A lot (if not all) of the guns featured in Cowboy Bebop are modeled after weapons that people use in the real world. Here is some information about some of the guns that have been featured most prominently in the series.

Beretta Model 92
Used By: Annie (Ballad of Fallen Angels)
Caliber: 9 x 19 mm
Capacity: 15 + 1 in chamber

Glock 30
Used By: Faye (My Funny Valentine)
Caliber: .45 ACP
Length: 6.77 inches
Barrel length: 3.78 inches
Weight: 23.99oz
Capacity: 10 + 1 in chamber

Heckler & Koch MP5
Used By: Faye (Honky Tonk Woman)
Caliber: 9 mm
Barrel length: 225 mm
Weight: 2.97 kg loaded
Magazine: 15- or 30-round box

Heckler and Koch USP
Used By: Gren (Jupiter Jazz Pt. 2)
Caliber: .45 ACP
Length: 7.08 inches
Barrel length: 4.015 inches
Weight: 875.06 g
Capacity: 12 + 1 in chamber

IMI Jericho 941
Used By: Spike (Ballad of Fallen Angels)
Caliber: 9 x 19 mm Parabellum
Length: 207 mm
Barrel length: 120 mm
Weight: 1000g (1 Kg)
Capacity: 16 + 1 in chamber

Sig-Sauer P226
Used By: Shin (The Real Folk Blues)
Caliber: .40 Smith & Wesson
Barrel length: 112mm
Weight: 840g unloaded
Capacity: 12 + 1 in chamber

Ruger P89
Used By: Spike (Ballad of Fallen Angels)
Caliber: 9 x 19 mm
Weight: 907 g
Capacity: 10 + 1 in chamber

Smith and Wesson Model 29
Used By: Jetts Partner (Black Dog Serenade)
Caliber: .44 Magnum
Capacity: 6 rounds

Walther P99
Used By: Jett (Boogie Woogie Feng-Shui)
Caliber: 9 x 19 mm
Length: 7.08 inches
Barrel length: 4.015 inches
Weight: 875.06 g
Capacity: 16 + 1 in chamber

Walther P5
Used By: Giraffe (Sympathy for the Devil)
Caliber: 9 x 19 mm
Barrel length: 90mm
Weight: 885g
Capacity: 8 + 1 in chamber