It's impossible to say from that brief look. In a deleted scene, Clemenza retrieves the guns he tossed to Vito through the bathroom window and brings them to "Agostino Coppola," a former gunsmith for the Italian Army. They were all revolvers, plenty old, and possibly foreign-made (maybe European military sidearms). Presumbly Vito used one of them on Fanucci. The gun he used was obviously of small caliber (maybe even a .22) because it left small holes in Fanucci and didn't make much noise. Vito'd want a small, relatively quie gun for that job. On the other hand, the small holes might simply have been to keep the on-screen gore down--military handguns would be much more powerful than a .22. At that close range, the first shot, to his chest, would have knocked Fanucci down. The second would have blown off a good part of his head.