British 4.2-inch Heavy Mortar

[horsa]

I've been slowly restoring a 4.2" mortar and finally managed to get some firing pins turned down from stainless steel round bar stock. Originals were firing studs as the launching cartridges each had their own firing pin in a clip attached to the cartridge so they were self contained for launching. Guess that eliminated the chance of a malfunction in the field due to a firing pin breaking or wearing down.

I wanted to load my own cartridges from 12-gauge shotgun shell hulls and prefer a fixed firing pin so the rounds light off on impact with the pin at the bottom of the tube. Plus I have no way of producing self contained caps like they had during the war.

Took my live barrel out over the weekend and used a normal infantry base and legs which I am storing for someone else since my base and legs are the towed pattern which is much larger and harder to mess with for just a few test shots.

Link below is a very short YouTube video showing a single launch of an original round using black powder. I only used enough to throw the round out 225 yards which is extremely short range compared to the wartime maximum ranges of 3500 yards. As such, the round is moving fairly slow so you can see it leave the tube. I also didn't dig in the base since I was using light loads.

http://www.youtube.com/watch?v=TshEHDD-GvQ

Figured it would be OK to add this to the existing message thread since its one of the types of mortars being discussed. And in case anyone is wondering, its a BATF approved destructive device here in the USA so I can legally take it out and get it dirty now and then

[alleramilitaria]

whats the range?

[horsa]

Hey Dave,

Mentioned the range in the first post....I was using only enough powder to get the rounds out about 225 yards. There is an old shack with overgrown trees around it in the field I was shooting from and it sits 150 yards from where I set up. Rounds were soaring over it before arcing back towards the ground. There was roughly a 15mph headwind I was shooting into and the rounds had no impact fuzes which left a 1.75" hole in the ends. Not very aerodynamic conditions so the same powder charge would probably get further range with calm conditions and thick tape over the holes. And more powder would easily toss them further but then there would be the issue of finding them.

With high angle and a reduced charge, we could easily set up the towed mortar and do a demonstration firing at Mullin's place next year. Probably have to do it for the vehicle owners and reenactors though as opposed to when the public is there due to liability.

Next time I head south for testing I'll have someone else along to take pictures and will be prepared with more rounds and power to experiment with. The video I posted was shot from my little digital camera being placed on the back of the pickup bed.

[lssah2025]

That's awesome Dave, still working out the details on my 3", think I found someone to do up my baseplates, Thanks again for the dimensions, also I was thinking of doing up a DD/mortar tube, are the firing pins the same as you made for the 4.2"? Or would I need to do something different? Looks like the Humber and Trailer will be here by the end of the year, if not Jan. 2010 when I get back on leave from the sandbox. Also I picked up a lot of 19 set stuff, so if you need anything for your carrier let me know.

[horsa]

Hey Lance,

I had an original 1942 dated 3" mortar firing stud. Problem is you need launching cartridges which have the caps with individual firing pins build into them. When the round is dropped down the tube, the stud is the stop against the center of the cartridge and it compessed the build in firing pin into the primer. I took the specs from the orignial and made actual firing pins which will work without the WWII style cartridges. That way you can use modern plastic 12-gauge shotgun shell hulls and load them with black powder or modern substitute.

I'll try and get photos posted in the next day or two of the 3" and 4.2" versions I made for my tubes along with an example of the WWII style cartridges with a firing stud for reference.

[lssah2025]

Excellent, that is what I am looking for, I might just do up one of the trap door mortar tubes for public displays etc, drop down the round with a 12 ga charge, big puff of smoke and pull the round out the bottom...I picked up 3 complete 19 sets for tank installation with all goodies so I have some spare stuff, also my 22 set is in and I worked on it the last time I was home on leave. I also picked up a complete ranging set with base which will be coming over in the container, same base as the 22 set to mount it in a vehicle, so I can install it in my A/B jeep. I also picked up an original wooden battery box for 10 volt that I will have to get you some pics of, looks like same dimensions as the 6 volt, plus 2 metal battery carriers 50's dated with original 6 volt batteries, so more pics there..OK patiently awaiting the mortar stuff....

[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.