Picked up one of Dean's 50 Thumper barrels and I'm learning more about cartridges that head space off the mouth of the case. I'm not new to this as I used to have a 44AMP that also head spaced off the mouth and required close tolerance on case COL length.

My question is how much space or tolerances do we allow for when setting the head space for a cartridge that head spaces off the mouth?

Here is what I have..... 50 Thumper (500-375 Ruger, basically a straight wall case) brass with a COL of 2.65" this is new unfired brass. I FL sized them all the way to the base in a 500 S&W die then trimmed them all to same exact length of 2.652. To set the head space of this wildcat I placed a .002 thick piece of tape on the bottom of a case and used it to set the head space with.

Now after firing the full power round the case measures 2.645, after resizing again all the way to the base with the 500 S&W FL die the case has grown/lengthened to 2.660 and of course my head space is too tight now, not allowing the case to chamber.

Maybe it is just the new brass "settling in" or maybe my head space is too tight? I suppose I can do one of two things...... trim the brass a wee bit shorter or...... back the head space out ever so slightly?

So my question is again, how tight on the tolerance should a case like this have? and anyone have any thoughts or knowledge?

Thanks

BT