Whats this??

[shaunx]

What is the little chamber that can be screwed in and out under the barrel on the regulator called?
And what does it do?

[Chuckles]

Little chamber? Depends on that marker you've got. On cheap spyders etc it will be a volume chamber. On electros it might be a LPR.

[Jawa_nz]

if you can get a pic pointing it out, then we can tell you what it is. Or if you are more specific as to what marker and exactly where it is.

[pbsniperassasin]

look on your instructions e.g exploded diagram or call the guy you got it off