Whenever you have the chance, pick up a Kak 98; the most powerful guns in the game. There is a scopedKak 98 in the last level (Dragon's Teeth) in the 2 bunker in the building.
The most common boltaction rifle is one of the most effective weapons against small groups of enemies. It may be a boltaction but it is a onehit kill almost all the time. Make sure to keep an automatic on hand due to the slow reload.
Smoke grenades provide instant concealment, proving very useful for getting close to tanks on the Russian front.
SMGs work best at close range; the PPSH 42 is recommended.
Rifles are useful at short to medium range, the best ones being the SVT40 and Gheher because they are semiautomatic.
Sniper rifles are great for picking off machine gunners before they set up. The Gheher sniper rifle is recommended because it is semiautomatic.