TOPIC:

Berenice
Sounds like a good story to me! I think the key lies in the part sentence "... open some cases of cartridges and SHELLS with picks". Even if one (assumed to be a rifle) cartridge was "struck with a pick" and the blow detonated the primer, the unconfined cartridge would go bang and simply split open. And the projectile would have no velocity at all. Even if the pick-wielder used the implement to bash into a full case of small arms ammunition, he might set off a primer or two - but unconfined, there would be no bullets whizzing all over the shop. Small Arms Ammunition is not liable to explode in bulk.
However, what about opening a case of artillery SHELLS with picks? Another thing altogether. What could POSSIBLY go wrong? It is much more likely that Cpl. Marchant or one of his comrades accidentally/carelessly set off some form of explosive shell (perhaps for a Pom Pom) and the Corporal's injuries included shards of the exploded shell, its packing and probably other fragments.
Regards
IL.

Here's a similar case to what may have happened to Snowball/Matthews. It's in a letter by Lance-Sergeant G. Read, a Woodbridge Volunteer in the 1st Suffolk Regiment, and was dated 27th September, 1900.

"Palmer is at Middleburg. When we were there he was stooping down over a fire cooking something in his mess tin; by some means a cartridge had got into the fire and exploded, the bullet or a piece of the case going through the fleshy part of one of his thighs; he was getting better when we came away, but was unable to march."
The Ipswich Journal, 10.11.1900