I am having troubles with a 1925 M12 feeding the shell into the chamber. I have changed out the carrier asmbly, even added one from a trench gun with a carrier guide. at first the shells would hit on the outside edge, now with the guide, they fall short and hit at the bottom of the chamber...any idea's

Dunno if this will help, but I once had a Mossberg shotgun that would feed anything but Federal shells. Seems they were - like- 1/32 longer or something..

Try a different brand?