The M14 is one of the most misunderstood guns in Call of Duty 4. People who try it get scared away by its massive recoil and immediately ditch it for another gun. For single shot assault rifles, most players will use the G3. Although the M14 has quite a learning curve to it, once you get the hang of it, you will absolutely destroy the opposing team.
The M14 is primarily a long-range gun. Although, depending on what you attach, it can be used for medium ranges, and even CQC once you catch on. With stopping power this gun usually kills in two shots which is great power. It is basically a "sniper rifle" in many ways, and some players will love the ability to put a silencer on a gun, yet keep the range quite high.
Attachments
The M14 has all attachments that the assault rifle class can obtain. Following are short explanations of pros and cons of each.
Iron Sights: Not ideal, but not horrible either. Get used to them so you can use a grenade launcher or silencer. Iron sights are slightly obtrusive into your vision, so position the crosshairs on an enemy when aiming, then aim down the sights and make slight corrections to the aim before firing. Recoil is manageable with no attachments.
Grenade Launcher: This is recommended when getting used to the gun. It helps if there is a group of enemies that the semi-automatic M14 wont be able to take out in time, so just launch the grenade, then finish off any survivors with the M14 or pistol.
Red Dot Sight: Opens up the sights, but makes recoil very high. With this equipped, the gun becomes most usable at long ranges, as the recoil is so high, you must wait for the recoil to back off before firing a second shot. In medium ranges this time you must wait for the gun to come back down gives your enemy a chance to finish you off.
Silencer: Surprisingly works great on this gun. The reduction of range does not matter for almost all maps, as the M14 has great range to begin with. The silencer reduces recoil to the point where you can fire off shots in rapid succession and remain accurate. If you are used to the iron sights on the M14, this is a highly recommended attachment.
ACOG Scope: Increases range (which isn't needed, but that's okay) but also reduces recoil. You can fire off multiple shots with only slight aim adjustments. With the ACOG on I find it works best for medium ranges, as for long range shots, the ACOG sways, and it is hard to get a good bead on the target. With practice though, it can be done.
Other Tips:
QCQ: With steady aim (or without if you're getting good) you can fire off multiple quick shots at an enemy close to you. Don't aim down the sights, just place the crosshairs over the enemy and hope for the best. Not ideal, but if someone gets the jump on you, it can help.
Perk Choices: I recommend claymores for perk slot one, as this gun excels in long range assaults. Players will catch on and try to sneak up on you're camping spot, so claymores will defend you, or at least give you a warning to enemies approaching. For perk slot two, either stopping power or UAV jammer work, depending on what style you play. And finally, perk slot three where steady aim is probably the best choice, just in case an enemy gets the drop on you.
That's the guide to the M14. Try it out, see what you like, dislike, etc. Make changes as you see fit, just know that the M14 can be a good gun if you learn how to use it. Happy fragging.