In case the op is contemplating the PSA as I was, I just received a PSA reos response to several questions I had:
"Hello,
Unfortunately, the link you gave me didn't have any product information, but to answer your questions in regards to our AR-9 system :
The hybrid G9 bolt in our newer AR-9 uppers will not hold open after firing the last round. Magazines will not affect this.
You are correct, the lower uses all the same pins, buttons, levers, and springs.
Handguards can be replaced at your convenience. Some handguards require the removal of the barrel nut. This will void your warranty with us.
The thread pitch on our AR-9 is 1/2x36
The hybrid G9 bolt is designed for use with both Glock and Colt style magazines.
If you have any additional concerns, please contact us.
Best regards,
Dylan D.
Customer Service
Palmetto State Armory
(803) 724-6950
Info@palmettostatearmory.com"