ringram

Ive had similar and a tweak on the right hand wing helps Ive also chamfered the right side of the barrel a tiny bit. Used to hang up about 1 in 50. Hopefully sorted now. I hand cycled a couple of mags no problem. As also mentioned see if you can fund ammo with a good roll crimp.

Would this also fit a BR99 lower?

Yep, it might look closed but it only takes a couple of mm to prevent it firing. When new this happens more often. Its pretty annoying. Use lots of lube on the bolt and rails, push the bolt forward before firing to make sure its fully in battery. And check its clean, maybe the bolt relief in the barrel is gummed up etc.