British 4.2-inch Heavy Mortar

[horsa]

Attached are two shots showing original and my homemade versions of firing pins/studs. I'm not sure what the British equivalent thread size is but both mortar balls that I have would accept a 5/8" coarse thread bolt. The original firing stud was 7/8" diameter so I used that for my homemade firing pins. The other shot has my original 3” firing stud along with two wartime demilled launching cartridges. One has the self contained firing pin in place and the other shows it in parts for comparison. Not something I could readily reproduce which is why I went back to a more traditional weapon firing pin using simple standard primed cartridges.

Material I used is 7/8" diameter stainless steel. The 3" firing pin is 3.25" long and the 4.2" firing pin is 4.5" long. Each of these measurements includes the 5/8" threaded section. To make the pin itself, drill a hole in the center of the face and tap in a short length of 3/32" round stock spring steel or you can use the shaft from a drill bit of the same diameter.

So far I haven't had a failure on any of my self loaded launching cartridges that I've test fired. I'm using 12-gauge hulls which I use a roll crimper to seal after filling. Powder is two F (FFG) Triple Seven by Hodgdon which is a clean modern black powder substitute. It burns dirty and greasy like black powder but cleans up easily with water. This is the same powder I've been using with my 2" mortar. I've tried several types of smokeless powder that burned really clean but they don't produce a satisfactory amount of lift for the volume of powder. Probably 1/6th the range at best compared with an equivalent load of 777 powder.

For augmenting cartridges, I planned to fill paper quarter rolls with powder and rubber band them to the fins which is similar to the wartime method where they used celluloid tubes held in place by springs. But while experimenting, I found you can simply pour the added powder charge down the barrel and it cooks off perfectly when the normal launching cartridge hits the firing pin and sets of the main charge.