I had a conversation with my Russian-speaking friend and he used this phrase to describe his impressions of the newly purchased rifle. "Я тут паре типов своих показал его. Они выпали с аппарата." I barely understand that he show the rifle, but the other part is complete mystery to me. Is this some slang gibberish? What the meaning of this phrase?
It's slang all right, but it's not gibberish. Without seeing the context, it can be roughly translated something like
I showed it to a couple of my mates. They were totally stumped.
(Note that I am using showed it, as you mentioned a rifle in there).
Word тип, apart from its main meaning type, kind is also used to refer to a person, usually in a somewhat derogatory manner, and usually a man, e.g. подозрительный тип could be translated as twitchy bloke (although word-for-word means suspicious man).
Выпасть с аппарата is one of a large number of slang phrases meaning to be totally stumped. Some other ones you may encounter are выпасть в осадок, офигеть и т д.Tweet