I will be discussing ACOG sniping. I will mention which guns it's best to use, perks, maps, how to steady the scope, and when it's appropriate to no-scope. You may be thinking- why would I use an ACOG on a sniper? All is does is lowers the range and it can't even be steadied! It has absolutely no benefits for a sniper.
I actually use it
because it lowers the range. It helps me bring the sniper, and the one-hitting power it holds- to a much closer combat environment. Because the acog let's me quickly bring up my scope, fire, and kill in a hit, I can be competitive with SMG and Assault rifle users on maps like Killhouse and Vacant. You want to use the regular scope for when you sit way back and camp. But when you don't have the ability to get really far away, or you'd rather move around as you snipe, you are going to want to whip out the ACOG.
First, you must decide which sniper rifles you are going to put it on. I prefer the M40, or the first sniper rifle. Due to a glitch of sorts, an ACOGed M40 does 75 damage per hit instead of 70. I will not do the math on here, but I will compare the one-hitting ability of an ACOGed M40 to that of a R700 or Barrett .50cal.
With stopping power on, a M40 will now kill anywhere, including limbs, in a hit. If they have juggernaut on, it will only kill in the head and neck. A R700 or Barrett .50cal, which do the same damage, will kill anywhere but the limbs in a hit. If they have juggernaut on, it will kill from the chest up.
The choice is up to you- would you rather kill in a hit anywhere on the body, or would you like to one-hit juggernaut users in the chest? Another thing to put into consideration is with a semi-auto rifle, you have the option to fire extremely fast at point blank range, so you'll waste more bullets, but likely have more success in that area.
Once you've got your gun chosen, you must select your perks.
Special Grenades: I prefer stun here just in case you are entering a building. It's going to be beneficial to know if someone is in there or not. You could also use smoke grenades to help you get away from sticky situations and get farther away.
Pistol: You won't be using it much if at all. If enemies get too close, you'll generally try no-scoping. Enemies should never be too far away in the levels I will recommend.
Perk 1: In some levels, like Chinatown, you are going to want to stick with good old claymores. However, in other levels like Vacant, you will be moving around a bunch, so you don't have the need for claymores. In these cases you can afford to use something different like bandolier or frag x3, which ever you prefer. I personally have two classes for ACOG sniping, one with claymores and the other with bandolier because whether or not you need them is highly dependent upon the level. If you had to choose between one, I'd go with either frag x3 or bandolier. You can always stay mobile and not have to worry about claymore at all.
Perk 2: In this case, you definitely want stopping power. At these ranges, where SMGs and Assault rifles are serious threats, you want to be certain they're dead in the first shot.
Perk 3: Here, you have two choices. You can go with steady aim if you find yourself up close and attempting no-scopes a fair bit. However, if you sit in the back a lot and are always moving to avoid point-blank confrontations, you may be better served by extreme conditioning.
Now you should have your class all set up. But, you must be at a close range to effectively use the ACOG scope. This means no ACOGs in Creek or Overgrown, unless you intend to move a lot and be way up in the enemy's face. Below is a list of the levels I use my ACOGed sniper on and a short explanation as to why:
Countdown: Although this level has some really long ranges to shoot down, I still prefer the ACOG scope. Very often during the map, someone will jump out from the middle and open fire on you. The ACOG is going to help you react fast to this situation and kill the threat. You definitely have to know how to steady your scope before you use it on this level, though.
District: The snipers in this level stick to the long street and pick off anyone brave enough to work their way down it. However, there are at least 3 directions the enemy can come from at any given time. And, they can get pretty close before you see them. Because of this threat, I use the ACOG to be ready for them. Additionally, I am not limited to only the main street with an ACOG and I am able to move around the perimeter if I need to.
Downpour: Nothing on this level is very long distance, so there's no reason not to use the ACOG. Additionally, if you are sniping near the mounted guns, people will occasionally shoot at you from right below it. It's not very hard to hit them with the ACOG, but nearly impossible with a standard scope.
Showdown: This level is really small, with very little cover. With short distances and little cover, the sway doesn't mean much here, but the reduced distance certainly does.

Vacant: This level is too small for sniper scopes, so you have to use the ACOG. The areas you'll be sniping from here are the exteriors and long hallways.
Chinatown: This is a rather large map, but it's got buildings
everywhere, so all the combat here will be experienced at close range. This is one place where claymores help out a bunch.
Killhouse: Another small level that ACOGs thrive in.
Optional levels: Broadcast and Pipeline. Both of these levels have distances that are borderline between ACOG range and regular scope range. You will do fine with either scope on both of these levels.
Steadying the Scope: While it is true that you cannot just press a button the have the scope stay still, there is another way of doing so. I started doing it subconsciously and I began to notice the value in this trick to making long range shots. If the scope sways left, you begin to look right, moving at the same velocity. If if sways up, you move down at an equal speed. The effect is that the scope stays essentially still and you can pull of longer and longer shots the better you get at it. The last thing I want to say about this is: If your scope is moving right, but your target is to the left of where the scope is steadied, all you have to do is overpower the scope's sway just a little bit and it will steady further left now.
The final technique I'd like to mention is when it's appropriate to no-scope. Sniper rifles have by far the largest hip-fire cross hairs, so it is really hard to land a no-scope at any reasonable distance. I will generally attempt to no-scope them if roughly half of my cross hairs are covered by my target. A 50% or higher chance is generally good enough for me and I usually luck out and get it to hit. I am aware that a G-shot technique of no-scoping exists, but I have no idea as to how it works.
That is it for my guide. I hope I inspired you to try out the ACOG on a sniper rifle next time you play on a small level.