I am looking to build 4 SBR's.  Actually, it will be 2 for me, and my brother is going for 2.  We are looking at a 5.56 and a 300blk each.  Both will be suppressed 100% of the time, and the 300blk will have subs and supers through it.  So, my thinking is that as long as I get a decent gas port on a 5.56 that is meant to be suppressed, there should be no need to go for an adjustable gas block.  However, since I will be running supers and subs through the 300blks, that it would be in my best interest to get an adjustable one for those.

Is my thinking correct?

Also, the 5.56s will be 11.5 inches and the 300blk will be either 8.5 or 9 inches.   Any idea one what gas port hole sizes I should be looking for with them being 100% suppressed?  

Thanks in advance.

Noveske switchblock.

There are other adjustable ones, but you dont need all the adjustment they offer

This is what I use on my SBR/5.56's, and they are awesome for the purpose... Should not be needed on the .300 BO's however.

Not to threadjack, but what tooling is needed to adjust the switchblock? If so, does Noveske include that or does it need to be purchased elsewhere? Who makes a good rail in the 10"-12" range that accommodates Noveske's switchblock?

For dedicated suppressed you just need a gas port that is properly sized. If you have a barrel with a small gas port you can enlarge it to what you need (something like a Colt barrel with a .062" gas port cut down to 10"). If you have a barrel with a large gas port you can use BRT gas port inserts to reduce the port size to what you need. The size of a BRT gas port is comparable to the size of an actual gas port (a .060" insert over a .070" gas port should function the same as a .060" gas port).

I believe your thinking is correct about the 5.56, if you can find a barrel with the proper gas port for suppressed only fire. The problem I see with that is your rifle would be a single shot rifle if you do not put your suppressor on and may not be reliable with all ammo. But with the right port size, right ammo and suppressor on you should not need to adjust. The BO set up will need adjustment since you are running different "strengths" of ammo.
Interestingly and surprisingly I found an advantage to a bleed-off system for adjusting gas pressure when you are running suppressed. Bleeding off the excess gas thru a separate port in the gas block rather than pushing that gas out of the muzzle and into the suppressor makes the suppressed weapon quieter. You can see that there is still occasional flash from the suppressor when restriction is used and none with bleed-off. This accounts for the louder sound associated with a restriction type of adjustable gas block. Here is a field test where I first discovered this advantage....https://youtu.be/Cy5XLr3igaU

Thanks for the info, I will look at that when I can.  What port size should I be looking at for a suppressed 11.5in 5.56 and 9in 300blk?

For an 11.5" 5.56, unsupressed would be .067"-.070" depending on buffer mass. Suppressed, maybe .060"? That's what's great about the BRT gas port inserts. You can go smaller or larger as you need.

I have no problems with my 10.5 inch 300 with subs and supers, with or without a can. No adjustable gas block needed. It's pistol length gas.

Same with an 18 inch 5.56, mid length on an A2 rifle lower, runs fine with or without a can

I do have an adjustable gas block on my 308 ar10. With a can it's just way way over gassed.

Shoot it a bunch before you fiddle with it I think