Would there be any reason a .420 size hornady headspace gauge attachment would not work on a 30-378? .420 appears to be as high as they go from what I can tell. This cartridge doesn't appear to be listed in the charts..
I don't see why it wouldn't if you are using it to set up your sizing die it's strictly a comparitor anyways. That is how I use mine anyways. Measure fireformed brass and then set up my full length sizing die to bump the shoulder back .001". A thousandth is a thousandth regardless if your datum point is in the middle of the shoulder or at some other point along the shoulder. Ideally the middle is probably the best place to measure too but which ever bushing is called for for the parent cartridge should work.
Thanks for the reply. Since I could not find anything larger than the .420 gauge, I went ahead and gave it a shot. Since I had some interesting findings, I thought I would share my results:
Using the .420 gauge on my 30-378, I found .003-.004" of variance in the shoulder length (using RCBS dies). Comparing this to my 280 AI (redding dies), I'm seeing ~.0015" of variance in the shoulder length after sizing.
I am not sure whether this is coming from measurement errors or actual shoulder sizing errors. Either way, with so much variance in shoulder length on the 30-378, it makes it rather difficult to bump each piece of brass back 0.001". Basically, what I ended up doing was adjusting the die to bump each piece of brass (large batches) back bit by bit until the bolt would close easily on each piece.
If anyone has some mentoring advice or recommendations, please feel free to share. Thanks.