Just because you are an engineer doesn't mean you are correct. I don't cut prefits for ARC actions anymore. They can't seem to hit their own specifications..... +.004" headspace from their own print is garbage.
Ern
Interesting, +0.004" over print dimension is certainly enough to cause an issue with a prefit and I don't blame you for not wanting to do prefits for them anymore... Nobody likes dealing with returns and reworks when it could have been avoided.
I'm sure you've measured a much larger sample size of ARC actions than I have, but between 9 ARC actions of 3 different models that share an identical tenon print that I've measured, they've all measured well within the print tolerance, and all 9 actions measured within a total span of 0.0012" even when swapping bolt heads around between all the actions.
The 9 I've measured were certainly good enough for prefits but like any chamber job regardless of action brand, if you want to hit an exact headspace dimension you either need the action in hand or accurate action face to bolt face measurements.
If I'm ordering a custom prefit, even for an action that supposedly guarantees an action face to bolt face dimension within +/- 0.001" according to the manufacturers print, I always provide the smith with a measurement from the action face to the bolt face taken with a calibrated depth mic measured multiple times in multiple spots on the bolt face on my action that the barrel will be going on. Doing this I've yet to find any out of spec dimensional surprises regardless of brand, but I'm sure you've measured many more actions than I have and can note trends and deviations by manufacturer.
If the smith knows what they're doing and I provide that action to bolt face dimension along with a headspace gauge or a couple of pieces of sized brass and tell them "please headspace to this provided gauge/brass +0.001" using this provided action measurement" they can usually hit the desired headspace dimension within a couple of tenths.
Of course, for the best tenon thread fit the smith needs the action in hand, but that's not always convenient if you're still shooting out the current barrel and don't want any down time.