Last year Mean Arms, the makers of the MA Loader, came out with a caliber conversion insert. The EndoMag converts a 5.56x45mm PMAG into 9mm. With the EndoMag you can use your standard AR lower to shoot 9mm and all you need is a 9mm upper.
Installation of the EndoMag is straightforward. Take a Gen 2 or Gen 3 PMAG and remove the internals. Then take the EndoMag and cut the rear tab for the desired capacity. The EndoMag comes from the factory with the capacity restricted to 10 rounds. But if you look at the photo below, there are indentations that correspond to your desired capacity. The highest mark is for a full 30 round mag of 9mm.
Since you are using a standard 5.56 AR lower, it won’t have the ejector like a dedicated 9mm lower. So the EndoMag has a built-in ejector just above the feed lips. You can see the rectangular looking block below.
When the EndoMag first came out I was a little hesitant about the idea of a polymer ejector. However Mean Arms told me, at the 2019 SHOT Show, they have thousands of rounds through their first production EndoMag without an issue. There is a slight change in the manual of arms due to the integrated ejector. If you want to eject a live round, you have to keep the magazine in the gun to do so. So if you want to “Empty and show clear!” in a USPSA match, put the safety on, lock the bolt back and then eject the magazine.
However, there is an alternative solution if the plastic ejector bothers you. Use a CMMG Guard upper receiver. The CMMG Guard bolt has a built-in ejector like a 5.56 bolt. If you have a CMMG Guard 9mm upper then Mean Arms can modify the EndoMag to work with the Guard system.
Here is a video of Mike shooting a CMMG upper on an AR lower using a modified EndoMag.
The EndoMag insert retails for $29.99 then add a PMAG and you are around $40-$50 for one 30 rd magazine. However, you saved money by not having to buy a dedicated lower or a pistol magazine adapter. With the EndoMag, you can quickly change calibers without the need for tools. Just take the magazine out and swap uppers.
I only have one issue with the EndoMag and that is the fact the capacity is fixed at 30rds. I would like an insert to work with the 40 round PMAG. Due to the EndoMag design, it won’t work with a 40-rd PMAG. For people in restricted states, they do make a fixed restricted 10-rd capacity EndoMag for $44.95.