Hmmmm, I'm now seriously confused, it souds like you want a boresight laser but you already said your not, or a laser sight but it seems like you're wanting to make a very specific one, but from understanding there isn't a lot of room for a laser to be "embeded" into a gun. The closest thing I can think of is your trying to make a laser sight mounted into say the handle of a pistol, not so much mounted under the barrel and in front of the trigger guard, sort of like...I think barret makes a gun with a built in laser sight.
I could be wrong on any one of those ideas.
Yea fenz, I was thinking the same thing since the dia of a diode is 5.6mm, but I think there are some 3.8mm ones but to my knowledge they are rare.