The following guns can be seen in Bullitt:
WARNING! SPOILERS AHEAD!
SFPD Lt. Frank Bullitt (Steve McQueen) carries as his sidearm a Colt Diamondback snubnose revolver.  According to the source, the Colt Diamondback snubnose that is carried by Steve McQueen was fitted with grips from a Colt Detective Special. (See the Colt Detective Special photo on this page.)
Colt Detective Special
It appears that the other two detectives who work for Bullitt, Delgetti (Don Gordon) and Stanton (Carl Reindel), carry the Colt Detective Special (either the 1st or 2nd generation with the exposed ejector rod).
Smith & Wesson Model 58
There is one scene in which one can see a revolver in a motorcycle officer's holster with either fake ivory (i.e. plastic) grips or genuine ivory grips.Target grips as well. Judging from the grip's shape, it possible that it is a Smith & Wesson Model 58 revolver. For several years, the S.F.P.D. authorized officers to carry the S&W Model 58 .41 magnum. The Model 58 was manufactured between 1964 - 1977. The S.F.P.D. gave its full cooperation to the production in the spring of 1968 and it's plausible that the two officers were the real thing using their actual equipment.
Winchester Model 1897
The two hitmen (Bill Hickman and Paul Genge) who are seen shooting Ross (Felice Orlandi) and later shoot at Bullitt from the Charger during the famous car chase use a Winchester Model 1897. The Model 1897 was designed by John Browning and had an exposed hammer like a lever action rifle. The version used by the blonde-haired hitman (Paul Genge) is what is known as a “takedown” model. After he shoots Ross, the shotgun is seen taken down when he separates the barrel/magazine from the receiver. It appears the hitman’s model has had the barrel cut down to at least 18". It also appears to have a full stock. The left side of the receiver is marked "U.S.". The shotgun appears to be a 12 gauge as red shells are loaded, but there is no way to make certain of its actual gauge.