Finally got around to putting my piston build together with all of this free time.
Build list:
Huldra Arms Piston upper
MTI Billet lower
Rise Armament Drop in trigger
Magpul Moe Stock

Not pictured is Eotech that is on its way. What handguards work on these? It seems like it is between a carbine and midlength?  Thanks!

sure looks like a re-branded Adams Arms.  counting the # of ribs, and then comparing to the AA mid-length HG on the AA web page, looks like mid-length.
https://www.adamsarms.net/adams-arms-handguard-and-cap-midlength

Do you have a triangle handguard cap?  if so, Midwest Industries mid-length 2-pc drop-in HG's should fit.  The MI pic-rail drop-in states compatible with piston systems.  I'm guessing the M-LOK version also, you should call MI to verify.

MAGPUL MOE and MOE-SL mid-lengths also fit.

https://www.midwestindustriesinc.com/MI-Gen2-Two-Piece-Drop-In-Handguard-Mid-Length-p/mctar-18g2-blk.htm

https://www.midwestindustriesinc.com/product-p/mi-18m-blk.htm

https://magpul.com/firearm-accessories/hand-guards/ar15-m4-m16-ar10.html

Thanks for the help! From my research it seems like Adams made the piston kit and then sold it to huldra/fleet farm. We picked up a few of them when they were dumping all of their AR-15 stuff 7 years or so ago. I want to say I paid around 250 for the complete upper.

Definitely there design.

Stag was blowing out there piston kits awhile back *rebranded AA kits* I snagged a couple @150 a piece and built a AR pistol 10.5 modified a KAC RAS II works like a champ and I have spares

Got mine in 2013 (complete AA rifle, what they call P1 today) and put on a Magpul MOE handguard. This was prior to it having the mlok slots. It fit without any modifications.  I'm sure the current version should fit. This is a mid length system.