It was my understanding that this combination is not ideal, but it would work.  However, I did a function test on my build today and everything ran ok.  I had two jams, both hit the edge of the upper head on and fucked up the crimping on the round.  Both happened with Remington UMC Cproducts SS magazines and I couldn't try it out with any others.

Does this mean the magazine is sitting too low in the receiver? If it had standard FRs I would assume it'd do the same thing.

What is a good file size for making the ramps on the receiver?

Dremel, but I'd be scared to do it myself.

Perhaps try some different ammo and put some rounds thru it first and the problem may go away.

I have done it with a dremel.  However, I have had formal armorer training and a good number of years experience.

Work  S L O W L Y  and cut just a little at a time, then polish your "homemade" M4 feed ramps to finish.

OR... you can buy an upper with M4 ramps, then sell the other or use it on another build.

Try a chainsaw file.
Take it easy, the aluminum comes off quickly.
Finish it with a smooth round file.

Weren't there some QC issues with CProducts SS magazines?

I'd try a known-good USGI mag before I'd attack my rifle with a file.

this

i had the same EXACT problem with an aluminum CProducts mag... the rounds were catching on the front of the mag and that extra drag was causing the round to nosedive into the upper... the mag body wasn't welded evenly, i filed the front of it down where the small lip was catching the rounds and it ran 100%... i pulled the spring and follower and trashed the mag body.

I have the same type setup (Colt M4 bbl and standard upper) and have never had a problem with GI mags, both full auto and semi.

ADCO does it cheap, just send the upper, no FFL needed.