How does a fella' adjust the pressure going to the bolt carrier assembly?

Its NOT adjustable in any way I'm aware of.  

It can be changed by a whole assortments of ways, that require modifying the rifle in some way.  But the mod will give you a different set pressure, no way to adjust it other than doing another mod to change the pressure again.

I have seen an aftermarket gas block that has an adjustment on it for gas pressure, NOT sure how well that works and what is required beyond swapping the gas block.

Changing gas pressure can be done by changing volume of the chambers that are around the gas port.  Either move the gas port on the barrel (not very practical, but keep it in mind if your getting a new barrel), change the size of the gas port (can only go bigger to reduce pressure, NOT smaller to increase pressure), or change the volume of the gas tube, there are Fatty Tubes and pig tailed tubes that are either wider or longer (wrap around the barrel) that increase the volume and thus reduce the pressure in the gas tube.

Mass of the BCG, Buffer and Buffer Spring can effect the reaction to the gas pressure, that is another way to "TUNE" for the effect you desire.   Perhaps there are custom BCG out there that also have different chamber volumes to change gas pressure, never heard of them, but I could see that being another way of changing pressure.

good info...thank you man

You could purchase a JP adjustable gas system.

Yep, JP was the one I saw, never used it, NOR heard from anyone that has ever used it.

I would worry that the adjustable orifice would foul with carbon, changing the pressure and require that you continually adjust it OR have to clean it.  BUT, I don't know that, I'm just guessing.

I drilled and tapped my own. Haven't got to shoot it yet, but I'll try and update in a week or so when I take it out.

Your own what?  Gas Port in the barrel or an Adjustable Gas System?

Let us know and how well it worked.  As well, let us know how long you can shoot on one setting without cleaning and if you experience any signs of reduced gas pressure, i.e. the valve clogging up.

You can try an MGI Regulated Gas Tube, available in carbine, middy and rifle lengths. Its a 5 minute drop-in part and totally reversible. The only challenge can be access to the adjustment valve, which requires a hole in the handguard for an allen wrench to reach through:

Again, how well does it work?  Don't get me wrong, this could be a great innovation, could solve a lot of problems for a lot of folks.  BUT, I also wonder will it just clog up with carbon and require constant adjustment to compensate for the carbon build up in the valve.  Which, if true, is your weapon really reliable if it fails to cycle properly after 300 rounds and have to make adjustments to get it cycling properly again.

Maybe it doesn't and that would be a great piece to add to any AR.  I keep reading the carbine cycles way to hard, thus a higher rate of stoppages than the Rifle version, throw in many civilian carbines out there have an extra 1.5" past the gas port on the barrel, making the gas pressure even higher than the military version, this might be a great piece to add to a carbine and reduce the cyclic rate and see if stoppages reduce.

Lots of uspsa rifle shooters use them in JP rifles to add just enough gass for function.  These rifles need to be maintained a little more than a normal rifle ass most of the rifle is low mass.

Clean rifle they will function easily over 400 rounds if set correctly.

Slight correction:  Opening up the gas port will INCREASE pressure, not decrease it.

Thanks, that's right, a larger diameter gas port will provide greater pressure, a smaller diameter gas port will provide less pressure.  BUT, you can't add metal to the gas port to make it smaller in any practical way I am aware of.

You can slow your action down by running a heavier BCG and buffer.

You can speed up your action by opening up the gas port. (carefully)