I've got a 9mm DI upper that's set up for colt mags (double feed) with normal M4 style dual feed ramps on the barrel extension. I use this on my SBR lower with an adapter block. I want to build a second lower (non-nfa pistol configuration) that uses glock mags (which are center feed) and use this upper on it.... but... Glock mags are center feed. I know about the BCG needing a different cut for glock mags and that's not a problem, I'll just get a spare BCG and mill it for glock mags.

The only issue I'm wondering is if cutting the center bump out between the twin feed ramps so that the glock mags will feed is going to cause the colt mags to misfeed. Has anyone tried to run double feed mags on a DI upper that's already had the ramp modified for glock mags? I don't want to screw up the feed ramps and not be able to run it with the colt mags again.

Who built your upper? I have 2 DI 9mm uppers a 10.5" and a 5" (from RMW XTREME) I have not tried to run them on a double feed mag but I would think it would still work depending on how your bcg is modified for the Glock mags. I know the 2 lugs on the bottom of the bolt need to be touched to go thru the Glock mag as well as the rib on the bottom of the carrier depending upon how high the Glock mag is in the lower when fully seated. I don't know if I still have a Colt mag adapter and a mag but I will look and if I do will hand cycle some dummies thru it and see if it works. (my DI 9mm is setup for Glock mags).

I built it myself. The BCG isn't a concern really, I'd rather just do a second BCG just for the glock mags and not touch this one because it's working with the colt mags now and I don't want to mess with it.

I'm not familiar with how the commercial builders do the ramps on the barrel, but as mine are now I just smoothed them out so they weren't leaving gouges in the brass while feeding, there's still two distinct ramps and not one big one like I would assume the glock mag guns would use.

I would suggest getting the Glock lower and testing it to see how the mag sits and get a new bcg and see where it contacts the mag, you will more than likely have to hit the outer edges of the 2 bottom lugs on the bolt and the bottom rib on the carrier to get the bcg to pass thru the glock mag. Then test some dummy rounds feeding by hand and see if you need to do anything to the extension. I will have to look but I don't think my extension was touched?

If it helps, take a look at CMMG's Guard 9mm which feeds from a Glock mag.  Although not a DI system, the geometry is basically the same.  They took an AR extension and shaved down the center lug but not completely flat.  The two original double feed ramps are clearly still visible (pic from badkarmaiii):



Badkarmaiii, myself, and others have gotten this BCG/barrel combo to work with Colt magazines.  So I think if you shaved the center lug down a little for the Glock, you'd still be ok with Colt mags.  However, as 475AR10 mentioned I'd first try feeding from Glock mags without touching the center lug.  You may be pleasantly surprised.