He welded a pipe closed on one end. Taking every precaution one can take when one is illegally creating high explosives, Sam was smart enough to let the metal cool before he put the gunpowder in the pipe. When he was done packing the powder, he realized that he had run out of welding rod. And so he set the half-finished pipe bomb on the scrap metal pile for later.
Pipe bombs, like any other half-done task, have a tendency to stay undone for a great deal of time. That's what happened with Sam's project. He simply forgot about the pipe bomb for six long months, summer and fall.
A few days before hunting season, Sam was loading his hunting gear into his pride-and-joy Ford Bronco, when he noticed that a shaft was cracked. Being an expert welder, Sam knew he could fix the cracked pipe himself. I hardly need to say more.
Sam reached into his scrap metal pile, pulled out a pipe, pulled down his welding hood, and struck an arc. He remembers a loud bang and not much else. Shrapnel embedded itself up to the rafters of the third floor of his house! Another piece of shrapnel blew through Sam's welding hood, missing his empty skull by half an inch.
I still have the photographic evidence.
No good ever comes from the phrase, "Well, I was building a pipe bomb."
"Hilarious Roger Cotte styled accident."