37mm anti tank gun build....non-DD

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Andy Cole
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Re: 37mm anti tank gun build....non-DD

Post by Andy Cole » Wed May 12, 2021 6:36 am

dpcd67 wrote:
Tue May 11, 2021 4:35 pm
Wow, super job! I'm going to study your ideas; hope you don't mind!

You could have legally made it a muzzle loader and fired with a fuse. Put a breech plug in the breech end; and have a firing cannon. DOM tubing makes a good barrel.
I have decided that since I have gone this far that I am going to submit a form 1 and complete this as an NFA DD.
Especially since no one that I have talked to can tell me whether or no it will be considered a DD if I complete it where it could fire.
At this point it has NO chamber for any known fixed ammunition and has a 1.5" steel rod welded into the breech end of the bore. On top of that the breech ring is incomplete and there has been no breech block machined. Right now it is a 900lb paperweight.
I have muzzle loading cannons of the Civil War variety. I had given thought about making this one a breech loading black powder cannon. Fired by a .22 blank used in powder actuated hammers. If I take to to WWII events, a breech pug would be obvious when people look at it.
My idea would make it kind of like a big Civil War Sharps rifle. Insert projectile (or not) , insert powder bag, close breech, prime, and fire.
I asked a class 7 SOT if it would be legal like that, and he said it depended upon the agent who looked at it. I asked an ATF agent, they said they would have to ask the NFA folks. I sent the Firearms Technology branch a letter with detailed descriptions, measurements, and excerpts from the NFA rules to see IF it would be considered a DD.
I got a very polite reply that they could not make a determination through descriptions, drawings, blueprints, photos, etc. The firearm would have to be sent to them in WV for testing and that they were not responsible for any damage that may occur during testing. Also the rules say that if it was determined to be a DD that I could lose possession of it and it could be destroyed.

I've got too much time in it for that. I would cry for sure.

For me $200 is nothing to sneeze at for the Form 1, but it is better than legal issues and/or losing a project that I have worked on for 7 years (off and on).

The hang up is that the bore is over .50 inches. I imagine they could say it met the criteria for a DD, even though it would fire using pre 1898 style loads and ignition sources, i.e no fixed ammunition. Unless I cross drill through the bore and put rods through it for blank fire only, but what's the fun in that? If I go through the trouble of completing all the parts so that it will fire a blank, why not make it fire a projectile? I am not sure that I will live fire it more than a few times ever. Even then it will be by remote means from a covered place. However, If I decide that I want to shoot it, then by God, I want to be able to shoot it. 95% of what I would shoot would be blanks though. I am just not willing to remove the possibility of live fire if made to fire blanks.

If I am going to build this to fire then I about as well rifle this barrel so I can hit what that really nifty sighting mechanism that I made is aiming at. Soooooo I have gone and gotten my fingerprint cards, and getting the passport style photos printed. While I am waiting on the Form 1 approval, I think I am going to make a contraption to rifle this barrel. It is supposed to have 1 turn in 25 calibers. So 1.5 inches times 25 is one complete rotation every 37.5 inches. I can do that. It is supposed to have 12 grooves. I can do that. What I am trying to determine now is how to machine an adjustable carbide cutter that I can drag down this 6 1/2 foot long barrel and cut the rifling. It'll take a whole Saturday or possibly weekend to cut, index 12 times, then adjust the cutter and repeat until the grooves are 1/16" or 3/32" deep. I am thinking that the groove needs to be at least 1/8" wide.

Along the way, I am also going to have to machine a lead mold for conical point hollow base projectiles alone the lines of a Civil War Enfield bullet. I am not sure that the rings from a Minie' Bullet (Minnie Ball) are necessary. I'll cross that bridge when I get to it.

Again, this is one of those projects that I often look at and wonder "what was I thinking?"
1943 Willys MB285433 D.O.D. 12-3-1943
1951 M100 trailer D.O.D. 3/51
1951 M37 Truck D.O.D. 5-28-1951
M3 37MM ATG
M3A4 Hand Cart
Wife D.O.Delivery 7-21-1979
Boy D.O.Delivery. 2-15-2007
Girl D.O.Delivery. 4-9-2009
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dpcd67
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Re: 37mm anti tank gun build....non-DD

Post by dpcd67 » Wed May 12, 2021 7:04 am

All good ideas; we can't own DDs in this state. As for a breech loading piece, that is how modern artillery is loaded; like the Sharps. I have made cannon molds out of aluminum; no grooves. Just smooth to fit the bore and an groove diameter obturator band at the rear. They don't go fast enough to need lube and with BP I clean after each shot anyway. One piece; bored in the lathe. No need to make it split; open at both ends. It uses a lot of lead though, I considered making them out of steel, which would be a lot cheaper. Or cans filled with concrete; need an obturator band of some type.
ATF rules say that muzzle loading cannons have to be replicas of some pre 1898 piece.
Oh, you don't need a hollow base if you have a chamber that will accept a projo with an obterabing band on it. And the Minie bullet rings are for lubrication. Don't need those either; British ammo was smooth. .58 Rifle-Musket ammo. Hollow base was only for muzzle loading.
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Re: 37mm anti tank gun build....non-DD

Post by dantheman » Sun May 16, 2021 12:55 pm

Probably not a bad idea to go ahead a form 1 it. Like you said, $200 is much less that the headache of trying to get a determination letter on. Did you use an eForm 1? They come back pretty quickly compared to paper, normally.

Truly an impressive undertaking! I wish I had the equipment and machining skills to do something like this. Not to mention the time lol. I’m working on a M2 mortar, that’s about the largest scale DD I can manage.

Speaking of mortars, how hard do you think it would be to bore the ID of an off the shelf piece of tubing to the correct ID for a 60mm barrel? Last I checked the piece of tubing I have is 0.020” undersized. I can’t find a shop around here that will agree to bore it. At least for a “reasonable” cost. Don’t mean to derail the conversation, pm me if you feel I’d like to discuss.


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Re: 37mm anti tank gun build....non-DD

Post by dpcd67 » Sun May 16, 2021 3:34 pm

Why is it important to remove .020 (the thickness of a matchbook cover); you aren't shooting real 60mm ammo are you? Cheaper to buy a piece of DOM tubing the ID of what you want.
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37mm anti tank gun build....non-DD

Post by dantheman » Mon May 17, 2021 4:36 am

dpcd67 wrote:Why is it important to remove .020 (the thickness of a matchbook cover); you aren't shooting real 60mm ammo are you? Cheaper to buy a piece of DOM tubing the ID of what you want.
It’s a DD. I have a registered cup. I know Marshall Morgan sells live barrels but I wanted to try to make one. The closest size tubing you can buy off the shelf is 2.375” ID. The bore spec is 2.392”-2.410”. So the exact amount of wall thickness that needs to be removed is 0.017/2 or 0.0085”. That is to meet the minimum spec. Not a lot of material but no one has been successful in doing so. There’s a thread on NFA talk http://nfatalk.org/forum/showthread.ph ... 84&page=31 where there are several component machinists working on M2s and they’ve given up trying to bore a tube. Just seeing if anyone else had any ideas. I’d like to shoot the many original m69 practice rounds I have without modifying them.

I’m guessing all this is why only one guy makes them. Boring that long of a tube seems to be an arduous task. If all else fails I’ll just order Marshals.


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Andy Cole
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Re: 37mm anti tank gun build....non-DD

Post by Andy Cole » Mon May 17, 2021 4:52 am

I am not a professional machinist. I am purely self taught and amateur for sure.
I do know that trying to bore inside of a tube that long will require a pretty large machine. Not for length, or difficulty in cutting. However a boring bar long enough to cut that will have lots of chatter unless it is Hawsy-gee-mo-gittum-wampas hawg-mack daddy big. My boring bar is 1" diameter and I get flex and sometimes chatter after a few inches. A lot of it has to do with cut speed, feed speed, cutter angle, depth of cut etc, LOTS of variables to get a clean, long cut on the inside. Those with more knowledge and experience may be able to provide more insight.

Unfortunately, I will not be able to help. Good luck with that! Post pics along the way.
1943 Willys MB285433 D.O.D. 12-3-1943
1951 M100 trailer D.O.D. 3/51
1951 M37 Truck D.O.D. 5-28-1951
M3 37MM ATG
M3A4 Hand Cart
Wife D.O.Delivery 7-21-1979
Boy D.O.Delivery. 2-15-2007
Girl D.O.Delivery. 4-9-2009
Girl D.O.Delivery. 10-23-2012

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Re: 37mm anti tank gun build....non-DD

Post by dantheman » Mon May 17, 2021 5:10 am

Thanks Andy! I don’t see how a boing bar would work any way you try. Like you said flex is going to be a problem. I had envisioned some type of piloted cutter that I’m sure would have to be custom made. I guess I should just break down and buy the dang ready made one. It’s just frustrating knowing it has to be possible because it’s been done.

At any rate, sorry to get off topic. Keep up the good work, I’m excited to see this completed!


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Re: 37mm anti tank gun build....non-DD

Post by dpcd67 » Mon May 17, 2021 6:48 am

Ok, what you want to do is super easy; we have been boring cannon barrels in this country for well over 200 years and before about 1840 the tooling was very primitive. You use a boring head; those support the cutter. The heads are adjustable so the cutter does not chatter; it can't move. Early boring heads were made of wood. It took like 3 days to bore one, and they were bored from solid castings. Google for boring head and you will see them.
Many times they used three or 4 cutters on a boring bar, which won't allow flexing. OD of the front of the bar was as large as possible too so that acted as a pilot.
Or just make a pilot in front of the cutter. Lots of ways to do it. Like a rifle barrel rifling head, which uses only one cutter, the opposite side rides on the bore. Then when you are done, use a cylinder hone but you know that.
I managed the Tank Cannon barrels for the Army (in TACOM) and visited Watervliet Arsenal many times and saw their tooling.
For the M256, 120mm Smoothbore, those are swaged with a big football pushed through hydraulically. After hot rotary forging to size and length from a short fat tube.
They also use big reamers. Removing your 20 thou is child's play.
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Re: 37mm anti tank gun build....non-DD

Post by ordnance » Mon May 17, 2021 1:52 pm

Without buying a bunch of expensive tooling, the easiest way to remove that material is to find a shop that does industrial honing. They can get it out to finish size and you will have a mirror bright, smooth bore in the process.
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Re: 37mm anti tank gun build....non-DD

Post by Combat Medic » Mon May 17, 2021 5:21 pm

Great work. You truly have a lot of metal working and welding skills. I have enjoyed you sharing your pictures and progress with us!

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Re: 37mm anti tank gun build....non-DD

Post by Lee Bishop » Tue May 18, 2021 1:46 pm

I am marveling at the scope of this build. Just goes to show you how complex something is when you decide to create it from the ground up.
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Re: 37mm anti tank gun build....non-DD

Post by brett20mm » Tue May 18, 2021 5:22 pm

Honing is the ticket and might even be able to buy something already the correct size. Find the hydraulic cylinder builder in your town and most have a gun guy working there already. That might give you some extra time and find the ID your looking for.
Good luck

Andy Cole
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Re: 37mm anti tank gun build....non-DD

Post by Andy Cole » Sat Jul 24, 2021 2:41 pm

My form 1 came back today.
Now I can complete the breech ring and block!
This may end up being the hardest part!
1943 Willys MB285433 D.O.D. 12-3-1943
1951 M100 trailer D.O.D. 3/51
1951 M37 Truck D.O.D. 5-28-1951
M3 37MM ATG
M3A4 Hand Cart
Wife D.O.Delivery 7-21-1979
Boy D.O.Delivery. 2-15-2007
Girl D.O.Delivery. 4-9-2009
Girl D.O.Delivery. 10-23-2012

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Re: 37mm anti tank gun build....non-DD

Post by mreyloc » Mon Nov 08, 2021 9:06 pm

Would you be willing to share or sell your CAD files?


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