2000fps. That same load, but in the .308, fired in the 22" barrel of my Savage 99, only gave me 1675fps... case capacity makes that big of a difference. I had 2 other powders loaded up, under the same bullet, and the results were similar. The closest I got was 21.5grn IMR4198 @1900fps vs 24grn IMR4198 for 1850fps (.30-30 and .308, respectively, of course...) before I moved on. This is an extreme case, of course, of the differences in case volume, but I have to admit I was surprised at the results.