I have a M85NP that I have always shot with the 4-piece Bulgarian brake. I read somewhere that one reason it was developed was to provide some back-pressure to reliably send the bolt back home when shooting the underpowered Russian 223, particularly in cold-weather environments. Question I have is how necessary is it? I would like to get something lighter like a birdgage F/H. I was wondering how much backpressure might be created by installing a 1/2-28 thread adapter with the standard AR birdcage on that. The adapter screwed onto the M26L threads at the end of the barrel would create a small cavity which might provide some of the backpressure created by the 4-pc Bulgy brake. What do you guys think?

Have you shot it without the bulgy 4-piece? I dont think the back-pressure thing has much to do with it, at least in semi-auto.  Russians/bulgarians dont traditionally use 223/556 so would not invest in making a muzzle device to increase the back-pressure for that round...maybe in 5.45, but I thought a back-pressure problem with 5.45 was an internet rumor...not an expert though.

With the M85 it is less necessary than the M92 and surplus rifles. The M85 gas system was made specifically for the 223/5.56 round and will function just fine with steel cased 223.