I used to have the same problem with handloads in my 742. The powder I used to load was too slow burning to allow the gas control system to work properly. My cases were still under high pressure (and expanded from the heat) when the action tried to open, and the extractor would rip my rims. I switched to a faster burning powder, and the problem went away.
Since you're using factory ammo in a relatively new gun (at the time), it's unlikely this is the exact problem you're having.
In the old days Remington used to supply a chamber brush with all new 742's, but I don't know if this is the case with the newer 7400s. This brush had a bent handle, and was intended for cleaning powder residue out of the chamber.
Autos still have some of the products of powder combustion under some pressure in the barrel when the action begins to extract the brass. This allows some blow back of this residue into the chamber. It's best to clean it regularly in a Remington auto. If you don't have a chamber brush, I recommend getting one - and using it.
I suppose there may also be a rough chamber in your rifle, but since the Remington ammo works OK this is highly unlikely.
