take a ramrod with a flat jag on it, run it down the barrel with the bolt closed and cocked, when it touches the boltface put a mark on your ramrod. Now take a bullet of choice, put it in the chamber, use the eraser end of a pencil to hold it against the lands, put your rod down the barrel while holding the bullet in place, you will feel the rod touch the bullet tip. put another mark on the rod. measure between the marks, there is you Max COL for that bullet. and why you worried about compressed loads? about all loads with those slow powders in that family of cases will be compressed.
RR