300 Blk Subsonic Issues

I recently built a 300 blk SBR upper and am struggling to get it running reliably with subsonic ammo, hoping someone on here has any advise/tips to share.

I'm guessing the gun is severely undergassed, even with the gas block fully open the bolt would hold open on the last round. Also constant failure to feed issues (2-3x per 10-round magazine), didn't matter which mags or ammo that I tried.

Build specs:

- Fightlite SCR lower receiver
- Gibbz side charging upper receiver
- 10.5" 1:5 twist 300 blk barrel - pistol length gas
- Faxon low profile adjustable gas block
- CAT ODB Ti suppressor
- Magpul 300 blk PMAGS and Lancer 300 blk mags
- Ammo: Winchester Super Suppressed 200 Grain Open Tip, Scorpion 220 Grain Subsonic, and Magtech 200 Grain Subsonic Full Metal Jacket