The idea of a vacuum bazooka is: you use a vacuum cleaner to evacuate a pipe, with a valve over one end. You stick a dowel in the other end, it accelerates down the tube, pushed by pressure behind it, shoots right past the vacuum inlet, through the flap, and flies off into the blue yonder. It's a pretty quick build.
I chose 3/4" (inside diameter) white pvc pipe because it's cheap, and fits a 3/4" pine dowel reasonably well. Smaller diameters are even cheaper but the dowels don't fly as far because they don't have a lot of inertia; larger diameters work great but a piece of 1" or larger wooden dowel can go through a window.
The traditional version uses a T-fitting with the long barrel leading to the breech going out of one end, and a short stub leading to a flat-faced fitting with the valve, on the other end, and the vacuum cleaner on the right angle of the T. I didn't like that as much because A: the dowel bullet could get sucked against the single vacuum inlet and get slowed down, and B: I didn't want to have a joint in the barrel, where the dowel could jam. I bought an X-joint instead and bored out the inside so the barrel slid right through it. However, while I was there I found a fitting like this
That would work great, and would be much simpler than my solution.
The bulkhead on the end is a 3/4"-to-1 1/2" adapter.
When I started, it was like this
I needed to use it reversed, so I cut off the back, basically. I also faced off the front on the lathe to provide a dead flat surface for the valve to seal against, but that's not actually necessary.
Here's a shot showing the parts, somewhat disassembled, with all the interesting bits exposed:
You can see the back-side of the cut-off adapter, the barrel with a hole drilled through, that lines up with the cross-arms, and the cross-arm assembly with two sets of 3/4" curves going to a 3/4x3/4x1" tee. All the fittings are connected with pieces of 1 1/2" long pvc cut from the end of the barrel. I didn't glue anything together because it's totally not necessary: they're just dry-fit and shoved together. The amount of air the shopvac pulls means minor leaks are completely unimportant.
Here it is, assembled:
There's an adapter I cut using a hole saw or three, that connects the 1" ID T to the 2" ID end of the shopvac hose. I have an adjustable-diameter hole saw, so I cut the OD of each tube halfway into a piece of pine, one from each side, and then a roughly 1" ID all the way through. This is totally unnecessary: a big wrap of duct tape would do a better job.
Also I didn't get the picture in focus, and also the piece of pine is a waste board I was using on the milling machine so it has holes and slots all over it.
The flap on the end is a piece of 0.006" thick aluminium gutter drip-edge, cut to about the same size as the faceplate, with a bit of electrical tape holding it on. (This only lasts about four shots before getting blown off. I'm certain you can come up with a more durable solution, as will I at some point.)
I cut off pieces of 3/4" dowel, about 1" long, on a chopsaw, and if I'm feeling fancy I run a file across the faces to remove splinters. Turn on the shopvac and put the dowel piece within about 1" of the breech, and it makes the most delightful THWOOOP-ptak! as it shoots down the barrel and knocks the flap open.
Firing from about a meter high, on a flat trajectory, it shoots about 15 meters before it hits the ground. When I'm not completely overwhelmed with work, I'll try setting it at a 45 degree angle and measuring its maximum distance.
One obvious and very exciting extension of this would be to find a big bucket full of superballs that are the right diameter for the barrel... and just suck it dry, firing a barrage of superballs all over creation. I have not yet found 3/4" superballs. However, when I find any source of cheap superballs I'm going to bore a piece of tubing to match them and try it out.
There are many other varieties and plans online for good cheap ways to do this, that don't even involve drilling holes in the tube. I wish I'd thought of some of them before assembling this.