First I apologize because I know this topic has been discussed many times but this is my first experience with 6BR. I have a Savage 12FV .308 and I ordered a prefit 6BR Super Match barrel from PacNor. They chambered it for the 6BR Lapua case. Not knowing anything about freebore I never asked and when the barrel arrived they had noted that freebore was .115-.120.
When I searched for freebore postings last night and reading the threads the general feeling was that any freebore above .115 might work with the 105 VLD bullets but may not have sufficient neck to grip the 70 grainers, etc. It seemed like .060 was an ideal compromise.
I have an OAL gauge and a Sinclair comparator as well as a Sinclair 6mm chamber plug to determine chamber length.
When I first received the barrel I was experimenting with all kinds of loads and bullets including 70 gr MK's, 69 gr Bergers, 87 gr V-max and so on and to be honest all of them had lots of neck to grip the bullet.
This morning I went out to the shop to try and figure out this freebore issue. I went back to my OAL gauge set up and here is what I came up with using standard cases trimmed to 1.555 which I calculated gave me a total neck length of .312 plus or minus a couple of thousand.
1. Berger 105 VLD ....they gave me an OAL of 2.620 to the lands and seated .284 into the neck which I calculated was .028 above the neck/shoulder junction. Would this clear a possible donut situation?
2. Hornady 87 V-max ....they gave me an OAL of 2.620 to the lands and seated .334 into the neck or .022 past the neck/shoulder junction,definitely into the donut area).
3. Sierra 70 gr MK ....they gave me an OAL of 2.634 to the lands and seated .215 into the neck or .097 above the neck/shoulder junction.
I know last night the general feeling was that the case neck should grip an amount of the bullet equal to it's diameter.
What I decided to do next was measure the actual length of my chamber using the 6mm chamber plug and a brand new Lapua case which measured 1.555. Needless to say I couldn't chamber the round as the plug was bottoming out on the case neck. The Sinclair 6mm chamber plug has a shoulder which measures .100
What I did next was to radically shorten the Lapua case, insert the plug and re-chamber the round. This time the bolt closed. When I removed the round the OAL was 1.578. If my math is correct when you subtract a case length of 1.555 the difference is .023 which if I am not mistaken, and I might be, should represent the actual freebore because the 6mm plug should stop when it touches the lands or entrance to the barrel.
I know this is a long posting and I apologize but am I correct in my calculations then in fact I don't have the .115 - .120 freebore that PacNor says and actually have a freebore of .023?
When I searched for freebore postings last night and reading the threads the general feeling was that any freebore above .115 might work with the 105 VLD bullets but may not have sufficient neck to grip the 70 grainers, etc. It seemed like .060 was an ideal compromise.
I have an OAL gauge and a Sinclair comparator as well as a Sinclair 6mm chamber plug to determine chamber length.
When I first received the barrel I was experimenting with all kinds of loads and bullets including 70 gr MK's, 69 gr Bergers, 87 gr V-max and so on and to be honest all of them had lots of neck to grip the bullet.
This morning I went out to the shop to try and figure out this freebore issue. I went back to my OAL gauge set up and here is what I came up with using standard cases trimmed to 1.555 which I calculated gave me a total neck length of .312 plus or minus a couple of thousand.
1. Berger 105 VLD ....they gave me an OAL of 2.620 to the lands and seated .284 into the neck which I calculated was .028 above the neck/shoulder junction. Would this clear a possible donut situation?
2. Hornady 87 V-max ....they gave me an OAL of 2.620 to the lands and seated .334 into the neck or .022 past the neck/shoulder junction,definitely into the donut area).
3. Sierra 70 gr MK ....they gave me an OAL of 2.634 to the lands and seated .215 into the neck or .097 above the neck/shoulder junction.
I know last night the general feeling was that the case neck should grip an amount of the bullet equal to it's diameter.
What I decided to do next was measure the actual length of my chamber using the 6mm chamber plug and a brand new Lapua case which measured 1.555. Needless to say I couldn't chamber the round as the plug was bottoming out on the case neck. The Sinclair 6mm chamber plug has a shoulder which measures .100
What I did next was to radically shorten the Lapua case, insert the plug and re-chamber the round. This time the bolt closed. When I removed the round the OAL was 1.578. If my math is correct when you subtract a case length of 1.555 the difference is .023 which if I am not mistaken, and I might be, should represent the actual freebore because the 6mm plug should stop when it touches the lands or entrance to the barrel.
I know this is a long posting and I apologize but am I correct in my calculations then in fact I don't have the .115 - .120 freebore that PacNor says and actually have a freebore of .023?