I've read a lot of disdainful dismissal of BC2's Magnum Ammo Specialization here. Some say it's noobish, that it's a waste of a specialization slot; some say it makes killing too easy, and that a truly skillful player will make better use of other specializations for the benefit of his team. I haven't heard any such denigration of the Marksman Training Specialization, another specialization that could fill that same slot. Is Marksman Training not as "noobish"? Does it not convey as much of an advantage as Magnum Ammo? I don't intend to explore the appropriateness of these specializations for any particular kits; I'm just going to take a short look at the mathematics of the situation to compare the two.
At first glance, it would seem like it's a wash: One increases your damage by 25%, one increases your accuracy by 25%; I mean, what's the difference, right? Well, Marksman Training increases your accuracy by decreasing your spread by 25%. A decrease of 25% is a larger relative change than an increase of 25%. I have a mass of about 70 kg. If I were to decrease my mass by 25%, then increase it by 25%, here's how big I'd be: (75 kg)(0.75)(1.25) = 65.625 kg. If I decrease by 25%, then increase by 25% (or the other way around; it doesn't matter), I'm smaller.
Furthermore, decreasing your spread doesn't just increase your accuracy by that same amount. When your spread is zero, such as when you're aiming down your sight, have been standing still, and haven't fired for a few moments, the bullet leaves your weapon heading exactly towards the point where you are aiming (see footnote 1). Now, when you have spread (due to hip firing, movement, recoil, or some combination of the three), that bullet could leave the barrel in any direction within a circle of deviation whose radius is your spread value. The area of this circle is related to the square of your spread value; hence, when your spread value is reduced to 0.75 of its normal value by Marksman Training, your circle of deviation is actually (0.75)(0.75) = 0.5625 times the size it would normally be. This is a decrease of not 25%, but 43.75%.
By this point, the numbers seem to be favoring Marksman Training; however, there are a couple of mitigating factors to consider. First, if you're taking a shot under conditions that wouldn't normally have any spread in the first place, marksmanship isn't going to have any effect; 43.75% of zero is still zero. Second, if the enemy is close enough, his entire body might fill your circle of deviation for a few shots, even without Marksman Training, so it's not going to be any help here, either.
Mathematically, there is no definitive answer to the question of which specialization is "better". What we can say, though, is that, in terms of pure numbers, Marksman Training wins hands down; however, those numbers become less useful and relevant as your opponent gets closer (and fills up more of your sight picture), but more useful in situations where your spread is larger (such as when you're on the move or you've been holding the trigger down for a few shots).
Personally, anecdotally, I have found Marksman Training to perform better for my style of play. I don't like to get caught in short-range cat fights; if I'm close enough that the enemy fills up my circle of deviation, it probably means one of us has surprised the other, that person is going to start shooting first, and neither Magnum Ammo nor Marksman Training is going to save the surprised.
Your mileage may vary.
1. Yes, there is bullet drop, so that after the bullet leaves the barrel, it will start to head for a point increasingly below your aim point; however, at the moment it leaves the barrel it is heading exactly where you are aiming. Furthermore, bullet drop generally only has a serious effect on long-distance sniper-rifle shots, for which spread isn't as much of a concern, and for which there isn't a Marksman Training option, anyway.