My memory is foggy but I believe I have read that a heavy gun, say a double rifle, would be stocked just slightly shorter than a sxs shotgun. So if my normal lop is 15" with a 6 1/2 lb game gun, a double rifle or an 8 lb sxs might be 1/4 or 3/8" shorter. I think the idea is to bring that heavier weight back closer to your body to ease in mounting. Does this make sense.

This ain't a dress rehearsal , Don't Let the Old Man IN