People conflating case head measurements and measurements at the .200” line, two different things with two different purposes. You need a blade mic for measuring the case head (just above extractor groove) but not the .200” line (but you could use one here if you wanted to). Measuring the case head is a hit or miss proposition to attempt to judge pressure/over-pressure. Measuring at the .200” line is used as an attempt to decipher brass, chamber, size die fit.
Not saying powder charge is the sole problem but you are still referencing book max (with hornady brass) as if it's applicable to ADG/Gunwerks brass and it isn't. 40.0 grains is about 4% less than 41.7 book max listed in a couple locations and last i calc'd it ADG brass is about 6% less capacity than Hornady. I'd venture to guess your "Almost 2 grains under max" is close to the same or maybe higher pressure than was used to select a book max with Hornady brass.
I see I see. I can try even lower and see if that resolves it.
A precise .2" spacer, or whatever offset needed for the contact points of the measuring tool to be at .2". I was just noting this measurement because it can help diagnose an issue, like if your sizing die is sizing it down far enough for proper clearance, or if the tail end of your chamber is under spec from something like a worn reamer, which I've personally experienced. But it has to be taken with the proper tools and at the spec'd locations, or it could be wrong. Like when people measure headspace with comparator inserts that have a tiny chamfer on the inside of the mouth, that changes the contact point a couple thou. But sometimes (like in my case) even with calipers a fired case was .002" under SAAMI min spec as close as I could get on the .2" line, so the issue was obvious.
Just file a touch off either side of the rim if you only have a standard mic, it won’t affect function of the cases
Someone did mention the narrow bore, but I'm not sure how to measure that precisely (ball gauge maybe?), nor what I would do with that information (I'm still going to try and optimize the load for the barrel). It's a Bartlein that I bought off Bugholes, no mention of bore diameter one way or another. I also don't quite see what the box ammo would do to illuminate the issue, since I wouldn't know what the equivalent powder charge is...
If it's a Bartlein it's most likely .237" x .243". Krieger does both .236" and .237". Rock Creek .237". I'd say .237" is probably the std in 6mm where Krieger tends to offer the option of small bores more than others.
Which brings up another point - if @solar shooter wants to squeeze the most velocity out of his short barrel, he’d probably be well served to use brass with more capacity. I think 243/260 cases get something like an 8% capacity increase when ackley improved? 223AI is 7-8% over standard 223? That should be a fair illustration that a 6% difference between different head stamps makes a difference.
As said above. Barrel in the lathe. Split dowel with sand paper. I use multiple pin gauges to quickly check for size as I’m doing it. No dinking with gages and micrometers that way. I will final check with a gage and mic. In the case of a PRC with a SAAMI chamber I’d run a reamer in with a larger back end. I’d choose this method because I have a PRC reamer witha larger back end in hand. The reamer solution can also be done without removing the barrel with the reamer run through the action with a T-Handle extension.
Random question as I don't know much about gunsmithing/chambering: can a freebore reamer be used through the action without removing the barrel? Curious if extending freebore is an involved activity or simple.
