Will Adams Arms .300 blk rifles and pistols cycle subsonic ammunition(factory remington subs)? I have emailed adams arms and messaged their FB page. No answer. Anyone who has tried it, please let me know. Thanks.

You're not likely to find anyone around here who will be able to answer this.  Everything I've seen around here related to this subject (300 BLK + piston) indicates that it simply does not work.  Yes, I know, there are other piston manufacturers who sell 300 BLK piston guns.  But, I believe those setups are specifically tailored for 300 BLK.  A drop in system like AA is going to take a lot of effort to get subsonic loads running/functioning at all, and even more effort to achieve reliability and consistency.

Actually the new XLP and the lightened carrier are designed to run 300BO.  With supersonics I have my gas set to the 3 of 5, and that's 1 over where I was at full reliability.  The low-mass carrier really makes the difference though, as low power from efficient powder burns is the issue.  Now I will say I"m running a 10.5" barrel because I wanted to maximize dwell time for exactly this reason, but as of now I've had no issues with subs, I just go to the max gas setting and go.  Sooner or later I'll make the investment in a can and it certainly won't be an issue then, but it's not one now without it.

Ok, so you are saying to go with the low mass carrier... What buffer are you using? I have a standard carbine buffer now with the DI upper and its flawless with subs. Same?

That's good to hear.  I tried running the regular carrier with pistol length XLP system on my 10.5" seekins 300BLK barrel and I couldn't even get sonic loads to cycle reliably on full gas.  I had the same thoughts on barrel length because my end goal was a piston driven 300 BLK pistol.  I have a low mass carrier on the way so hopefully that will be the solution.

What buffer are you running in your setup?

ETA what size is the gas port on the barrel?

Standard carbine buffer and spring.  Until I have an issue I have no reason to change it.  If you need to go lighter though you can always pull the weights out of the buffer.  With the adjustable gas on the XLP there should never be a reasonto go heavier on any components.

Couldn't tell ya on the gas port.  It's a Bergara so maybe you could check with them?

Are you using carbine or pistol length gas system?  I'm assuming pistol length, but if it's carbine, I'd really like to know the process you used to decide your components.  I'm currently spec'ing out an AA upper and would really prefer carbine to pistol length, but may not be in the cards...

Yep...10.5" barrel with pistol gas.