Read this before buying. compatible with cmmg 9mm rdb system. if i could rate 3.5, i would. - New Frontier Armory Pistol Caliber Gen 2 Stripped Upper Receiver

You're probably looking at this upper receiver because you bought a lower that lacks a last round bolt open, like I did. I additionally selected this to be used with the CMMG radial delayed blowback (RDB) 9mm kit. I'm happy to report this works. No modifications necessary. I paired this with the KE-9 lower receiver; this requires removal/modification of the magazinestop/case ejector. This has the cutout for the cam pin that you would normally find on a normal AR15 bolt carrier group, which you will need for the RDB system. The only reason I went with this pairing is because I wanted ambi mag release and KE-9 is the only one that provides this for a decent price. Since it lacks the LRBHO, this upper is the only one I can find that retains the cam pin cutout.

I've used this upper with Korean, ETS, Promag, Magpul and 2nd Amend magazines. I think the 2nd Amend was the one that held the bolt open while there was still one round in the chamber. However, I did not have a magazine stop installed, so it can be actuated by upward pressure on the magazine. Other than that, no issues with the upper. I've only had failure to eject issues during rapid trigger pulls that is characteristic with using the RDB kit w/o CMMG upper or lowers.

Here are my 3 cons for this upper.

So here is...the cons, which are not too bad, but its enough to mention. I can tell that the upper receiver is of lower density material compared to the KE-9 Lower. You can hear it and feel it, which makes me concerned about durability.

After testing, the screws that holds the last round hold open xfer bar became loose and I lost 1 screw. Thankfully New Frontier sells small parts for a moderate price. I paid over $11 for screws and a ejector plate (I'm going to modify the plate to use as a mag stop compatible with the BCG).

Lastly, the fit between the KE-9 and the NF upper is a little sloppy. Granted, it's meant for an NF lower, but I feel like it would just be as sloppy. Again, I don't expect (and didn't demonstrate) MOA accuracy, just something that bugs me whenever I pick up the gun.

In conclusion, this is appears to be the only upper for AR-9 builds with the LRBHO that requires no modifications for use the the CMMG RDB bolt. Aero Arms EPC requires cutting out a space for the cam to swing into. I wish NF had cerakote options like FDE.