The original Bullets and Beer had no fewer than six pages dedicated to "The Guns of Spenser," representing a running dialogue between contributors Dennis Tallett, Jim Colburn, Jerry Smith, Mark Cook, and 'Matt from St. Paul.' The discussion centered around the fact that RBP provides somewhat sparse and occasionally inconsistent details about the weapons used by Spenser and Hawk. Smith, an experienced gun owner, complained that at times RBP's characters will do something "really dumb, or worse, impossible, with a firearm." Several people noted that Spenser doesn't use speed loaders or spare magazines, often carrying extra bullets loose in his pocket (when he has any). Bob Ames summed up the author's apparent approach to firearms:

"I know that somewhere in one of the early books Spenser said that he used a short barreled revolver because any gunfight he was involved in happened at such close range that four or five shots were more than enough. I still think Parker carries an image of Spenser as a more sensitive Sam Spade with a steady girlfriend, and he still pictures Humphrey Bogart settling the score with a single shot from a smoking gun in a dark alley."

The bottom line is, Spenser normally uses a snub revolver because it fits his literary heritage, and (from a plot point of view) he is frequently outgunned as a result, forcing him to demonstrate greater resourcefulness/heroics. Hawk uses an array of enormous, deadly weapons because that suits Hawk's persona better.

That said, it's possible to identify many of the firearms referred to in the Spenser series.

Spenser's primary pistol of choice is a .32 Smith and Wesson revolver with a 2-inch barrel. He carries it because "it's small and comfortable, and it's usually enough." Dennis Tallett provided images, which are unfortunately lost. Jim Colburn later provided the following updates/clarifications:

"Spenser's .32 Smith and Wesson is most likely a revolver; the only 32 caliber automatic S&W made prior to the writing of "Crimson Joy" is a seldom-seen piece with a reputation for less than reliable functioning. The Iver Johnson 12 guage shotgun is problematic. The book describes it as " a 12-gauge Iver Johnson pump gun". Iver Johnson made lots of 12 gauge shotguns but I have found no mention of a pump-action shotgun of any gauge manufactured by or for Iver Johnson. Spenser's Colt .32 automatic cannot be a revolver--Colt didn't chamber the revolver's for the automatic cartridge. Colt made a "Colt Pocket Model Automatic Pistol" in 32 auto from 1903-1945, this would be the best fit for the scant description. The 25 automatic is most likely a Colt "Pocket Model Automatic", often referred to as the "Vest Pocket Automatic", or a variation of the same design produced by Browning in Europe. It could also be a European piece of a similar design--many of the European WWII military officers were issued sidearms chambered for the .25 or .32 auto cartridges.
Hawk's guns are more... diverse. His 44 magnum, at least in the early books, would most likely be a Smith & Wesson. Colt didn't introduce a 44 Magnum until the early 90s if I recall correctly; other makers were available but were either a bit less than reliable (such as the Dan Wesson) or very bulky (Ruger Redhawk and Super Redhawk) or were so unusual as to merit more description by Parker if that was what he had in mind (the examples that come to mind here are the various single-action revolvers--which would require manually cocking the revolver for each shot-- and the Automag--large, bulky, ridiculously expensive and a bit hard to obtain). Other than the S&W and the Automag, they lacked style and class...
Hawk's Colt .357 Magnum would probably be a Python (the Anaconca is a large-frame gun chambered for cartridges such as the 44 Magnum). It could also be something like the Trooper, Lawman, New Service, or Three-Fifty-Seven. Given Hawk's propensity for the finer things in life, I'd bet on the Python."

The slideshow below includes pictures of the firearms identified. (I freely admit I'm not a gun expert and anyone with more expertise who wants to weigh in is welcome.) 

  • .32 Smith and Wesson revolver with a 2 inch barrel. "Comfortable to carry, and most shooting is done up close."
  • Colt .32 caliber "New Police" revolver.
  • Spenser's Colt 32 "pocket model automatic".
  • Hawk's .357 Python. "Good for elephant hunting."