Okay, these are my final thoughts on the shotgun. I've read some interesting stuff from teleportation to being really skilled at a fast reload. So...I've come up with something that (IMO) sounds pretty good. Well, anyway, here they are:
As I mentioned before, when you look in Anvil, the WSTE-M5 fires a burst of 10 (In other words, it fires 10 projectiles at once) Since it's a double-barrel, this led me to believe that it uses 5-gauge shells for ammunition.
Now, when you pickup the shotgun it comes loaded with two shells in the chamber. Now I can't really back this up with proof from Anvil because I can't remember the clip size off the top of my head (I think its 20 per gun) But, this would explain how you are able to fire off so many shells with no reload animation. See, when you pickup more ammo for it, you know the sound it makes? Well that could be the rounds teleporting into the shotgun when they come in contact with your suit. As it was pointed out before, they do have highly advanced teleportation technology, so it isn't that implausible.
So, our marine fires the shotgun, it spits out the shells when it comes around using the lever-action reload and then new shells are put in its place from its clip and any remaining shells are warped in to take the place of any empty shells. This also explains why he just puts it away once he runs out of ammo. However, this doesn't explain the size of the shells that you pickup. In defense of THIS theory, the creators had to make an image big enough so that you could see the ammo you're picking up.
Okay, the shotgun still uses 5-gauge shells (shells with 5 friggin' BIG slugs in them), but instead of holding a clip/drum of ammunition it just holds a total of 2, 1 for each barrel. Now, when you pickup ammunition instead of teleporting it into the gun, it teleports it somewhere (Hammer Space anyone?), perhaps to Durandal or whatever AI is commanding you at the time sends it somewhere for safe keeping. So, when you fire the spent shells are spit out and then as before new shells are teleported in. This reasoning could also explain how you can carry so much ammo and up to 9 weapons on your person (Hammer Space or the warp theory)
BTW, Hammer Space is this dimension of space that has unlimited capacity that can be opened by some people at will. Its called "Hammer Space" because most people use it to store large objects to pound people with...Like 20-ton hammers and blue whales.