I have 2 300 blackout guns.. Its an excellent cartridge for heavy subsonic loads
one is an older style Ruger American bolt action with the rotary mag and the other is an SBR AR15
The Ruger would feed any cartridge up to and including just about any 220 grain bullet but not reliably the NOE 311-247 HP which is what I wanted to shoot
same with the AR.
Polished the lugs back a little on the AR15 (opening the ramp wider for the fatter profile bullets) ect ect
Tryed just about every mag I could get my hands including on MagPul P mags ver M2 and M3, multiple steel mag companies ect ect.
Nothing would feed 100% reliable most not even 50%
well finally got my hands on a Lancer 20 rd mag . To my amazement it feeds the NOE 311 247 HP powdercoated bullets like butter
a happy camper I am
probably going to convert the Ruger bolt action over to the new AR mag fed version
for those that want to try it part # 99-000-4280-06 order directly from Lancer and choose USPS shipping ($4.40 shipping) . had mine in 4 days