Subsonic and ARs

[RWBlue]

Is it easier to get heavy or lighter bullets to fly subsonic and make the AR-15 work?

My AR works fine with supersonic rounds, but I have yet to get it to function properly with subsonics.

Should I be working on 165gr bullet and then the 220-240s or should I try to get the 240gr to work and then play with the 165gr ers.

I have a pistol length gas system.

With the adjustable gas system, I assume I will get them all to function, but so far, no joy.