Have a subsonic short barrel load to share?

[CTinNV]

I finally got my Noveske 10.5" barrel with pistol length tube and tried out some loads using 8.8gr of AA#9 and 240gr SMKs. Velocity was where I wanted it to be (about 1000fps), but the operation was erratic. (i.e. short strokes, usually ejecting with no next round feed). The rifle is using a standard buffer and the lighter AR15 semi carrier.

My first instinct is to drill out the gas port and use an adjustable tube to dial it back, but I'm wondering if the load is just not right for the barrel length. Is there a different powder I should be using for this setup? Thanks!