first press the ~ key to get into console. Activate the cheats by typing Giftfrompterodon. Then type the following to get the weapon you want:

chtweap #
Where # presents a weapon number from 1-30:

1 M16 (Automatic Rifle)
2 AK-47 (Automatic Rifle)
3 --
4 M1 Garand (Rifle)
5 --
6 PPS-41 (MG)
7 Colt M1911 (Pistol)
8 Tokarev (Pistol)
9 Makarov (Pistol)
10 Revolver 38 (Pistol)
11 Remington 870 (Shotgun)
12 Winchester 70 (Sniper Rifle)
13 --
14 SVD Dragunov (Sniper Rifle)
15 SKS Simonov (automatic Rifle)
16 --
17 M60 (Heavy Machinegun)
18 Dagtarev (HMG)
19 US-M3 (MG)
20 --
21 Thompson (MG)
22 SW Modell 39 (Pistol with silencer)
23 PPS-43 (MG)
24 --
25 M1 (Automatic Rifle)
26 Mosin Nagant (Rifle)
27 M79 (Granade Launcher)
28 Baikal IZH-43
29 Survival Knife
30 Knife
Example: chtweap 1