We all love specialists, from the meltagun wielding space marine to the imperial guardsman with a flamer, or the dark eldar warrior with a blaster, specialists are a common sight in our hobby. I've got a few thoughts on the perils of concentrating too much on these weapons when formulating tactics before or during games, and the idea of synergy in their use.
GW writes codices in a very particular way most of the time, which involves infantry squads being mainly armed with a 'standard' weapon, such as the humble bolter/splinter rifle/las gun etc with options to select a special weapon for, usually, one member of the squad. The epitome of this style is of course the space marines (they are after all GW's signature creation) so I'm going to use them as my example.
Starting from basic principles then, a typical tactical squad requires a unit size of ten in order to take a special weapon, of which there are three options, the meltagun, plasma gun and flamer. Each of these weapons has a clearly defined role, with secondary options I'm going to consider later on. The meltagun is an anti-tank weapon, the plasma gun is a jack of all trades, and the flamer is very definitely anti infantry. In our example, the rest of the squad will be armed with boltguns, which have almost no anti-vehicle capability against the majority of vehicles, so if you are taking a meltagun, it has precisely zero synergy with the rest of the squad (there are ways round this, see more later). What this means is you've just bought a 175 point unit for one chance to take out one vehicle per turn, once you're within range. At this point I should make it clear, I'm not suggesting you don't take the meltagun, just be aware that if you're using it to shoot at a tank every turn, you're wasting the fire of the rest of the unit.
So, onto the ways around this problem then. Well first off, your ten strong unit of tactical marines can be split down into combat squads when they deploy, so immediately you're reducing the wasted firepower if you do use that melta down to 4 bolters instead of nine. Secondly, you can also take a heavy weapon marine with a multi melta and put them in the same squad. Granted, this makes the unit less mobile as in order to fire at full effect the guy with the multi melta has to stay still, but at least the others in the squad can still move under 6th edition rules, alternatively you can fire the multi melta as a snap shot, which makes this a much more viable option than in previous editions. Thirdly, every sergeant has the option to take a combi weapon, giving you a third melta shot, albeit only once per game. Of course, if you've got the spare points, the selection rules for the sergeant actually say they can swap both their bolter and their bolt pistol for combi weapons, so for 20 points, you could actually give your sergeant 2 combi meltas. Combining these options with the points above, you can actually create a specialist tank hunter unit able to bring three melta shots to bear on any target in a single turn, thus only 'wasting' two bolter shots from the accompanying marines who make up the combat squad. I've used this tactic before to bring down one of those pesky heldrakes in a single turn - chances of rolling at least one six from three dice are much better than from one!
If you haven't got the points to create a unit like this, the only secondary option for using the melta is to shoot at an infantry target with the rest of the squad. As we've partially discussed already, the range of the melta doesn't really synergise well for this purpose, but when it is in range, S8 and AP1 does give some bite to your shooting.
Obviously this method doesn't work quite so well for the flamer specialist, for two reasons, first - there isn't a similar option in the heavy weapon category for the tactical squad (seriously GW, why can't tactical marines take heavy flamers?). Secondly, and I accept this is a personal view, is that flamers actually work better under 6th edition as defensive weapons. Think about charging a marine squad with your fragile but deadly assault troops (I'm thinking wyches, banshees, hormagaunts etc) and then realise that if you do, that flamer (when added to the sergeant's combi flamer) are going to put 2d3 automatic hits on your unit, before you count any other over watch fire. That makes me think twice, even more so if the unit I'm charging is sternguard and they've got several combi flamers and a heavy flamer! But I digress. Flamers are exceedingly useful at thinning out more numerous opponents as they can generally hit far more models than any other weapon you can take, once they're within optimum range. The main disadvantage I can think of to taking a flamer is its range, which doesn't synergise at all well with the rest of the unit. It can lead to you being charged, though this is at least offset by the previous point.
Discussion of range brings me neatly onto the last special weapon, the plasma gun. This weapon is my personal favourite, as it synergises beautifully with the bolters carried by the rest of the squad. Same range, same weapon type, and it just supplements nicely the killing power of the squad. Realistically, you're not going to be taking down too many vehicles with it anyway, it's better focussing on the enemy elite infantry that its AP2 will make mincemeat of. You can also pull the combat squad and sergeant trick described under the meltagun section earlier, though the blast type of the plasma cannon does mean you can't fire snap shots with it on the move. That enemy unit of terminators will wet themselves when they see it though, especially if you can get them as soon as they deep strike so you're hitting the maximum number possible.