question regarding gun modification legality?

There is nothing that would prevent you from adding a longer barrel to a handgun if it were mechanically feasible to do so. However the moment you added the stock you'd have problems - legally - which you already seem to know.

Potentially even using something like a single point sling and shooting with that wrapped around your body, could cause you to be in violation of Federal laws. (Look at SAS/SF shooting techniques for info about this shooting style)

On a side note - if you have the stock in your possession you could be charged with something called Constructive Possession. Something you want to be aware of.

http://www.guntrustlawyer.com/2009/08/florida-man-...

if its a rifle depending on the state their are certain length restrictions.... so you can if you maintain the proper length...in ohio its sixteen inches... if you add the rifled barrel and chop the stock and it is still sixteen inches then your good...check state officials for any other regulations though i dont believe there would be any

pistols are much more difficult due to the fact of the stock being difficult... there is something about it and a sling