I really think you haven't done as much research as you claim.
If you know of most blasters, you've heard of the Big Salvo. Well, besides using the tanks, it is a self contained blaster. It's four shots, staggered fire (pull the trigger a little, fires the first tank, pull more, next tank, etc), and the tanks with a plugged pump can launch three to four 1/2" darts 80-90' easily. It's not really that war practical, but it sounds almost exactly like what you want, with little work (re-barreling and plugging the pump). (Draconis also suggested this solution)
(Thanks to Carbon for the pic)
If you really need it to have a semi-auto trigger and be an airgun, take a hornet and replace all of the tanks with big salvo tanks. It has been done before and works pretty well. If you want to fill it faster, switch out the pump.
^^Ice Nine's beasty
Lastly, the most practical (and coolest) solution is already in the directory.
out. This thing with 1/2" darts is a monster. (Boot suggested a similar solution in his last edit)
Broken SVT recently very kindly introduced me to the triple strike tank. It is a large backpressure tank, larger than that of a Big Salvo, but smaller than a titan tank (it is rare though)
And weaker than you think.
If you really are going for semi auto you should use a back pressure tank. With a pin tank semi auto setups require multiple processes and valves, while in a setup like the Mirage-SS a single simple valve can be used to efficiently trigger a backpressure tank semi automatically.
It's actually only one more process, which is easily linked. The trigger pulls the pin, when the trigger goes forward again, it hits the fill button. Some might even call that easier than the check valve setup and finding particular valves with the right flow rates. Neither setup is difficult.
Regardless, to get a semi auto shotgun spread you will need a Massive fill tank, anything large enough has to be fitted to a backpack, otherwise I doubt any more than about 3 shots per fill is possible (unless you are using some insane HPA or Paintball tank with a regulator.)
There is too much blanket statement here. See the solutions I posted above in regards to how simple the project actually is.
A final possibility could actually be the marshmallow blaster, I'm not too familiar with them, but I think they operate off a backpressure system or something similar.
There is only one line of these that uses an airtank (most are push-pull) afaik and it's a pin-valve.
Also there is this off brand BBBB equivalent called the "blast bazooka" (or words to that effect) which has a tank just shy of the BBBB and operates off a backpressure system.
The LBB (blast bazooka) is virtually identical to a 4B, except that the pin is made of plastic and the tank body is a different color. The Buzzer Bazooka you may be thinking of does have a similar shell
to a 4B, and is
backpressure, but the tank is tiny. Smaller than a hornet tank I believe. I've owned a few.
Edited by Split, 23 February 2011 - 09:38 AM.