I have a new upper for my AR (A2 style from Bushmaster), and a friend of mine asked a good question.

Can you use a foaming bore cleaner in a gas gun like an AR? My concern in not being able to get all the cleaner out of the gas tube.

A little bit of foam in the gas tube won't hurt anything.

A blast of air-in-a-can into the gas tube from the receiver end will clear out the foam.

The gas holes are on the top or the bottom of the barrel in a semi auto.

If I put copper solvent in a semi auto, I have the hole pointed up, so gravity helps me keep from loosing my solvent down the gas hole.

No problem in a direct gas impingement gun-just dry it out. Not sure about piston guns.