But when you take into account fireing on burst, magazines go empty pretty quickly. Then throw in hordes.
I personally am not using the shot selector as written anyway, as even pictures of the Deathwatch bolters don't have multiple mags sticking out of them. They have one mag with four buttons on the side. The way I am using them is that each of those buttons has a round loaded in it, and the Marine presses the button, which unchambers the current round and chambers the round loaded in that button.
Well, if you aren't using the Shot Selector as written, then of course your Marines are going to go through ammo a lot quicker than normal. I'm not sure how we are supposed to know that you've changed other rules unless you tell us when you post. Altering something else in the mix is going to create a different balance anyway, making a lot of commentary pointless. If you create a situation where the Marines run out of ammo a lot faster than in the rules, then there's going to be a lot more need to reload faster. If the excessive reloading then becomes a game concern, why not use the original rules, which would require less reloading? I'm not being cranky there (at least I'm trying not to be), but I don't see any particular benefit from your system, and it introduces a problem which then requires another rule change to address.
So let me make sure that I understand how your bolter works.
A marine has a magazine of one type of ammunition (say standard bolter rounds). This magazine is what the bolter normally fires.
A marine has other types of ammunition requisitioned. He gets a clip worth of each specialty ammo he has required, but he can have 4 rounds of ALL of those ammo clips available, in total (not each). They are removed from the magazine and individually placed in the four buttons on the bolter.
The bolter fires rounds from the magazine, except when you press the button and it then fires the one round from that button.
How does the bolter work on full auto, or semi auto?
Does it fire the one round from the button you pushed, and stop?
Does it fire the one round from the button you pushed, and then go back to the magazine ammo for the rest of the shots?
Can the marine rapidly push all four buttons to allow 4 specialty rounds to be fired in full auto?
How does the Marine normally reload the single bullets in each of the button selectors?
How many shots does he waste when a jam is rolled when firing a bullet from one of the button selectors?
It seems confusing and ineffective to use the shot selector in this case.
I'm not sure why so many people feel that a bolter would have to have 3 magazines sticking out of it for there to be three clips of ammunition in the bolter. I had just assumed that a normal bolter had a relatively small magazine which held 28 rounds in a compact packing. I assumed that the shot selector meant that the bolter was modified to have a larger magazine which would hold 3 clips worth of ammo, and which could keep those clips separate. You could have all of them filled with the same ammo, if you wanted. For example, a sickle shaped magazine might hold 28 rounds of ammo in a single curved stack, with two other, separate stacks of 28 rounds on either side of it. The "magazine" would only have to be the width of 3 bullets, and the shot selector would allow the bolter to feed ammo from any of those three stacks. Given the massive bulky size of the Asartes bolter, 3 bullets doesn't seem like an implausibly wide magazine. A drum style magazine, found on some of the miniatures, could have three circular hoppers of ammo, each only containing a "disk" of 28 rounds, and a selector which fed from any of the three "disks". Some of the Marine miniatures show Marines (often veterans) using weapons with larger ammo magazines (sometimes a box, sometimes a drum, sometimes even a belt of ammo), but none of them show a Marine with three separate clips sticking out in different directions, so I am confused as to why so many people assume that is how it would work.