1. Try the possibly bad ammo in another rifle, preferably as similar to yours as possible, and see how it performs.
2. Check the crown and the headspace of your existing barrel.
Printable View
1. Try the possibly bad ammo in another rifle, preferably as similar to yours as possible, and see how it performs.
2. Check the crown and the headspace of your existing barrel.
That's two folks that have suggested head space. I know that headspace issues can crop up with really long term rifle use, but wouldn't throat erosion occur LONG before that. In conversations with IG and a couple other armorers, they had very few occurrences of rifles having headspace issue before the barrel was worn out. What is worn out headspace? It's got to be totally hosed to not pass a field guage, and very few people have access to a set of guages to measure actual headspace.
I'm betting it's ammo related.
I'd be very, very surprised if headspace-due-to-round-count was the issue. I've sectioned several barrels with 20K plus through them and you can still see toolmarks on the shoulder of the chamber. Which is not to say the barrel extension and bolt could not have worn / stretched, but in these cases they had not.
It's the same thing that gets every barrel-- the throat. Some ammo will be more sensitive to it that others.
I had14,000+- a few hundred through my S&W 1/9 twist barrel before I replaced it. It would shoot 3-4" group all day with 55gr ammmo till it hit about 12,000 rounds then it opened up to over 6". I finally broke down and replaced it with a Rainer arms select barrel. Now it shoots 2" and under all day with 55gr and under 1" with BH 75gr match.
At 6,500 rounds, the barrel may not be worn out, yet there may be a combination of Throat Erosion, and Bolt Lug wear to decrease the accuracy of the shorter 55gr projectile... As some have stated not everyone has Head Space Gauges, or Calipers to check the Bolt Lugs...
As the round is fired in the chamber, the Bolt will get pressed back from the pressure, if the Lugs have some wear, in conjunction with Throat erosion, this could cause poor accuracy with the shorter 55gr projectiles, yet not with the slower less pressure, and longer 69 gr match loads...
At this point, if it can be deemed that the throat is indeed the issue, I would compare the cost of replacing the barrel versus selling the 55gr and using heavier loads for the foreseeable future. Given the current ammo situation, it may be more cost effective to replace the barrel.
New 16" barrel and BCG on the way--it was faster than leaving the rifle with the shop, and cheaper than buying more high grade ammo. I'll zero the new barrel with match grade 69grn and try out the 55grn XM193 again. Then the old barrel and BCG will go to the gunsmith for evaluation.
Using the ammo it still shoots well, it's gonna at some point go bad like all at once, that is, over a 200-round stretch.... that's my prediction. Not gonna say which 200 rounds...... that would be risky.;). Speaking of chamber throats wearing and causing accuracy issues, I......
<<static sound>>
We interrupt our regular program to pass on this little tidbit from my days with a guy I apprenticed with, who was a benchrest legend (Robert Simonson). A barrel that had proved itself a good grouper, he would shoot it a season or two, and could notice accuracy dropping off (like, 100-yard groups "opening up" to .290"). At that point he would remove it, and sometimes set it back a certain distance (I don't remember, I didn't say I was a good apprentice, but I'd say maybe 1/2"), but sometimes he would cut the chamber end off, crown it, and then thread and chamber the other end! Mind you these were barrels that were uncontoured and untapered-- the outer diameter was the way it was when it came as a blank-- like, 1 1/4" diameter-- they maybe had a light turn to make it concentric to the bore. These were "rail guns": a barrel, an action, a custom-made machine rest and a couple dial indicators.