The co2 pistols are pretty loud in .22 cal, dunno about .177. The pump pistols in .22 cal are halfway between them and my hw45. The hw45 has some mechanical(gun) noise but no pop(air/gas) noise. In the bush where there is no sound, the hw45 is the quietest of a crosman 2240 and a benjamin hb22 fyi. I have buddies these guns and when we shot them together it was obvious which one was the quietest...and deadliest <img src="http://jsramsbottom.co.uk/bbs/e/icon_wink.gif" alt="Wink" width="15" height="15"><!--graemlin:--> <img src="http://jsramsbottom.co.uk/bbs/e/icon_biggrin.gif" alt="Big Grin" width="15" height="15"><!--graemlin:-->.
Hope that helps.
B