jr_roosa
Member
I'm a big fan of knowing the size of your chamber, sizing brass to fit, and seating bullets with an understanding of how far you are from the rifling.
I just got a Garand rebuilt with a match chamber and Krieger barrel. It shot GI ball with reasonable accuracy on the test target, and I had some leftover loads from a recent match so I headed to the range to see what it could do.
I got bizarre results. One load shot two wonderful groups in one ten shot string, one group an inch above center, and the other 2 inches below...high high low high low low high low etc. Others shot worse than GI ball, with unexplained flyers, and these loads were all with match kings that shot great groups out of my other Garands. GI ball shot its usual 2-3 MOA groups.
Then it occurred to me...these were sized and seated based on the loose GI chambers and long chamber throats on my other Garands. Oh no!
I got out my gauges and found out that my new chamber headspace was 0.007" shorter than my other rifles, and that my bullets were going between 0.050 and 0.075" into the lands. Fortunately, the force of the bolt closing handled the oversize issue, and I had no pressure signs or slamfires...thank god.
To get my brass into the chamber with the bolt all the way closed, I had to take 0.003" off the top of my shell holder with a bastard file.
My GI chambers headspaced at 2.054" and the new one is cut to 2.049" which is minimum SAAMI. My RCBS resizing die would only take the shoulder back to 2.050" or so depending on headstamp. I shortened the shellholder to get to go down to 2.046" which is still a little snug in the chamber.
SAAMI specs for the case length at the headspace datum line is 2.0456 to 2.0526"
SAAMI chamber headspace at the same datum point is 2.0487 to 2.0587"
GI 30-06 case length at the datum line is bigger at 2.0482 to 2.0542"
So, except for the bullets being way too far out there, everything (reloaded cases, GI chamber, match chamber, sizing die) was within spec, but still apparently off enough to really degrade accuracy. I bet that the bullets getting slammed into the lands was the big player.
So, keep close track of what you're doing. Even full-length sizing your brass with cam-over can still leave brass bigger than your chamber depending on what you have.
Now that I have some reloads that are actually the right size for my rifle, I have to go and see what it can really do. I'm also really glad I didn't blow up my new rifle with an out-of-battery slamfire. There but for the grace of god...
-J.
I just got a Garand rebuilt with a match chamber and Krieger barrel. It shot GI ball with reasonable accuracy on the test target, and I had some leftover loads from a recent match so I headed to the range to see what it could do.
I got bizarre results. One load shot two wonderful groups in one ten shot string, one group an inch above center, and the other 2 inches below...high high low high low low high low etc. Others shot worse than GI ball, with unexplained flyers, and these loads were all with match kings that shot great groups out of my other Garands. GI ball shot its usual 2-3 MOA groups.
Then it occurred to me...these were sized and seated based on the loose GI chambers and long chamber throats on my other Garands. Oh no!
I got out my gauges and found out that my new chamber headspace was 0.007" shorter than my other rifles, and that my bullets were going between 0.050 and 0.075" into the lands. Fortunately, the force of the bolt closing handled the oversize issue, and I had no pressure signs or slamfires...thank god.
To get my brass into the chamber with the bolt all the way closed, I had to take 0.003" off the top of my shell holder with a bastard file.
My GI chambers headspaced at 2.054" and the new one is cut to 2.049" which is minimum SAAMI. My RCBS resizing die would only take the shoulder back to 2.050" or so depending on headstamp. I shortened the shellholder to get to go down to 2.046" which is still a little snug in the chamber.
SAAMI specs for the case length at the headspace datum line is 2.0456 to 2.0526"
SAAMI chamber headspace at the same datum point is 2.0487 to 2.0587"
GI 30-06 case length at the datum line is bigger at 2.0482 to 2.0542"
So, except for the bullets being way too far out there, everything (reloaded cases, GI chamber, match chamber, sizing die) was within spec, but still apparently off enough to really degrade accuracy. I bet that the bullets getting slammed into the lands was the big player.
So, keep close track of what you're doing. Even full-length sizing your brass with cam-over can still leave brass bigger than your chamber depending on what you have.
Now that I have some reloads that are actually the right size for my rifle, I have to go and see what it can really do. I'm also really glad I didn't blow up my new rifle with an out-of-battery slamfire. There but for the grace of god...
-J.
