The reason is quite simple; if it happens that you require an extra .006” of sizing as my 375 Weatherby does, as indicated by running feeler gauges under the case head, then .010” is a nice easy round number to take off the shellholder. Then you simply adjust the die until you get the extra .006” shoulder bump required. ALL shellholders are supposed to be .125” in height, many aren’t exact, but the ones I trim ALL measure .115” so I KNOW where I’m measuring from with the feeler gauge.I'm glad you said that; I don't know why 10 thousandths was the number getting thrown around.
The cases came out of the gun, it shouldn't take 10 thou to get them back in.
What you say about the cases came out of the gun so it shouldn’t take .010” to get them back in is EXACTLY the problem. The case elongates during sizing in head to shoulder datum length. If my chamber is .001” above minimum dimension in head to shoulder and the die can only move the shoulder back .004” LONGER than that minimum length.....how are the cases ever going to fit back in? This is a case of short chamber and long die, the cheapest and easiest remedy is to turn .010” from the shellholder, or use a feeler gauge under the head of EVERY SINGLE CASE EVERY SINGLE TIME YOU LOAD ONE, this gets tiresome quick in the number of loadings I do.
I have deliberately short chambered many rifles, the reason being that match chambers are already small and I want absolute minimum head clearance. For instance, I use A191 chambers for my comp 300WM rifles, these have the shoulder .008” over un-fired cases as I chamber them, not the .016” that they normally have. A SAAMI 300WM chamber grows the shoulder forward from unfired anywhere from .018”-.028”. I don’t want this much brass movememt EVER.
The fatter and longer a cartridge case is, the more metal that is displaced, the Weatherby’s are fat and long right to the shoulder, hence the elongation is more.
Another noteworthy aspect to fat cases is the fact they are harder to size, in that it takes more sizing force to displace the metal back to closely resembling the original size. Weatherby’s are notorious for springing back in the die, you need to size and leave the case at full stroke for a few seconds so the brass doesn’t spring back and ruin what you just tried to achieve, most would wind the die in more, creating more unnecessary head clearance.
I bet there are many WSM users out there pulling their hair out because their cases refuse to fit back in the gun after being FL sized.
I will even change a shellholder if it sizes too much before I get cam over in the press, cam over helps keep the head to datum length consistent, as there is no ‘slippage’ of the press pins or linkages as everything is ‘bound up’ case after case.