What's the best tube to use for a sb brace that won't cause the brace to twist? A tight fit!

I don't know. I researched this same question and never found a suitable, to me, answer. That is why I went the SBA3 route. Uses standard buffer tubes with a standard pin setup to retract. Removes the twisting issue. Probably no help, sorry.

Phase 5 Hex tube. I pushed the brace far enoughforward to get onto the hexagonal portion, and it stopped all rotation.