Accuracy Degrading in 6mm Creedmoor...

[Lost_Vegas]

So its been awhile since I've shot this rifle; it has a total round count of 456 rounds fired on this barrel (chambered in 6mm Creedmoor) 24" barrel. When I did my load development my best accuracy was with Sierra HPBT 107's at .010" off of the lands and 42.1 grains of H4350. I settled on 42 grains of H4350 as this seems like somewhat of a "hot" load (still is at 42.), but that's where the accuracy was...

Fast forward a couple of years... I took the rifle out and was confirming zero and getting 1 1/4" groups. This was pretty far off of the 1/2" groups I was getting at first. I realize that some of this is me and my lack of time behind the rifle recently, but I couldn't accept total responsibility. I checked the length of ogive to lands and it has grown a lot; 0.41" increase. I have changed nothing else other than this throat erosion, which is pretty much par for the course in the 6mm game unfortunately.

My plan is to chase the lands and not seat my bullets as deep and go back to my original seating depth of .010" off of the lands; increase my case base to ogive length from 2.148" to 2.189 and see if my accuracy returns. My question is, should I be worried about making this change without changing my charge weight or anything else? I don't want to go back to starting at load development again. I don't see any hazard as I am not compressing the load, rather going the other way with the bullet seating depth, and I am going back to my original distance from the lands from when I developed the load originally. Hopefully this brings back some sort of acceptable accuracy from this barrel.

Input and factual opinions appreciated!

 

[Roach]

You'll now as soon as you shoot it, but technically you're increasing capacity (albeit small) so you may have to change the charge to get the same velocity.

 

[Lost_Vegas]

You'll now as soon as you shoot it, but technically you're increasing capacity (albeit small) so you may have to change the charge to get the same velocity.

Yeah, I will probably lose a little velocity, but that is okay. I was at the upper end of speed to begin with, which is probably why my barrel is dying so fast. I'll have to see if my old chrono still works and tester out. Problem is you can burn through hundreds of dollars and half a barrels life with all of that "testing"...

My only real worry is increasing pressure and having a case failure or explosion, but I should be going down in pressure by expanding case volume. I am however getting closer to the lands, which being bullet against lands increases case pressure I believe. Although a few dozen rounds done the pipe will remedy that issue.

 

[Roach]

YMMV but IMO chasing the lands isn't going to cause any added pressure or wear. It's essentially where you started and all you're doing by seating the bullet out further is increasing capacity which I don't see how that is going to contribute to any failures. I only mention velocity because it seems like there are sweet spots where the harmonics just seem to work well. for my 6.5 thats around 2750FPS.

Yeah, I will probably lose a little velocity, but that is okay. I was at the upper end of speed to begin with, which is probably why my barrel is dying so fast. I'll have to see if my old chrono still works and tester out. Problem is you can burn through hundreds of dollars and half a barrels life with all of that "testing"...

My only real worry is increasing pressure and having a case failure or explosion, but I should be going down in pressure by expanding case volume. I am however getting closer to the lands, which being bullet against lands increases case pressure I believe. Although a few dozen rounds done the pipe will remedy that issue.

 

[Lost_Vegas]

YMMV but IMO chasing the lands isn't going to cause any added pressure or wear. It's essentially where you started and all you're doing by seating the bullet out further is increasing capacity which I don't see how that is going to contribute to any failures. I only mention velocity because it seems like there are sweet spots where the harmonics just seem to work well. for my 6.5 thats around 2750FPS.

I agree.

 

[LongRange]

i know im late to the party here but just curious what you ended up doing and how it worked out?

 

[Lost_Vegas]

i know im late to the party here but just curious what you ended up doing and how it worked out?

I remeasured the distance to the lands by taking many measurements using a case with a movable bullet; starting long, slowly closing the bolt on the round as presses the bullet into the lands, then carefully removing and measuring base to ogive. I then average the results and came up with a new COL that was .010" off of the lands.

I loaded up ten rounds of the new length ammo and went to the range. I fired five rounds of the old length ammo first, then five of the new; I did this series two times. I compared the groups and the first cold barrel shots with the old ammo were about a 1/2" group. The longer length loads were grouping about 3/4 - 1" at 100 yards. I decided that I would just keep loading the ammo at the original length and letting the SMK's jump to the lands. As long as I can keep the rifle at 3/4 MOA or better I am happy. When I shot last time when I was not getting as good of accuracy I was forced to shoot from a bench, this time I was shooting prone; I had better stability and thus, better overall accuracy. I guess I was "jumping the gun" on determining the barrel was going south.