I am getting to the point where my current 18" rifle gassed barrel on my 3-gun rifle is getting shot out and in need of replacement. I have an 18" Larue Stealth barrel ready to go. It is the perfect balance of weight and accuracy for this rifle. I have no doubts that it will perform to where I need it.

However, being a midlength gas, I'm concerned I may not be able to get it shooting as soft as an 18" rifle gas barrel. I will be running an adjustable gas block, so theoretically it should be similar as the gas only serves to unlock the bolt, but I'm still unsure. Does anyone have experience with 18" mid gas vs 18" rifle gas?

Also, I will be giving a MicroMOA Govnah gas block a try. Does anyone have recommended port diameters for a 18" midlength? I was going to get the 3-position with high (open), low, and suppressed settings.

I have one that I run on my SPR/3gun with the Larue provided gas block and have no issues with it so far. Cycles fine and accurate as nuts!

I'm not really worried about reliability. I'm basically wondering if I can get it shooting as soft as a rifle length gas system. FWIW, I'm running a lightweight carrier and A5 system using the lightest buffer. Adjustable gas is a must.

It's not going to be as soft as intermediate or rifle gas length. With your setup, running a adj gas is probably the best bet!

It's not going to be as soft but if you're using an adjustable gas black and not shooting them back to back, most of us probably wouldn't be able to tell the difference, IMO.

FWIW check out the Odin Works gas block.  Adjustment screw is on the front so don't have to take off the handguard to adjust no matter the handguard design, and has detents to keep it from moving.  It's not as simple as a 3-position so may not be your preference, but it does allow for more fine tuning if you're trying to minimize recoil to its bare minimum.

If you are going to go with an adjustable gas block on a LaRue Stealth barrel. I would start and stop with the SLR Rifleworks Sentry Seven. They make a .750 diameter block with .450 set screw spacing. That will allow it to mate perfectly with the 3-dimples LaRue Stealth barrels already come with.

Odin Works block does as well, fwiw.  The SLR is supposedly the gold standard but pricier.