.260 30 imp. help

[Gr8Horses2003]

I am in need of some guidance. I have a .260 imp 30 AR. It is relatively new, now on 2nd firing of Lapua brass. I am fire forming the brass by loading it and shooting it.

Today, since it's definitely NOT shooting weather, I decided to figure out how much the shoulders had moved on the 2x fired brass so I can figure out how far back toward the 1x fired brass' shoulders I need to set it back. I was surprised at what I found and want to check with y'all to see if this is consistent with your findings.

I had a set of Whidden dies made for it and the set came with a comparator, so I was using that to measure with.

The 1x fired brass measured an average of 1.649761 from base to datum line on the shoulder, the range was 1.6485 to 1.6505. The 2x brass averaged 1.656313, the range was 1.6530 to 1.6575. The difference between the averages was 0.006552. Does this seem correct?

Since this is a semi-auto gas gun, I am planning on setting the shoulder back 0.005.

Thanks for your help, in advance.

Harry

 

['Freak]

Working from the highest number, resize/bump a fair sampling with the die adjusted lookin’ for a case head to shoulder datum measuring ~ 1.654”, then shoot ‘em to see if those will grow even longer. If so, then the case head to datum was still shorter than the distance from a locked bolt face to the chamber’s shoulder, or, maybe the bolt is unlocking too soon and moving rearward before pressure drops back to safe levels.

 

[Gr8Horses2003]

Thanks for your reply. A couple of questions: first, why the ~1.654? Is that your number or something derived from mine? Second, you must be a Jedi because I(was) am doing load development with a supressor and am having a fit with it.

Since you were kind enough to reply, and it sounds as if you may have some experience with this, I will tell you "the rest of the story" in hopes that you may have more sage advice. I have a 22in barrel w/ +2 gas tube and SLR adj gas block and TBAC Ultra 7 suppressor. I (was, am, might continue depending on what happens next) using a JP LMOS bcg and high pressure bolt. Then...I took off the muzzle brake and added the suppressor. So, with my regular load of IMR 4064 in unfired Lapua .260 brass, I fool with the adj gas block and get the bolt to lock back, but this is not happening with AA4064 or Varget. I had been at around 6 or 7, out of 15 , on the gas block adjustment, but now even if I turn it all the way open the bolt won't lock back...so I turn it down thinking it may be over gassed and cycling too fast but nada. So I says to myself, maybe I need a heavier bcg and have ordered the JP variable. So your comment about the bolt unlocking too early may be insightful. If you have any advice, I'm all ears.

Thanks,
Harry

 

[jdj71]

Thanks for your reply. A couple of questions: first, why the ~1.654? Is that your number or something derived from mine? Second, you must be a Jedi because I(was) am doing load development with a supressor and am having a fit with it.

Since you were kind enough to reply, and it sounds as if you may have some experience with this, I will tell you "the rest of the story" in hopes that you may have more sage advice. I have a 22in barrel w/ +2 gas tube and SLR adj gas block and TBAC Ultra 7 suppressor. I (was, am, might continue depending on what happens next) using a JP LMOS bcg and high pressure bolt. Then...I took off the muzzle brake and added the suppressor. So, with my regular load of IMR 4064 in unfired Lapua .260 brass, I fool with the adj gas block and get the bolt to lock back, but this is not happening with AA4064 or Varget. I had been at around 6 or 7, out of 15 , on the gas block adjustment, but now even if I turn it all the way open the bolt won't lock back...so I turn it down thinking it may be over gassed and cycling too fast but nada. So I says to myself, maybe I need a heavier bcg and have ordered the JP variable. So your comment about the bolt unlocking too early may be insightful. If you have any advice, I'm all ears.

Thanks,
Harry

Harry PM sent

 

[justinp61]

Harry, just out of curiosity, what are you using that'll measure .000001?

 

[Gr8Horses2003]

I use Mitutoyo calipers and the calculator did the rest ;>)