I've been using a lot of different boppers lately and here's what I've found/learned.
I take the copper pipe caps and hammer the ends over a piece of steel rod until they are sufficiently rounded, lots of info on the net regarding that so I won't belabour the method. Then I clean the inside of the cap with emery cloth and also clean the end of the copper pipe until both are nice and shiny. Then use some soldering paste on each piece. I melt the lead in an old (used be new!) soup ladle over a small butane torch then add some soldering paste and stir it around with a stick until all the impurities have floated to the top and skim them off with the stick. Then pour the lead into the pipe, you'll notice it draining out from the joint a bit and that's a good thing as it's "tinning" the joint which means it's actually being pulled into the joint between the cap and the pipe. These last really well because the lead filling the end "consolidates" both pieces of copper.
A word of caution. Melt lead in a well ventelated area, wear safety goggles-a face shield is even better. When you pour tihe lead into the pipe tilt the pipe away from yourself. If there's even a tiny bit of water inside the molten lead will spatter out the end and into your face. I've never seen it but the possibility is there. I repeat don't tilt the pipe towards your face. Wear gloves and use pliers or vicegrips etc. to hold the pipe.
I've made some boppers where the whole pipe is filled with lead but i find that they are too heavy and "brutal" and really crush the flint platforms-that aint good! A couple I made are only filled 1 1/2"-2" with lead and have about 3" of bare pipe. These I whittled a wooden handle to slip into the pipe and glued it in. The wooden handled ones have great balance and I can get a lot faster swing to them. I think most knappers realize that a fast swing is what's required for good long thinning flakes.
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