[45_Fan]

First off, I don't currently have any firearms based on the AR platform.
Second, I have above average mechanical inclination and tooling.
Third, I live ITP and 25 yards is "long range" around here.

I've done enough research and reading to have a basic understanding of AR parts and functions but have questions about caliber vs barrel length vs twist as well as gas port sizes and placements vs caliber.

Subsonic velocities are expected -- and I reload.

What are the min/max barrel lengths I'd want for a 300 blackout AR pistol?
What would be optimal twist for those barrel lengths?
What would be optimal gas port sizing and placement for those lengths?
Is a gas piston something that can be added into an existing upper?

 

[rmodel65]

Adams arms makes retrofit piston conversions

 

[ber950]

9" is about the optimum length with a pistol gas system for subsonic

 

[KingTiger]

1. 7-9"
2. 1:7
3. Pistol length gas, not sure about port size, < .1" I think.
4. Stick with DI, people have experienced difficulties trying to get piston systems to run with subsonics.

 

[Dirty J]

1. 7-9"
2. 1:7
3. Pistol length gas, not sure about port size, < .1" I think.
4. Stick with DI, people have experienced difficulties trying to get piston systems to run with subsonics.

Yes. (don't know about the port sizing though).

Just know my 8.5" pistol gas DI 300bo shoots great.