decrease in muzzle velocity after 3500+ rounds

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rugbydave

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I keep pretty detailed logs of my rifles' round counts and muzzle velocity for whatever reason. Maybe I have mild OCD. Anyways, I like to shoot my M14S as far as our range goes (475 yards) on at least a weekly basis so I figured that keeping track of muzzle velocity relative to ammo temp helps you keep good Density Altitude based drop charts (and the extreme spread and standard deviation let you know you're putting together decent handloads) so that I can push it out further when I find a good spot.

I've been using a Shooting Chrony Alpha (I think?)with remote for many years and recently got a Magnetospeed Sporter. 2 weeks ago (July 23) I decided to test them simultaneously to see how the readings differed. At that point I hadn't measured muzzle velocity since June 24. Over 5 rounds, the Crony measured 2575 fps (ES:29.01; SD: 10.90) and the Magnetospeed 2578 fps (ES: 33; SD:11). Over another 9 rounds (while re-zeroing the optical I have to remove the front sight base and muzzle brake to use the magneto speed) the Shooting Chrony averaged out to 2580 fps over a total of 14 rounds. All this at temps from 20.1C to 21.7C. Interestingly, in the data that I have logged since November, the lowest MV was 2614 fps @ 12.0C and the highest was 2650 fps @ 32.4C. Looking back at the load development for this to last summer, the velocities were all around 2625ish. This is all the same batch of powder and primers and neither the load (168 gr Hornady HPBT over 40.3 gr Benchmark in Fed cases with CCI 200s) nor the seating depth (2.800) has varied.

Any ideas on what would cause the velocity drop? The rifle's round count is at 3540. The accuracy is still great (for a M14) with the vast majority of 5 rounds groups below (or well below) 1.5 MOA. I mean the last 3 rounds of my zeroing today were sub-MOA. What would cause these slower MVs? Throat erosion? I always kinda figured these stock chrome-lined barrels were good to about 10k rounds?
 
Headspace are known to expand with these rifles. In short, the slightly larger chamber would mean there is a greater distance between the bullet ogive and the rifling where the gases can escape. A smaller gas volume behind the bullet means less velocity. To make up for this you could seat your bullet less deep but the only way to know is to measure your chamber and compare the difference from the time it was new.
 
At 3000 rnds I would expect any 308 bolt rifle to be "past its prime".

At the start of your barrels life span, I expect you saw an increase in velocity for the first 2-300 rounds. I would expect that the velocity would slowly start to diminish at 2500 or so rounds and then start to drastically drop off till 3500-4000. At 4000 I would expect a visible difference in group size. At 6000 I would expect to see noticeable (to the naked eye) reduction in rifling in the throat and muzzle.

That said. It should in no way be inferred that the rifle wouldn't shoot after 4500. I have seen RCMP model 70's with no appreciable rifling left after 100,000 rounds still hit a man sized target at 100y.

I am not surprised that you are seeing a decrease in MV after 3500 rnds. If you were seeing 1.5" groups in the barrels prime, I would expect you will still be under 3" up to 6K.

My $.02.
Cheers
 
notsorichguy said:
At 3000 rnds I would expect any 308 bolt rifle to be "past its prime".

At the start of your barrels life span, I expect you saw an increase in velocity for the first 2-300 rounds. I would expect that the velocity would slowly start to diminish at 2500 or so rounds and then start to drastically drop off till 3500-4000. At 4000 I would expect a visible difference in group size. At 6000 I would expect to see noticeable (to the naked eye) reduction in rifling in the throat and muzzle.

That said. It should in no way be inferred that the rifle wouldn't shoot after 4500. I have seen RCMP model 70's with no appreciable rifling left after 100,000 rounds still hit a man sized target at 100y.

I am not surprised that you are seeing a decrease in MV after 3500 rnds. If you were seeing 1.5" groups in the barrels prime, I would expect you will still be under 3" up to 6K.

My $.02.
Cheers
Click to expand...

I kinda figured as much for the barrel on a precision rig(that starts off much more accurate to begin with) but was under the impression that more of a rack grade, chrome lined barrel wouldn't drop off that quickly? Looks like I might finally have an excuse to re-barrel with something nicer and heavier :)
 
Your barrel is Chrome lined and most militaries consider them to have a life span of 10-12000 rounds. So the barrel is only 1/3 of the way through its life span.
I would look to your ammo as the cause, is it the same lot number or identical reloads?
 
Did you load all the ammo at the same time?

Are you measuring and dispensing your powder charge the same way?

Is it the same brass reloaded? Stretched and then trimmed = larger internal capacity?

Has your reloading method changed or are you using different dies?

Kneck sized, collet sized, partial length, full length... regular or small base sized?
 
Dake21 said:
Headspace are known to expand with these rifles. In short, the slightly larger chamber would mean there is a greater distance between the bullet ogive and the rifling where the gases can escape. A smaller gas volume behind the bullet means less velocity. To make up for this you could seat your bullet less deep but the only way to know is to measure your chamber and compare the difference from the time it was new.
Click to expand...

I measured 10 spent cases from my last session and all measured within 1.635-1.638 (1.638 was the headspace as measured when the rifle was brand new)
 
Lee Enfield said:
Did you load all the ammo at the same time?

Are you measuring and dispensing your powder charge the same way?

Is it the same brass reloaded? Stretched and then trimmed = larger internal capacity?

Has your reloading method changed or are you using different dies?

Kneck sized, collet sized, partial length, full length... regular or small base sized?
Click to expand...


The ammo wasn't all loaded at the same time. The data I have is from November 2015 to now. Same scale, same powder throw. It's all the same batch of Federal cases, that have now all been fired 3X. Same jug of powder, reloading method has changed in the past few months from Full Length resizing to Body Sizing (0.002 bump) and neck sizing. When I made that change though I tested both different ways against each other and noticed no noticeable change in muzzle velocity for the 2 different methods over a sample of about 50 rounds for each method.
 
rugbydave said:
I measured 10 spent cases from my last session and all measured within 1.635-1.638 (1.638 was the headspace as measured when the rifle was brand new)
Click to expand...

Did you measure with the gas system off? How much the brass measured last year?

When you say same powder, you mean from the same jug? If not it could be a different lot number, performance may vary a little bit between the same powder but with a different lot number.

Lee Enfield has a good suggestion about the larger cartridge case volume. You could measure the new brass thickness vs the older one.
 
Ganderite said:
The velocity change does not look big enough to be statistically significant.

That said, I doubt it is the rifle. Powder changes as it dried out and neck tension reduce.
Click to expand...

Honestly, you're right when you say it isn't statistically significant. Even punching it into Shooter and comparing it to the charts I have made up from data from JBM and verified, the difference, at least as far as going out to 500 yards is within the accuracy of the rifle so pretty insignificant. I guess I just took note as it was the first time I saw the MV duck under 2600 and I got curious. I'd like to put in a new trigger group(USGI) and one of those aftermarket triggers from Shootingsight before I re-barrel. I've been getting spoiled shooting my bolt gun with a super light trigger, haha. Thanks for everyone's insight!
 
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