Selection of Pipe

High-density polyethylene, also known as HDPE, is the only type of plastic pipe that should ever be used for launching fireworks such as aerial shells.  You must never use anything such as ABS and PVC.  Such pipes are extremely brittle (despite how thick they may be), and cannot withstand large explosions, such as those from the burst charge of shells.  If for some reason the shell failed to leave the mortar (if it was loaded upside-down, or got lodged in the tube), the burst charge would shatter the pipe and send razor-sharp pieces in all directions. 

HDPE, on the other hand, is a very dense pipe made from a flexible polymer.  Though a small piece of it may seem rock-hard, is is actually very bendable in larger lengths, making it ideal for use as underground piping because it can withstand the shifting of the earth.  This also makes it very safe to use with fireworks, because if a shell were to explode in the tube, the HDPE would absorb much of the shock.  Though the tube would likely crack or become distorted, it would not create shrapnel.  In addition to being safe, these tubes are also very durable - if cared for, they can last for decades.  More and more shell kits are now being sold with HDPE mortars rather than cardboard ones (which are only good for a few shots).

Unfortunately, most plumbing stores don't carry HDPE - often times the store employees have never even heard of it.  If you do manage to find it, be sure to get size DR-11, with an inside diameter of 1 7/8" - no more, no less.  It's important that you use that specific type, because it's exactly the size necessary to launch consumer firework shells.  If it's too small, the shell won't fit, and if it's too large, the lift gases will escape and the shell won't go as high.

If you cannot find it locally, however, you can buy it online.  You can also buy pre-plugged fiberglass tubes (another common material used for making mortars) on my store page.

People have also asked me about the length of the tube.  For single-break shells, 1-foot lengths are usually long enough, even though about an inch of that will be taken up by the plug in the bottom.  The remaining 11 or so inches is long enough to send the shell to a safe height when fired.  If you want them to go a bit higher, just make the tubes about half a foot longer.  This will cost more, however, and the racks will be heavier.  For multi-break shells, the mortars should be at least 15" long, and ideally 18" long.

Do not be tempted to buy ABS or PVC pipe because of low prices and availability.  It is not worth the risk.

Plugging

All HDPE mortars need to be fitted with a plug in the bottom to prevent lift charge gases from escaping.  The plug is usually made of 3/4 - 1" thick wood cut into a circular shape.  Unless you bought pre-plugged mortars, you will need to make your own plugs.  The best way to do this is to buy a hole saw (shown at right) that has an inside diameter that's the same as your mortars (about 1 7/8").  Remember that the walls of the bit have some thickness too, which will make the plug slightly smaller than the size of the drill bit.  Because of that, you may want to use a bit that's 1/8" or so larger.  A good test is to try to insert the bit into the tube - if it fits in at all, the plug it produces would be too small.

Once you've got the right bit, find some scrap wood that's at least 3/4" thick.  Do not use any sort of pressed particle board.  I like to use 1x4s.  While you can drill them out using a hand held drill, it's much better to use a drill press.  Remember, drilling completely through the piece of wood will usually cause the plug to stick inside of the bit, in which case you'll have to turn off the drill and pry it out.  To make it go faster, set the depth stop so that the bit comes just within a fraction of an inch of cutting the plug completely out (it may take several attempts to get this just right).  This way, the plug will stay behind, and you can just go on to the next one.  When you're done, just pop the plugs out of the wood using the handle of a screwdriver or hammer.  

The plugs should fit into the tube with little to no free space around the edges.  To permanently mount them, run a continuous bead of Liquid Nails construction adhesive around the inside, as close to the edge as possible.  Then insert the plug.  The glue will help to secure the plug, and will create an airtight seal.  If you want, you can use the glue to fill the small pilot hole through the center of the plug, but I've found that this doesn't make any difference.  Before the glue dries, you need to fasten the plug into the tube from the outside using either screws or staples.  You can drive in a couple of flat-headed screws, but a much faster way is to use an air-powered staple gun and 1" staples.  Two or three staples is all that you need.