You're right Di-Pole Maglocks are only useful on meele weapons and then only when you got the Counter Attack Talent.

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.

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".

Deepstriker said:

You're right Di-Pole Maglocks are only useful on meele weapons and then only when you got the Counter Attack Talent.