The 124 has a very weak breech lockup mechanism, and a long barrel that compounds the problem. If your breech seal is new, or too thick, it can cause lockup issues and make the barrel point downward as well.
Or you could just have droop. I had one that did. I fixed it by filing the breech until it locked up correctly. You can also bend the barrel a smidge, but it takes a LOT of force and a solid way to hold it while it's removed from the gun.
But all that crap aside, are you sure it's shooting at the right velocity? It could just be super slow. Especially if the seals are rotted, the breech seal is bad, or the spring is broken. All of which are common 124 issues.
As for the mounts, I just use BKL mounts. They don't move and don't need a stop pin. If you want to spend money on a band-aid solution, they make a droop compensated mount or an adjustable mount. They are expensive but very well made.
Personally I'd check velocity first, then remove the breech seal and see if the gap at the top of the breech tightens up. If it does, size your breech seal. And don't sand the face of the seal, you need to take material off the diameter.
Good luck.