First let me answer the question about why Remington redesigned the 7400 and call it the 750: They redesigned the gas system for two reasons one more reliable, second they will be chambering it in Short mags soon (next year maybe).

More guys than not shoot them at the club I belong, and they all jammed real good. I spent every evening the first season cleaning there guns and have worked except a few time in the last 7 years. Because the bore and barrel can't be reached with a cleaning rod from thebreach people neglect them. Additionally they clean them from the front and push all the fouling into the bolt area, solvent gels up and they stop working. So my experence is you will either need to strip the gun to parade rest and clean the gas system and boltwith karosene or deisel, then 91% isopropel. Scrub the barrel and chamber with Sweet's 7.62 bore solvent alot with bore brushes and jagsuntil it's clear or green and black, then use 91% isopropel alcahol until it's clean of all Sweets 7.62 and let dry. Clean again with Sweet's and alcahol. Then clean the inside of the action and trigger assemblywith regular petro based powder solvent, tooth brush and qtips, clean with alcahol, thendo it again. Reassmble using dry film lube on the inner workings of the bolt and trigger assembly, useA LITTLE Mil-comm TW-25B or Wilson Combat ultima lube grease on the rails the bolt cycles on and piston/o-rings of the gas system.

If your not compfortable enough to do this take it to a good gunsmith and have it done.