Beginning in Season 2, Detective Danny Reagan (Donnie Wahlberg) carries a stainless Kahr K9 Stainless as his sidearm. In "The Job" (S2E12), Danny lets his son handle the Kahr after the family car was shot at. Danny has his son in the episode recite the three rules of gun safety: 1 - Always point the gun in a safe direction, 2 - Never touch the trigger until you're ready to shoot, and 3: Assume every gun is loaded.
Glock 19
The Glock 19 is the main handgun carried by NYPD uniformed officers in the series including Officer Jamie Reagan (Will Estes). It's also carried by Danny's partner Jackie Curatola (Jennifer Esposito).
Colt Government
Frank Reagan (Tom Selleck) is seen firing a Colt on the range in "Critical Condition" (S2E03).
SIG-Sauer P228
A gunman opens fire with a nickel SIG-Sauer P228 on Danny and the family car in "The Job" (S2E12).
SIG-Sauer P239
The bank robber is armed with a SIG-Sauer P239 in "Critical Condition" (S2E03).
Rifles
Colt M4A1 Carbine
The M4A1 carbine is seen used by officers of the NYPD's Emergency Services Unit.