The whole thing:

Internals:

The pump hooks up where the bolt would usually slide- you pump the bolt handle to fill the tank.
Trigger bridge detail:

The trigger is bridged with a small LEGO peice- it's the right size and shape for what I needed. It's gooped to the LS trigger and the pin on the air tank fits through a hole in it, and is epoxied in place.
The barrel I'm using is 1' of 17/32" brass, nested inside 1/2" PVC and stuck in a coupler. I don't know ranges (it's too dark outside), but they should be the same as a singled At3k. This is a lot more comfterable to use than an At3K, and takes fewer pumps and has a lower chance of misfires than my LBB.