I built a 350 Legend upper as I have an invitation to hunt deer in Ohio and they have "straight wall only" type requirements. Shot the upper to zero scope. Low recoil - better than 9mm AR upper. Got ready to reload the 350 legend brass with 147 grain 9mm bullets for practice loads (350 Legend is 355/356 not 357) so it will be "pistol range steel" friendly. Had a thought - could this be a viable PCC round for AR platform? 1) Uses gas not recoil operated. 2) Can use common 9mm pistol projectiles. 3) Uses standard AR platform - uses 223 bolt, upper, lower, etc. - difference is barrel and magazine. 4) Assuming high cap mags become available (currently 5 and 10 round available). 5) I can see playing with adjustable gas, different buffers, light weight BCG = very low recoil.
450 Legend is in many ways 9mm "Very Long". Will be interesting to see.