Explain "gas block"

[Greg0]

This is clearly a naive question, but will someone explain exactly which piece on the AR is the "gas block". This piece seems to be mentioned in the technical description in most AR adds. I gather that this is the hunk of metal that serves to mount the front sight, but I would like to understand this better. Is this were the gas tube terminates?

TIA,
Greg

[bayo]

go to the bushmaster sight they have a good parts diagram that
should help you out

[Circuits]

The gas block and front sight base are the same piece on a standard AR-15.

Some people use a gas block only, which is basically a cut-down front sight base when they have optics, and the "dissipator" style rifles uses a regular front sight base at the end of the barrel, with a gas block under the handguards where the gas tube is actually installed.