Took the MCX out for the first time today.

I purchased the factory 9" 300blk FDE SBR in March and it was approved last month. I changed out the 9" barrel for the 6.75" and installed a DeadAir Sandman S suppressor.

Went with Fiocchi subsonic EXACTA 220gr SMK rounds. Had 20rnd Magpul 556 mags, 30rnd 300blk Magpul mags, and 20rnd and 30rnd Lancer mags.

The MCX is very accurate and was easy to zero. I'm running a FDE Aimpoint T2 with Magpul Pro backups. Had no malfunctions with the gas setting set to "+" as instructed in the manual. Ejection did seem week on a few rounds though. Might just be this ammo. Will try some different ammo on next outing. Anyone else running this setup having great luck with a specific round? Will probably try the Sig 220gr rounds next.

With this setup, it was extremely quite, and ringing steel plates really showed how quite it was. Love the VIRTUS trigger. Will be nice to save a little and not have to buy a Geissele. Charging handle has quite a bit of vertical play when extended and strikes the top of the thin folding stock.

Only issue I ran into was the bolt would not lock back on the last round when using Magpul mags. Both the 556 and 300blk mags failed to lock bolt back on last round but the Lancers had no issues. Not sure if anyone else is having this issue. Might just need some breaking in???

Overall I am pleased with this system, and look forward to putting many more rounds down range with it.

Same issue with LRBHO on my Magpul 20 rd'ers (which is all I use for .300)

Got to the range again this weekend to put some various types of ammo downtange. Also installed a Geissele MCX trigger. Tried Remington, Winchester, Sig, Hornady, Fiocchi, and American Eagle. All subs.

All grouped well and were accurate, but some left quite a bit of unburnt powder in the receivers. Not sure if it's because of the 6.75" barrel or what.  The Hornady 208gr AMAX was very accurate but has week ejection and would not lock the carrier back in the correct location on the last round. So inserting another mag after was not possible without running the bolt all the way back with the charging handle.

Anyone having any good luck with a specific subsonic ammo and the 6.75" barrel and not getting a ton of unburnt powder left behind?