Not commissioned from the ranks but, some time before joining the RAF, I was a TA REME craftsman; trade: gunfitter, 25 pounders.
The only thing I remember was the procedure for removing a misfire which had jammed in the rifling. Personally I'd just have suggested to the gunners that they insert another charge (full three bags), use a very long lanyard, and blow it out. That, however was not the prescribed procedure
