It is, I think, a wellknown fact that basic weapons, especially rapid fire weapons in 40k are so bad, that you might as well not bother to fire them. Now, for many people, this is solely a prejustice. Here I hope to back it up with some mathhammer.
As a sample basic unit, lets take 10 tactical marines. They wield bolters, (24", S4, AP5, RapidFire), the most basic of all basic weapons. As a test, lets assume that they have taken some position that they want to hold, such as an objective. Still, they want to contribute to the game, so they are firing on another unit holding an objective, and therefore not within 12". How long will it take them to clear said unit of the objective?
As a test, lets assume the other unit is
1. Grots
2. Guardsmen
3. Marines
In all cases, the marines will generate 10 shots. They will hit on 3+, meaning that 2/3 will hit.
1. Grots. Bolters wound grots on 2+, so 5/6 will wound. The grots will go to ground, and being as small as they are, will most likely be able to claim a cover save. So, they will save on 3+. 1/3 wounds will kill. This taken together (2/3 * 5/6 * 1/3 = 0.185). So, per game turn, the marines will kill 1.85 grots. In the maximum 7 turns of the game, the marines will kill 13 grots. So, as the 40 pts 10 grot unit is fairly common, a full tactical marine squad should be able to clear an objective of one of these, per game.
2. Guardsmen. Against guardsmen, the bolters will wound on 3+. The guardsmen will have the option of going to ground or actually contributing to the game. We assume that they will not fire back, so they go to ground. Same save as for the grots, then. (2/3 * 2/3 * 1/3 = 0.148). So, the full squad will kill 1.5 guardsmen per game turn. In the seven turns, that is 10.4 guardsmen. So again, by spending an entire game shooting the same squad, 10 marines can wipe out 10 guardsmen.
3. Marines. Wounds on 4+. Now, the marines do have 3+ save, so they do not need to go to ground, but can actually do something. So, can the tacticals hold of another unit that is actually moving about and doing stuff? The calculation still stands (2/3 * 1/2 * 1/3 = 0.111) So, each bolter kills 1/9 marines per turn per shot. In the seven turns of the game, each tactical marine, remaining stationary and shooting, will have 7/9 of killing one opposing marine.
Of course, I simplified stuff, such as morale tests and the like, but I do believe the point stands. If a unit is armed with only basic weapons, even the two lowest units in the game need not fear them. Power armour can effectively ignore the bolters, if the have the numbers to absorb the occasional casualty.
Okay, the bolter is not exactly a weapon of mass destruction. But what about other the other basic weapons? The other basic weapons are, from memory
1. Lasgun (24",S3,AP-,RF,Orders)
2. Pulse Rifle (30",S5,AP5,RF)
3. Gauss Flayer (24",S4,AP5,RF,Gauss)
4. Splinter Rifle (24",X,AP5,RF,Poisoned(4+))
(I do not know the tyranids, and orks/eldar do not use basic weapons)
The humble lasgun. A marine costs 15+ points. A guardsman cost about 5. So, you can get three lasguns for the price of one bolter. Add in the useful order rules, that allows lasguns to fire twice on long range, and you get 6 lasgun shots to 1 boltgun. Even through the commad squad to give that order costs extra, the marines will not be able to match this fire with bolters in any cost effective way. Lasguns can be considered somewhat useful, not because they are better than bolters, but because they are aggresively priced.
Lasguns are S3, so they will not be able to hurt even AV10 vehicles. Not that the odds are great for a bolter do do something (2/3 * 1/6 * 1/2 = 1/18), if we assume that you need 4+ on the vehicle table to actually cause any meaningful damage (ie. minimum prevent the transport from moving next round).
The "great" pulse rifle. Slightly longer range than the bolter, and better strength. Surely this must be a great weapon. Well, no, it's not. It is wielded by a 10 pts model with BS 3. So, when shooting at marines it kill (1/2 * 2/3 * 1/3 = 0.111) the same as bolter wielded by a marine would. However, unlike the marines, who knows that their bolters are a joke, and try to negate their use, the fire warriors cannot take any special weapon and have stats comparable to grots.
Certain voices will attempt to convince you that the pulse rifle is useful to stun-lock vehicles. The typical vehicle that pulse rifles can hurt is the rhino. So (1/2 * 1/6 * 1/2 = 1/24) of the shots will stop the rhino or better. To be fairly confident in stopping the rhino, you would thus need 240 pts of firewarriors firing on it in one turn.
And yes, if you move close to double-shoot it, you will die. Even if the thing is broken, the contents will be able to charge your unit.
Gauss Flayer. A bolter that autoglances on a roll of 6. Hurray. See the text above for how not-impressive this is. To make this weapon most efficient, shoot the vehicles you would never be able to penetrate, but you are still stuck with 1/18 to actually do anything meaningful. It is possible to make the warrior units relentless, which does some good things for this use of the flayer.
Splinter Rifle. For most intents and purposes, a bolter. Slightly worse against guardsmen, and with a big fuck you to tyranids. The poison is useful, in that it allows the unit to take on targets it would otherwise have to leave alone.
So, now you know, in 40k do not expect the basic weapons to actually contribute to the game. It is the specialists that kill the enemy, and the basic weapon guys who die to the enemy.
