Velocity and barrel wear?

[Elwood]

I thought I read some where (probably here) that as a barrel nears the end of its accurate life the velocity will drop slightly? Can anyone confirm this?

I know my 7mm Shehane was doing 2940 fps, but when I put it over the chronograph a few days ago (Oehler 35p) it read 2912 fps. I understand that you will get different changes in velocity on different days but my elevation to 1000 yards also confirmed the drop.

1700 rounds on the barrel.

I know at 1700 rounds it is very near the end and the borescope confirms as much, but I'm curious regarding the velocity drop.

 

[GrocMax]

Gasses escaping around the bullet in the worn throat and first few inches when pressure is highest would reduce velocity a bit.

 

[fdshuster]

Elwood: I also experienced a loss of velocity as several 223 barrels (AR-15) neared the end of life and had to be replaced. As much as 150 fps, from when new to 4000 rounds, same exact load.

Throat erosion confirmed with my borescope. New krieger barrel(s) installed and with the identical ammo, velocities returned to the as new barrels.

 

[Laurie]

Ian,

the erosion of the throat forwards also has the effect of increasing the combustion chamber volume even if the user doesn't increase the COAL. (The primer explosion alone pushes the bullet out of the case as far as the throat allows and before the charge burn really gets going.)

So, in addition to 'chasing the throat' to keep the bullet in the lands or keep the amount of jump constant, one would have to increase the powder charge a little in theory to retain MV levels.

Just out of interest, I ran this through QuickLOAD. As my version doesn't have the Shehane, I used the straight 284 with three extra grains case water capacity added from 66.0 to 69.0gn. With the 180gn VLD at 3.150" COAL, 58.5gn Viht N165 produced an estimated 63,610 psi PMax and 2,871 fps from a 30-inch barrel. Increase the COAL to 3.250", 100 thou' throat erosion (reasonable for a 1,700 round count Shehane?) and they reduced to 60,055 psi / 2,839 fps solely from this cause.

back in the days when I was even more impecunious than now and ran very old 1st gen 7.62 TR rifles with staggering round counts bought for pennies, the rifling often started around a half-inch from the chamber. Velocities were pathetic with milspec 7.62 and I remember a Schultz & Larsen Mauser 98 rifle I had that I loaded with 185gn Lapua D46s seated in the case-neck by a twentieth of an inch (and still well off the rifling) and a literally case-full of H. BL-C(2) that was somewhere around 5gn above the max load quoted in reloading manuals. Despite that, there were no pressure symptoms at all. Heaven knows what the MV was as none of us had chronographs back then, but none too impressive I'd bet. (It shot very well for around 18 months, then collapsed compeletely halfway through a match one weekend.)

Were you using your 'worn-out' Shehane at Diggle last weekend? If so, you did pretty well with it!

 

[Elwood]

Ian,

the erosion of the throat forwards also has the effect of increasing the combustion chamber volume even if the user doesn't increase the COAL. (The primer explosion alone pushes the bullet out of the case as far as the throat allows and before the charge burn really gets going.)

So, in addition to 'chasing the throat' to keep the bullet in the lands or keep the amount of jump constant, one would have to increase the powder charge a little in theory to retain MV levels.

Just out of interest, I ran this through QuickLOAD. As my version doesn't have the Shehane, I used the straight 284 with three extra grains case water capacity added from 66.0 to 69.0gn. With the 180gn VLD at 3.150" COAL, 58.5gn Viht N165 produced an estimated 63,610 psi PMax and 2,871 fps from a 30-inch barrel. Increase the COAL to 3.250", 100 thou' throat erosion (reasonable for a 1,700 round count Shehane?) and they reduced to 60,055 psi / 2,839 fps solely from this cause.

back in the days when I was even more impecunious than now and ran very old 1st gen 7.62 TR rifles with staggering round counts bought for pennies, the rifling often started around a half-inch from the chamber. Velocities were pathetic with milspec 7.62 and I remember a Schultz & Larsen Mauser 98 rifle I had that I loaded with 185gn Lapua D46s seated in the case-neck by a twentieth of an inch (and still well off the rifling) and a literally case-full of H. BL-C(2) that was somewhere around 5gn above the max load quoted in reloading manuals. Despite that, there were no pressure symptoms at all. Heaven knows what the MV was as none of us had chronographs back then, but none too impressive I'd bet. (It shot very well for around 18 months, then collapsed compeletely halfway through a match one weekend.)

Were you using your 'worn-out' Shehane at Diggle last weekend? If so, you did pretty well with it!

Yes it was the same barrel Laurie. My 1000 elevation at Bisley is 23.5 MOA, I always expect that to drop by a half MOA at Diggle but this time it didn't.
This barrel has been awesome and normally I would consider a half MOA vertical to be the norm at the very maximum! Last weekend it was 1 MOA. I did notice people suffering large elevation changes, but all my shots bar one that showed elevation were very subtle, either a high 5 or a low 5.
Believe this or not but the throat has only moved 6 thou from chambering, a Berger 180 gr VLD was 2.475 +15 thou into the lands and this 6 thou increase occurred over last weekend at Diggle.

Possibly time to test some 180 gr Scenars in the spare Shehane barrel.

 

[Laurie]

Hmmm .... the headwind conditions last weekend may have caused both the lower fall of shot and at least some of the verticals. When it's like that there, it's difficult to tell what causes what.

Possibly time to test some 180 gr Scenars in the spare Shehane barrel.

I ran 60 odd through the callipers + comparator base to ogive check yesterday and I've never seen such consistency. They were all in 0.001" and most had variations on or under a half thou'. Weighing 50 odd saw a 0.3gn spread. Lengths are a little inconsistent, and if you've not already got hold of some, you'll find they have very wide meplats compared to Bergers and SMKs. Definitely offering a good return on trimming / pointing. Lapua's now announced BC puts them just a shade below the Berger 180gn VLD, with a 3% BC improvement from pointing (probably understating it), they move to around midway between the quoted Berger VLD and Hybrid values.