AGBs definitely can improve at-ear sound (Potss described this very well).
The bleed-off can be helpful for a couple reasons and can also work against you if you don't know what you're doing. There's a point where bleeding off too much on a short host can cause so much backwash sound that it's louder than the port. Like a piston dump on some systems. SA also makes a very good drop-in piston system. It gives you a huge spread--from very hearing safe to very NOT hearing safe (but with all the gas you need to run the system in adverse conditions).
I believe this is very host dependent, but this is what I measured on my 16" Ballistic Advantage barreled upper.
There are two sweet spots:
1. The first is where you've just limited gas (1 to 2 rotations out). If you take the suppressor off, the action gets too dirty, or you switch to under-powered ammo, then this might not work for you.
2. The second is where the bleed-off is balanced with the needs of the action. This typically won't get quite as good at-ear results as the option 1, but it really depends on how your weapon is set up (buffer, extension tube length and spring, etc.). Typically this is 7-8 revolutions out. The benefit is that the weapon will typically still run if the suppressor is removed. A short barrel might negate the at-ear goodness if you dial it up too much.
Note there are 4 clicks/rotation. For this study, I shot 1 bullet at each click point. Also note that there's a certain amount of conservation of energy. As the port gets quieter, the muzzle can get louder (the energy has to go somewhere, right?). The bleed-off helps with this effect. Note that this is a game some manufacturers have used to get stupid-low marketing numbers for at-muzzle measurements. A rifle with a huge gas port that unlocks early will give a few dB advantage in such a test. That's for all you dB chasers that get all hot and bothered over a .5 dB difference.
Enjoy....
SA DI GRAPH
Todd Magee
Dead Air Engineering
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