Rifles
General Strategy
Rifles come in two different forms: assault rifles and sniper rifles. The buy menu does nothing to distinguish between the two. The first four rifles (AK-47, SG-552, Colt M4A1, and the Steyr Aug) are assault rifles while the last four (Scout, AWM, G3/SG-1, and SG-550) are sniper rifles.
The AK-47 and SG-552 are Terrorist only weapons while the M4A1 and the Aug are only for the Counter-terrorist. The rifles for both sides match up well against each other. For example, you could consider the AK-47 to be the Terrorist's M4A1 and the M4A1 the Counter-terrorist's AK-47. There are enough differences between the two rifles to make them unique. The M4A1 can be silenced giving it a big advantage in stealth operations, while the AK is one of the loudest guns in the game. The AK, however, is more powerful than the Colt, but also suffers from horrendous recoil.
Sniper Rifles
The AWM and Scout are both single shot bolt action rifles whereas the G3/SG-1 and SG-550 are automatic rifles. All sniper rifles have two levels of zoom, and they are generally the most powerful weapons in CS for each round fired. Keeping your distance while using a sniper rifle is a no-brainer. Sniper rifles are just as powerful at close range, but far less effective since you'll be an easy target for enemies wielding SMGs and assault rifles. Having a good pistol is never a bad idea when you plan on sniping. Just don't wait too long to whip it out when enemies start closing in on you.
All rifles can fire rounds through walls and other thin surfaces. The amount of damage inflicted is difficult to calculate since you never know exactly where you are hitting the enemy. Damage through surfaces is a fraction of what it would normally be if you had line of sight. We tried to get exact percentages for every weapon capable of firing rounds through walls, but the results were never consistent. Either way, you can expect shots to do at least 60% of whatever the normal damage would be.