If anyone is interested, I have the beginnings of a working group to develop projectiles for the hobbyist 60mm DD owner. My aim is to develop a number of easily manufactured and cheap projectiles including chalk rounds with spotting charges, smoke, camera, and other rounds. All designs will be released under Creative Commons license and available to all for non-commercial use. We have a very basic impact detonating spotting round designed, with the final prototype design forthcoming within the next two weeks.

Anyone interested PM or e-mail me and I'll get you on the e-mail list.


ETA: Some of these fuzes may translate well into use for 81mm or foreign mortars as well. And all designs are going to be centered around < 1/4oz (100gr just to leave a bit of head room) of explosives for spotting charges so everyone stays legal.

Here is the basic jist of what we're trying to accomplish here. I've added the new folks to the list, and will include all the new guys with any new developments.




First, most modern mortar fuzes utilize a setback-armed, tube-safe design, meaning that until the lift charge has gone off and the round exits the tube it is safe.

http://uxoinfo.com/blogcfc/client/includes/uxopages/Mulvaney_Details.cfm?Ord_Id=PF39



The M525 fuze used during WWII is a good, albeit complicated example of an impact detonating fuze.
Only being able to use 1/4oz of explosive per round to keep each projo from being classified an explosive DD, leaves us able to minimize the mechanical paranoia of the HE fuzes.

Because this is only a hobby to me, I am not out to profit commercially on any mortar-related work. As such, I want to keep any work open source and freely distributed via a CreativeCommons license http://creativecommons.org/licenses/by-nc-sa/3.0/ meaning we'd get attribution for any derivative work but no one else could profit off it without authorization. Basically, I'm in this for the fun of it and to benefit the artillery shooting community.

We're basing our family of designs around the aluminum Bud Light bottles that just happen to be 60mm in diameter. The first prototype will be wholly contained within the bottle and require no further holes to be punched.

It would consist of 5 parts that should be easy to produce:

Perforated steel tube that would run the lenght of the bottle and be insertable through the mouth, requiring no other holes to be drilled.

Plug on the 'nose' end of the tube that would act as a fixed firing pin

Weighted carrier that slides freely in the tube that would contain the spotting charge itself

Plug on the 'tail' end of the tube that would contain a threaded area for M69 fin assemblies

Collar to join the lip of the bottle and lip of the 'tail' plug (Not shown in drawing)

The concept of operations would be that an empty bottle would be nearly filled with chalk or flour, the carrier would be loaded with the spotting charge, placed in the tube and capped on both ends. The tube assembly would be wrapped with metallic tape to prevent chalk inclusion into the perforations and inserted into the bottle, being secured by the collar. The projectiles would be stored vertically immediately prior to use.

Upon firing, the carrier and charge cartridge would be forced to the rear of the tube assy and should remain there through the ballistic arc. On impact, the carrier and charge would slam forward, contacting the firing pin, detonating the spotting charge and spreading the chalk payload.


After initial research, it looks like the spotting charge will have to be self-contained within the weighted carrier as no commercial cartridge has the volume neccessary to contain the 100gr of FFFG which I calculated should be about .40 cu in. Even a .444 marlin case only has a volume around .34 cu in. So, constructing the weighted carrier to carry a shotgun primer and hold 100gr of FFFG with a copper burst disk to hold it all in.

I am going to be reloading the lift charges using shortened 20ga shells, but for the fuze, everything has to fit through the neck of that beer bottle, so 12 and 20ga are out, and .410 doesn't have sufficient case volume. That's why the weighted carrier will have to be designed to contain the spotting charge, also it allows for higher pressures to develop with the inclusion of a burst disk instead of a roll crip on a brass casing.

As far as flour as a payload, the material doesn't appear on the ATF List of Explosive Materials (2011R–18T) http://www.gpo.gov/fdsys/pkg/FR-2011-10-19/pdf/2011-26963.pdfand, as such, cannot be deemed to be an explosive as it relates to projectile net explosive weight. And I think the case would be tenuous at best if anyone were to suggest that flour was an incendiary substance.

Regarding the apex transition, after the apex of flight, then the round and weighted carrier inside are all falling at the same rate, allowing the weighted carrier to maintain its relative position inside the round until such time that the round impacts, stopping the projectile and allowing the carrier to move toward the nose.

I also have a simple design for a setback-armed firing pin, but that will have to wait until later iterations.

Will 28 gauge shells work or a shortened .50 cal black powder cartridge? The 50-90 held 90 grains of fff with a bullet in the case, with no bullet and finer powder you should be able to get 100 grains in there. If that won't work a 50-110 would certainly hold all the powder you can legally put in there.

About the flour, this is the same organization that put people in prison for shoe strings. Before you discount the power of flammable dust look at some pics of destroyed grain elevators. A couple pics f the aftermath of grain explosions would be all it would take to convince a jury that the flour was part of the destructive payload. Even better check out youtube videos of people demonstrating the power of airborne flour.

That should be pretty easy to accomplish, I'd like to try the design without anything first, but the addition of a simple straight spring to maintain a minimum of friction to hold the carrier at the tail of the projo until impact is a definite possibility.

I'll get proper drawings done after next weekend. I'm up against a gradschool deadline, but will have a couple weeks afterward to play around with this idea. In fact, I may have the setback armed feature drawn up by then. I'm keeping to the watchwords: Safe, Simple, and Cheap

I've gotten swamped with my twin boys, a new job, and grad school. I am still conversing with folks about design particulars, but I lack the extra time to finish my drawing package. I'll put some time into it this weekend as my wife and kids are out of town.

OST

Maybe this will motivate me to get off ky Ase and start my build.

What is the ATF regulation concerning the quarter ounce of explosives? Is that BP , Smokeless, flash powder , TNT, PETN , RDX , or what?

I'm absolutely loving the ideas in this thread, I've just started looking into building a 60mm DD mortar.

it is Net Explosive Weight. The most common explosives in a normal scenario would be smokeless or black powder, mostly because they require no particularly onerous storage requirements, are easily purchased, and require no federal explosives license. Not only that, black powder produces a respectable detonation velocity above 1300 ft/sec. and you can store a can of it in your house. The issue only arises when you load more than a quarter ounce in a projectile or other explosive device.

Wingnut mentioned this project to me yesterday, so while enjoying my nightly insomnia I cobbled something together. Hope you guys find it of use. It's just a simple inertia percussion fuse assembly. Uses .38 special blanks to eject the chalk dust.

Cheap, thin, mild steel tubing and sheeting might be the most economical route.


Get some proper hard alloy steel dies made to do the forming in a hydraulic press.