How do you determine the trim-to length for a particular non-SAAMI chamber?
SAAMI chambers are normally so sloppy that you just pick a common trim-to length and you'll be fine.
But on a benchrest chamber, you want the gap between the end of the case and the start to the throat to be minimal (like 0.010") because the cast bullet is unsupported as it crosses the gap. The gap is also bad for jacketed bullets because it accumulates carbon. OK, so how do you measure that gap?
At times I have used pin gages to determine the location of the throat entrance relative to the breach. That system works well enough on single shots where you have easy access to the breach end of the barrel, but it doesn't work so easily on a bolt gun. On a bolt gun, you can use pin gages to measure the distance between the throat entrance and the end of the barrel, but more measurements and some math are required to calculate the distance between the bolt face and the end of the barrel. On some benchrest actions that distance is standardized and recorded so you don't have to re-calculate it every time you chamber a barrel. I should probably do that with my M700, since I already use standardized dimensions to fit my barrels.
But in the meantime what I've been doing is inserting a bore scope through a modified dummy case to eyeball the gap between the end of the case and the start of the throat. After I ream the chamber and the neck and verify headspace, then I firmly push the dummy case into the chamber and insert a borescope to verify that the gap looks about right. If there is not enough gap then I ream the neck a little bit longer. And that's how I set up the chamber neck length on the 308x1.75".
But ... what if the dummy case does not go all the way in to the chamber, due to friction? Then it might appear that there is sufficient gap between the end of the case and the start of the throat, when there really isn't. I try to cut my chambers very tight, so there may well be some friction that makes it difficult to push the dummy case in all the way with finger pressure. It's not a super precise system.
So where is this conversation leading? Today the 308x1.75 did not want to shoot, even with loads that had shot decently previously. It seemed like something must be wrong, so I started checking all the things that might go wrong.
I had been trimming cases 1.747", and my dummy case was 1.745", yet when I pushed the dummy case into the chamber firmly (using a cleaning rod guide as a pusher) I could not see a gap between the end of the case and the start of the throat. So I gradually trimmed the dummy case a little and tried again -- still no gap. I kept trimming and checking until finally a definite gap appeared at 1.724".
Now, there may be some measurement error for all the usual reasons -- it depends on how hard you push the case into the chamber and on exactly where you position the bore scope, but I'm pretty sure the 1.745" cases had been jamming into the throat. That could definitely play hell with accuracy. Was that why accuracy sucked today? Maybe, maybe not, but the bottom line is that the problem needs to be eliminated.
Either the cases need to be trimmed to 1.725", or else the chamber neck needs to be cut 0.020" - 0.025" longer. Cutting the chamber neck is more work but that's the right way to do it, so I'll schedule the job for this Saturday. Shooting is on hold in the meantime.