You will need one of the 1/2x28 thread models of the JP Enterprises Tactical Barrel Compensators for a S&W M&P 15 Sport 2 in .223/5.56. The different size outside diameters of the JP Enterprises Tactical Barrel Compensators are for different profile barrels. I recommend using one of the .750" outside diameter 1/2x28 thread model.
You would need the .308 version, which has an exit diameter of 0.350:
https://www.opticsplanet.com/jp-enterprises-tactical-barrel-compensators.html?_iv_code=J5-BCM-JPBCTRE-JPTRE-412S&_iv_threads-per-inch=5-sl-8x24
The JP barrel bolt combo does not include the firing pin. You could use their comps that have an exit diameter of 0.350" and 5/8x24 thread pitch:
https://www.opticsplanet.com/jp-enterprises-tactical-barrel-compensators.html?_iv_code=J5-BCM-JPBCTRE-JPTRE-412S&_iv_threads-per-inch=5-sl-8x24
For the .350 Legend, you would need the 9mm version, which has a .406 exit hole. This is the model: J5-BCM-JPE3PBCMP-JPTRE3-236S
You could use the models that have a .350 exit diameter: https://www.opticsplanet.com/jp-enterprises-tactical-barrel-compensators.html?_iv_code=J5-BCM-JPBCTRE-JPTRE-412S&_iv_threads-per-inch=5-sl-8x24
Yes, they would be compatible.
It is .281".
No, it does not.
Any of the 1/2x28 threaded versions should fit your barrel.
Any of the models with the 1/2x28 thread pitch will fit your rifle.