Would barrel throat wear cause loss of velocity?

[GSSP]

In January I started then completed my load development for my 6.5x47 Lapua using the 130 Berger VLD, 40.25 gr of H4350 sparked with the CCI 450 primer. Velocity in my 26" Krieger, 1:8.5 barrel was an even 2800 fps. Today, I logged it's 828th shot down the barrel. Today I was completing a test where I compared 10 annealed cases and 10 non-annealed cases that had 5 firings on them. What was shocking to me was the two velocity strings ran 2735 and 2739 fps respectively. That's a loss of 65 and 61 fps. I've been seating my bullets 10 thou into the lands but am guessing the throat has worn enough that my velocity has dropped off.

Would you concur?

I'll more than likely measure my throats wear but for now kind of hate to mess with things as it's shooting very good. One groups ran .111" and the other .375".

Alan

 

[RDavies]

Yes, normal throat wear (heat erosion) does cause velocity to drop. Once I have a load for my barrel and it has settled down after 200 shots or so, I leave it until it starts to lose accuracy. When the acuracy drops off, it is usualy a combination of a change in seating depth and the velocity dropping out of the node. With a 284 (and your 6.5x284 would be similar) I would usualy seat the bullets back out and bump the powder charge up a tad after 600 shots or so to keep it in tune.
You probably only need to bump your powder charge up .2gn, then seat your bullets back out to the same jam/jump to keep things happy for another 600 rounds or so.

 

[Erik Cortina]

CR500 is right, You have to keep "chasing" the lands, as the throat erodes you have to seat your bullets farther out.

 

[MarinePMI]

You probably only need to bump your powder charge up .2gn, then seat your bullets back out to the same jam/jump to keep things happy for another 800 rounds or so.

CR500,

I'm not knocking you, and am just curious; but are you serious? A 0.2gr increase will alter the internal ballistics enough to affect accuracy in a 45-50gr charge? Wow, sounds really sensitive...

 

[RDavies]

Yes, every now and then I bump it up .2-.25 grain to keep it at the same velocity. Over the life of a barrel, I will have bumped it up 1.5gns or so. Each .25gns only lifts it up another 10-15 fps or so, but it keeps it right in the node. I do the same for my 6x47 after less rounds, so with a 6.5x284, he might want to do this every 4-500 rounds.

 

[GSSP]

Yes, every now and then I bump it up .2-.25 grain to keep it at the same velocity. Over the life of a barrel, I will have bumped it up 1.5gns or so. Each .25gns only lifts it up another 10-15 fps or so, but it keeps it right in the node. I do the same for my 6x47 after less rounds, so with a 6.5x284, he might want to do this every 4-500 rounds.

I recall reading in Glen Zediker's book he does basically the same thing.

Alan

 

[fdshuster]

Absolutely! Proved it to myself years ago with a new Krieger barreled upper for the AR. When new was getting a consistant 3025 fps. At around 2500 rds. began to see velocity fall off, with the same ammo. By 4500 rds. fired, velocity was down to an average of 2840. Throat wear verified with my Hawkeye borescope. Could not seat the bullets further out and maintain the 2.260" magazine length required for the rapid fire stages of NRA Hi Power & CMP matches. Could not increase powder charge, it was already near max.

 

[GSSP]

So, food for thought. When having a new rifle made up which has a short mag length, think about not only throat wear and how it effects velocity but bullet seating depth and how you might not be able to chase the lands due to short mag length.

Note to self; don't be afraid to have reamer cut a bit short to allow for such stuff.

Alan

 

[AckleymanII]

GSSP, I learnd that the hard way in the p. dog wars....darn good advise.

When having a reamer made, get one with zero freebore and a seperate throater if necessary...then you can change the seating depth on barrel set backs.