I replaced the noveske switchblock on my 12.5 5.56 uppers. I don’t have to adjust it suppressed or not, but then again I don’t shoot subsonic anything through it obviously.

With such huge bullet weight range on the 300, is an adjustable gas block necessary?

It was for me. Have a 10” barrel and I use it with a can, so I set it up to work with those, and it cycles anything else with or w/o the can on it.

Definitely nice to have.

Required? Not in my experience. I have shot 125 and 150 gr supers, 190~220 gr subs, suppressed and non, without a hiccup. PSA upper

Nice but not mandatory for me. I have a 7.5” bbl .300 Blk but almost always run it suppressed. No issues with subs or supers but it can get a little gassy with the supers.

10.3" Wilson Combat 300blk upper here. Fixed gas block.

My first couple dozen rounds were unsuppressed, both supersonic and subsonic. No cycling issues.

Since then I've shot everything suppressed, mainly supersonic but some subsonic. No cycling issues.

Surefire SOCOM RC2 mini suppressor. No gassing issues.

Another option is to change the buffer weight. For the BCM 16" upper - "These barrels were designed to run with a carbine buffer [3 ounces] with subsonic ammo and an H3 buffer [5.4 ounces] with high-velocity ammo."

I built a .300blk 10.5" barrel, pistol length gas system without adjustable gas block and a PSA ar pistol lower and it flawlessly with or without suppressor and with either sub or supersonic. My Huntertown Arms .30cal suppressor isn't as quiet as my brother's srd762qd. I was very surprised it worked that well though. Then again, I was told that .300blk was specifically designed to do that. Other calibers could have issues with the gas. I bought the upper from sportsman's guide so I'm not sure who actually made that portion of it. Used a nickel-boron carrier/bolt and trigger group to help with reliability in dealing with the extra dirt and grime.

https://flic.kr/p/2qxY7uF

The top one is the .300blk. Actually an old puc I need to update it. I put a sig lpvo 1-6x24 on and put a skinnier forend on it.