I think that Lawson's description of the cartridge manufacture serves two purposes. The first is that it adds a level of realism to the story. In describing the cartridge in stark detail, the reader is able to envision this object that has caused fascination in Tommy, dread in the men, and represents the focal point of the story. To a great extent, the cartridge is what binds the men to Tommy and he to them. If he is not running around with this cartridge in his mouth, there is little conflict evident in the story. For this reason, Lawson devotes time in describing this object, one that binds even if it its purpose is to separate through destruction. The second reason is that its placement in the story is used for dramatic effect. Consider the lines leading up to the detailed description of the cartridge:
They [the other dogs] kept at a respectable distance round the nasty yellow dog, for it was dangerous to go near him when he thought he had found something which might be good for a dog to eat. He sniffed at the cartridge twice, and was just taking a third cautious sniff when----
Through including how well the cartridge was built and made at this point, Lawson is building a sense of the dramatic. His detailed description of how the cartridge was made and the craftsmanship was evident in its construction is used to convey its destructive capacity, and specifically what it will do to the "nasty yellow dog" as a result. His detailed description is almost like a buildup to a punchline to a good joke that a comedian will employ. In this vein, Lawson has featured a detailed description of the cartridge.