Boxlock or sidelock. How can you have dis-assembled the gun and not seen: inner wood damage-splinters in the jaws of the stock or anywhere else, dirt, gummy oil, gunk and crud need to be removed if and when you either field or detail strip the action of any shotgun--heavier shells can cause this problem as well in older side-by-side double guns. I have a 1944 era 12 gauge LC Smith Ideal grade in FWE- 28" M&F- with RST 1140 fps loads, or Remington STS 1140 fps loads- no problem- but if I use a AA Handicap load at 1350 fps in either or both barrels, 50% of the time it will double- and it has DT, not the troublesome Elie HOT, so to avoid any stock cracking issues with a double gun that is very expensive to have re-stocked, I use RST 1140 fps shells 100% in that later Ideal grade, as well as the 4 other 12 gauge Elsies I own. is your shotgun an european mfg. gun?If so, have you verified the chamber length- You want to see a 70mm chamber stamping- equals our 2 &3/4" lengths shells-RWTF
