1 general rule of thumb: higher power = higher recoil but less hits per kill.
There are 3 families of 1 hit kill sniper rifles. The L9, L11's, and the M107's. If any other sniper rifle kills in 1 shot it's either a head shot or a fluke.
Once you reach the rank of Sgt., I believe, you can upgrade your backpack. I suggest carrying 1 sub-machine gun so you can run fast and still defend yourself, 1 assault rifle with an Acog scope, and 1 sniper rifle.
Don't buy Extended II magazines. They're 400 GP, double the cost of the Extended I, and only provide 5 additional rounds.
Accuracy and control of your weapon is key. Once you rank up enough you can use the Itech Reflexive scope. From there on every weapon is almost brand new when you use it. Even the AK will seem like it has less recoil.
Don't aim for the head. Aim for the chest/neck area.
Don't stand in 1 place. You're just a target. You can't fire while sprinting but you can still sprint then stop and fire while still moving fast.
While moving your crosshairs spread out and make your weapon less accurate. However, rushing someone will undoubtedly confuse them and put them in defensive mode and you'll still win the fight. A sub-machine gun like the MP7, UMP, or my favorite the PP-19 Camo will chew people up. No need for scopes on these weapons. They're not accurate and they're meant for speed. Since you can't run while scoped just leave those things at home. What you want to do with a sub-machine gun is get right up under someones chin. Doesn't matter what weapon they have. They'll be missing you. If you want to mod your SMG's just use extended clips.
AND THE MOST IMPORTANT THING OF ALL:
Never listen to what anyone has to say, myself included. If you feel comfortable with a certain weapon then use it. NEVER SWITCH. Any single weapon in the game, including the default M16, is capable of owning. I play with Captains, Majors, Lt. Colonels, etc. who still use it. So use what you feel comfortable with. So if you like the UMP then use the UMP. When a new SMG gets unlocked don't even bother trying it. This is just an example but if you become a UMP pro you'll beat someone who is just a casual PP-19 user. Get it? Know your weapon.

Plus using discipline and saving GP by using the same weapons will help you when you reach the higher ranks. If you know what weapons you like you can buy them for 90 day terms and save massive amounts of GP.