Back when the CMMG RDB came out around 2017, I was still running 5.45×39 uppers and noticed that the bolt face was about the same as 9mm and dropped a 5.45×39 BCG into a CMMG RDB upper and it made it a straight pull single shot. I posted about it several times over the years on AR15.com. Bolt action AR uppers have been around a long time as well but you don’t see them for pistol caliber. The issue is that the case is so short on a pistol caliber that the ejector tries to kick it out as soon as it leaves the chamber and usually falls into the action and that gets old having to shake the rounds out.
I bought this BCA builder’s kit: https://www.bearcreekarsenal.com/bas-upperreceiver-builders-kit-450-458.html since at some point, I would do a configuration for 45ACP and I could re-use the bolt it comes with for that caliber. While they don’t offer anything that would be drop in for 9mm.
I first tested it with modifications and it fed fine with Endomags, Exomags and with a CMMG Glock lower as pictured below.
As expected, it did consistently drop the fired case into the action. I had an extra fixed ejector that I use for my PhaseII builds so I decided to modify it for the fixed ejector.