Sterling Archer's primary sidearm is a Walther PPK. In the early episodes, the pistol lacked detail, but by the end of season 2, there was so much detail, the serial number was visible. In season 2's "Tragical History" Archer correctly describes the pistol as having a seven shot capacity. According to the companion book How to Archer: The Ultimate Guide to Espionage and Style and Women and Also Cocktails Ever Written, his PPK is chambered in .32 ACP.
M1911
M1911A1 pistols appeared in season 2. It is the sidearm of Ray Gillette. Archer uses it in "Placebo Effect," mainly as a tribute to Magnum P.I. None of the M1911s in the series appear to have safeties, however. In the three part season 3 premiere, Rip Reilly carries a nickle plated M1911 with white grips, and the gun ends up being used by several characters over the three-episode arc. There are numerous continuity problems though, as the grips change back and forth from white to black. In the season four premiere a KGB hitman carries one in the assassimation attempt on Archer/Bob.
Mauser M712 Schnellfeuer
It is revealed in flashbacks that Malory Archer used a Mauser M712 Schnellfeuer when she was a field agent ("Double Deuce").
Beretta 92F
Lana Kane carries a pair of Beretta 92F pistols in a dual-pistol shoulder rig.
Tokarev TT-33
In the Archer universe, the official sidearm of Soviet Forces is still the Tokarev TT-33 Pistol.
Webley Mk IV
Woodhouse's old World War I squadron mate brings by a Webley Mk IV that had belonged to his old friend, Captain Reggie Thistleton in "Double Deuce." This is one of the few firearms referred to by name on this show. This Webley is later used to knock Archer out.
Smith & Wesson Model 629
Since retiring from the field, Malory Archer's primary handgun is a Smith & Wesson Model 629, tricked out with a scope and laser. She sleeps with it by her beside in "Dial M for Mother." She mentions that it is a .44 Magnum during a flashback in "Pipeline Fever."
Desert Eagle Mark VII
Archer's rival, Barry Dillon, carries a nickel Desert Eagle Mark VII, which he talks TO, not WITH. Lana Kane also uses a nickel Desert Eagle on occasion.
Luger P08
Krenshaw/Kremensky uses a Luger P08 in the pilot, "Mole Hunt." Lugers later show up in "Double Deuce," "El Secuestro" and "Fugue and Riffs."
M1917 Revolver
A M1917 Revolver is shown on the a table in "El Secuestro" and "Stage Two."
Smith & Wesson Model 67
A sky marshal pulls a Smith & Wesson Model 67 when Archer pulls out his sawed-off double barreled shotgun ON A PLANE.
Colt Python
Woodhouse wields a Colt Python in a flashback in "Blood Test." Malory uses one in one of her flashbacks in "Job Offer."
Shotguns
Sawed-off Double Barreled Shotgun
When he doesn't have to worry about concealment (and even when he does) Archer carries a Sawed-off double barreled shotgun that he "borrowed" from Woodhouse in "Pipeline Fever." Apparently, Bama McCall in Gator made a big impression on him. Woodhouse is also shown using (presumably) the same gun in flashbacks.
Submachine Guns
TEC-9
Lana Kane is often seen with a pair of TEC-9's, which would be a good fit for her Johnny Bench-ian, steam-shovelly, Truckasaurus hands. According to the dialogue, they are TEC-9s, and fire full auto, but lack the sling rings of a TEC-9. Later in season 2, she can be seen carrying them in her shoulder rig. Archer takes one in "Swiss Miss." In "Double Trouble", she carries what appears to be a pair of nickle plated or silver TECs, after Katya ruins her main pair by shooting the barrels. They are not seen again.
MP40
The MP40 is the SMG seen most on "Archer." ODIN agents use it almost exclusively, as do gay interior decorators/assassins for hire Charles and Rudy in "Honeypot."
PPSh-41
KGB defector Katya Kasanova uses a PPSh-41 to rescue Archer in the Soviet Union in "White Nights."
Uzi
Other members of the Cuban hit squad are armed with Uzi SMGs ("Diversity Hire.") The Cuban hit squad returns with Uzi's in "The Man from Jupiter" (S03E04). The Yakuza have them in "Drift Problem" (S03E07)
MAC-10
KGB agents use MAC-10 SMGs in "Fugue and Riffs" (S04E01).
Heckler and Koch MP5A5
Archer uses a Heckler & Koch MP5A5 with four position Navy trigger group in "The Wind Cries Mary" (S04E02), after first filling out the required paperwork at the ISIS armoury.
Rifles
AK-47
The AK-47 shows up numerous times during the series. They appear to be based on real AK-47s, with the milled receiver and open-topped front sight.
M14
Cyril channels Private Pyle from Full Metal Jacket with an M14 after Lana decides to sleep with all the men at ISIS as a form of revenge in "Dial M for Mother." Cyril and Calzado hunt for Archer and Lana with M14s in the Colombian jungle before Cyril catches up with Lana in "El Contador" (S03E05). Randy Gillette has one in "Bloody Ferlin" (S03E09) that fires full auto. Luke Troy uses a scoped M14 in "The Wind Cries Mary" (S04E02).
Heckler & Koch PSG-1
Except in one instance, the Heckler & Koch PSG-1 is the primary sniper rifle of ISIS.
Walther WA 2000
Archer uses a Walther WA 2000 sniper rifle in "Placebo Effect" to take out some guards.
Mauser Gewehr 1898
A German sniper sees Woodhouse and Reggie after Woodhouse braves No Man's Land to save Reggie in "Double Deuce." The bolt is on the wrong side of the rifle.
Archer lodges a Mk 2 hand grenade into the anus of an Irish gangster to persuade him to divulge the location of his boss in "Placebo Effect." Several are thrown, only to be caught and tossed back by Archer lacrosse style, in "Heart of Archness, Pt. II". The animators also used the same model for a flash bang grenade.
RPG-7
Among the ISIS equipment that Cyril, Krieger and Pam fence to gangsters in "Jeu Monégasque" is an RPG-7.
Miscellaneous
The ISIS Armory
The ISIS armory has a large collection of weaponry on its walls, much of it accurately rendered, except for scale. The Sa. Vz.61 Skorpion for example, is ludicrously large.