I have three Colt 20rd 9mm mags. Two of them look like the one on the right. The one on the left (the pretty one) will not fit into the mag well of my GPI 9mm lower. It goes in about halfway and then stops (and I do not want to force it). If you look closely at the mags, there are some slight differences. The one that will not fit has a slight upward contour to the feed lip that is obviously part of the design. The one that does not fit also has a shorter indent line (not sure what it's called) and the one that does not fit does not have that little "bubble bump thingy" on the follower.

I bought the one that does not fit from a well-established and trustworthy member here who said it came in the sealed box of an old unfired Colt 9mm (with A1 style lower) that he got in a trade. He cleared some hogs off a farmer's property and the farmer game him the un-touched carbine in return. He said he's used the mags (the on on the left) several times and it fits perfectly and that it runs perfectly. This makes sense because the one that does not fit is in really great shape with fantastic finish. I tested it in a friend's Colt 9mm lower (a newer one) and it fit fine. For some reason this particular style of the one on the left appears to fit Colt lowers fine but will not fit my GPI.

Does anyone have any info on the difference in these 20rd Colt mags? They're expensive as hell so I don't want to keep buying them if I'm going to run into this issue again. (Also, again for the record, the other two Colt mags fit my GPI lower perfectly as do seven 32rd converted UZI mags that run flawlessly).
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Maybe it's just an optical illusion, but the mag on the left looks slightly longer (in bullet direction) in the Top View picture.

Difference in the feed lips makes me think right mag (flat) is probably Metalform and left has a flare which is probably C Products (out of business). Both have made factory Colt mags in the past. Normally I'd recommend Metalform since that's been OEM Colt for the majority of their production years (I think C Products only supplied Colt for a few years before going back to Metalform) but if the raised lip mags are running well I'd look for some new C Product Defense mags ( which is NOT the same company as the now defunct C Products). Or shop the EE for older C Products mags.

Those are both metalform mags.  One is the raised feedlip design and the other is not.

I think they only offer flat feed lip currently.

Thanks for the feedback guys. I guess I know what *tell* to look for going forward buying new (used) mags to fit this GPI lower.

I was wrong, above is correct, both mags are stamped with an 'M' making them original Metalform production. Wasn't aware Metalform ever did a raised feed lip. Colt did use C products back around 2010 with the same baseplate markings minus the 'M', and based on pics I searched thru all of them had the plastic orange follower.

You may have to do a little filing on your magwell..  I had the same problem.  There is a thread somewhere on it.