I was looking at a 16" PSA upper (with carbine gas system) and they included the following statement:

"Notice - This upper is recommended for supersonic ammunition only to ensure reliable cycling.  This upper will run with subsonic ammunition however may require tuning of other components (buffer, recoil spring, or BCG) to improve reliability due to the lower pressures of the subsonic 300 AAC cartridge."
http://palmettostatearmory.com/psa-16-300-aac-bo-1-8-nitride-13-5-keymod-freedom-upper-without-bcg-or-charging-handle.html

I already have a std mil spec LPK, BCG, buffer...What "tuning" would be needed?  Or would I just be better off with another upper for running subsonics?

It could require a different combination of buffer & spring, or possibly enlarging the gas port IF NECESSARY (the last course of action).  Unless you try it first, you won't know exactly what would be needed for it to cycle properly and lock open on the last round.

Additionally, you may look into a suppressor for quieting it down for when you do shoot subs, as there isn't much of a point to shoot subs without a suppressor.

My 2 cents?

Purchase a barrel with a pistol length gas port location.  It will be less hassle with subs.  Oh and buy a can.  

fwiw- I have an 8" SBR w/ an Aero 300 barrel with a pistol gas location.  It cycles and locks back on all combinations.

Super/Unsuppressed
Super/Suppressed
Sub/Unsuppressed
Sub/Suppressed

eta- I'm using an LWRC UCIW stock kit on that SBR.

Yes, the intent is to shoot it suppressed.  
Thanks for the reply...instead of the carbine length gas system, looks like I should pursue something with pistol length gas system for best reliability with the subs.  Right?

Mine works with standard carbine buffer and spring and can. YMMV

Basically they are giving you sound advice..............reduced power buffer spring and light buffer may be required to reliably run subsonic ammo..............Most people will say pistol gas as opposed to carbine gas is the most reliable set up if you intend to run subs.........................

Yes.


I have a 14.5 w/pistol gas and its been 100% with subs, no can, no drilling, trimming, or screwing with anything.

carbine spring/"H" buffer on two lowers.

Brass has always looked good even shooting some nice hot Barnes copys.(110gr) No signs of overgas with supers.