A PVC crossbow is something between a serious weapon and a toy. While serious crossbows have draw weights of 150+ lbs, most PVC crossbows are less than half of that. This is enough to be accurate at short ranges and is definitely enough to do serious harm so BE CAREFUL. A crossbow is fun but it is not a toy!
If you choose to build a PVC crossbow you will have a few design options. Available tools, budget, skill level, and desired performance will all play into your consideration. Of course, another major consideration is style preference. You may choose to build a medieval style crossbow…
…or you might choose a more modern style:
Of course, since I am talking about a PVC crossbow, I will assume that a compound bow is out of the question. The point of a PVC crossbow is to have fun. It aint fun if it takes a year to build… in my opinion.
The design I chose is a bit of a hybrid. The stock is a relatively modern rifle design with a modern trigger. The bow has a recurve that looks absolutely medieval when cocked.
I suggest beginning your design at the heart of the crossbow, the trigger mechanism. If you are a machinist with the ability to form steel or aluminum parts then you can make an elaborate metal trigger. If you are like the rest of us you will have to get clever and build the trigger of wood or some piece of scrap metal.
I chose to build a finger trigger from an old wrench which I drilled and ground into 2 pieces. It functions flawlessly but has it’s disadvantages. The tradeoff for this design is that my crossbow will not shoot nocked arrows. The trigger holds the string at a single point and the arrow sits in front. Thus the string contacts the arrow only after the trigger is pulled.
After you have chosen how your crossbow will be triggered you can consider the stock and prod. In fact, the trigger design will likely dictate much of the design of the other elements.
If you have wood working skills and tools you can craft the stock (or tiller) into an elaborate rifle shape. This is largely an aesthetic choice. A crossbow made from a straight 2X4 would perform just as well, though it would certainly not be as comfortable to hold or aim.
I chose to build my crossbow from 2 1X8 pieces sandwiched together with the trigger in between. I shaped the wood with a jigsaw and belt sander then finished off with a random orbital sander.
If you choose to build a rifle stock style crossbow then do your homework. Review rifle stocks paying close attention to how they are shaped around the hand grip and butt. Note that the stock is basically a pistol grip with a butt added to it. The grip is also contoured to the shape of the palm and thumb. This is important if you want the weapon to be comfortable.
The bow or prod is made of PVC that is flattened by heating with a heat gun, torch, or even over a stove. It can be made from 1 1/4 or 1 1/2 inch schedule 40 PVC. Do not try any other type of plastic piping. Black plastic is brittle and CPVC is too thin to give any good draw weight. The condition of the PVC is also important. Sun damage can cause the prod to be weakened and brittle.
I chose to use 1 1/2 inch PVC because I wanted a longer prod (about 44 inches) and I chose to bolt the prod to the stock with 2 3 inch lag bolts. Because of this the prod is completely flat and requires a second short piece of PVC to brace the middle. If I had chosen to lash the prod to the stock with windings (something I plan for my second crossbow) then the PVC would be tapered from about an inch thick in the middle to flat on the ends. This would have made the prod more powerful, perhaps giving it over 100 lbs draw weight. Also, shortening the prod adds to the draw weight.
There is a point at which PVC will fail. If pulled too far it will fold. Thankfully, it does not violently snap so do not be scared to test it’s limits. You can always build a new prod. The best way to get the right size and design of prod is to break a few.