Ah Ash Wastes, the only rules I didn't bother to check. I even got the Vampyre rules out looking for one.
Now, I'm going to waffle for a bit, so sorry in advance. By all means ask more questions. I'm going to try some Rules Systems 101 to help, but this might be more info than you needed. I'm an Ork player, so my attitude to accuracy is to add more. Oh well...
There's always been a bit of a problem balancing out weapons and time constraints. Necro doesn't do a great job of doing it, but there's two things to consider, which are balance and turn structure.
The turn itself is an estimated concept. In any wargame, it is broken down to easy to manage phases, and sorted by convenience. Whichever way a turn works, it still usually follows some kind of logical order, movement, preceded by an action/s. It breaks up the time into a number of phases (usually 3) you might not use, and it is unlikely that in a real situation (unless you were in a choreographed military drill) you'd move the same distance, and fire one shot from your gun every few seconds. It's just a bit more convenient and easier on the mind to break it up.
Part of it is a quite hefty divide, but the philosophy behind Necro is one attack per weapon per turn. This can seem hazy because you have one shot guns like a blunderbuss and a gun capable of rapid fire (such as an autogun) firing once each turn, despite the fact that logically, you could put three bullets in the head of a blunderbuss user whilst they put in the new load.
It tries to estimate an amount of time, so you could logically see a rapid firing gun fire at most a few shots during each turn, whilst in real time, there'd be a lot more bullets. It is also possible that rapid fire weapons are actually firing more than one bullet per turn, and what you're following is the estimated likelihood that one shot of say, three, that has the highest likelihood of doing something. The Rapid Fire skill would then represent a fighter getting used to the recoil of his gun and holding his accuracy a bit better. It is also worth noting that most of the weapons in this game that fire more than one shot per turn have some pretty insane fire rates.
What it ultimately comes down to is balance. Trying to time every weapon exactly would take a pretty hefty (and most likely clunky) system to handle it, and would detract from the game too much and cause headaches rather than ammo rolls. It would also probably make any weapon capable of rapid-firing break the game. Where do miss a turn weapons fit in, well like I said previously, they're usually complicated weapons that have a good reason to take quite a while to fire again.
It doesn't just come down to loading; Necro sees no difference between a standard two-canister shotgun and a pump action shotgun that can hold more rounds. It sort of estimates reloading, because every gun is going to have to do it at some point. Even Lasgun users carry around multiple 'power packs' that slot into the bottom like ammo pack would.
Why the crossbow misses a turn, well it isn't just about putting the bolt on, you need to prime the string too. There's no reason to say that in-between turns, you'd load the crossbow and move. It just means that a crossbow user isn't going to go to the trouble of loading the bolt to fire it off quickly. Handbows are move-or-fire, representing that you sacrifice your movement in order to fire the weapon. It is the same for all heavy weapons, so there's a lot of the complicated nature of those guns that comes into it, even just getting the damn thing ready to fire.
Another thing is that the Ash Wastes rules are mostly written by submissions, so the logic behind the blunderbuss may not be shared by the person who wrote the rules the crossbow comes from. Personally I still think a crossbow is more cumbersome than a blunderbuss. Historic Crossbow users were used for their penetration abilities (not to mention the fact that those things can pelt heavier bolts than a bow can), and they usually shielded themselves whilst reloading.
I said previously that traditionally crossbow users stand on the bow to pull the string, but it isn't always necessary. Most of these Crossbows are primitive, so in 40k you'd hope there'd be some advancement.
I'd personally consider move or fire, but as the handbow has that, it could be a balancing factor to up the firing time of the crossbow to not render the handbow unnecessary. From what I've seen of weapons that miss a turn, they usually have a really good reason to miss a turn. You'd be free to do anything during the rest of the turn, even movement. The logic is that at some point, those few extra seconds would matter. It depends on the weapon, but remember the rules are an estimation.
I still think a Blundabuss is a lot less complicated. The charge is just a small bag/canister of gunpowder, and the wide barrel is a convenient funnel to just drop whatever load into it and then fire the contents. It's about the same time to load as any other weapon would be, you're just never going to be able to rapid fire with it, but then the chaos it causes is enough to make up for it. You wouldn't need to be that sophisticated, and I doubt many Blunderbuss users would bother to pat the load in.
I'm not quite sure I agree with the can't shoot another gun logic, but then the writer of those rules views a Crossbow as very fiddly, and it is, I suppose that makes a bit of sense. You can move though, you'd have enough time to do that. You'd be able to move in the following turn as well, although that would affect accuracy. You have to remember that most things happen at the same time, shooting and movement aren't that separate except in the ruleset, so a turn could represent 30 seconds, if a heavy bolt takes a minute, that's two turns.
As far as I'm aware only the Crossbow has a rule which prevents you from shooting a weapon on the other turn. I suppose the emphasis is that you'd still be reloading it, whereas a Plasma Gun would be cooling down, so you could chuck a pistol out and pop a few shots off. Weapon swapping is tricky in the Necro ruleset, so you are only able to change the contents on one hand during the same turn, so you could only pull a pistol out anyway. A crossbow is at all times a two-handed kind of weapon, so I suppose this makes sense, whereas a plasma gun could be flung around your back on a holster, although not too quick - don't want to burn ya back...
Boy, that was more detail than I planned. I'm not trying to be harsh or anything, its just that I make my own gaming systems, and I've put quite a lot of time into thinking about games logic such as this.