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Discussion Starter · #1 ·
Somehow a chunk of 4140 lying around the shop caught my eye and and I started noodling around with a Hotchkiss Mountain Gun action scaled way down (hence the "Molehill" designation).

The tapered cross bore and matching breech block are fitted, the extractor track and loading holes are in the block and I have an idea for the otherwise awkward extractor T-slot. I'm still debating doing a proper handle lock thread or just a slot & tab on the other end.

For ammunition I'm plotting a 45LC or .44SPL-based custom case with external antique ignition and fire-flap valves similar to the original. The finished product will be small enough I can easily send it to ATF for a determination if that loading & ignition method is officially 'antique' or not which has come up here a time or two (with an ATF manufacturer license, it would be OK no matter what they decide, although this perhaps might need to be posted elsewhere in the event of an unfavorable ruling).

Anyway, a question came up that I've not been able to find any details on regarding the rest of the tube. The usual sources say the trunnion is 'screwed on' which I assume means threaded and attached to the tube like a giant nut & bolt, but I've not been able to come up with any documentation of what that looks like internally and it would seem like a nuisance timing it correctly on a one-piece tube like the originals. Does anybody happen to have that info (and/or a copy of Koerner's book on the Hotchkiss Mountain Gun, which I've found dead links to but no live copy yet online?)
 

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I think you should look up the rules for destructive devises. https://www.atf.gov/firearms/firear...-firearms-national-firearms-act-definitions-1
In general, bores under 1/2" appear to be exempt without a requirement for sporting purposes. Accordingly, I don't think you need primitive ignition unless you want to. You might want to consider a custom case based on a rifle cartidge like a cut down 45/70. I think it is wise to exclude standard ammo by chamber dimensions.
If your project is questionable legality wise, you need ATF approval before getting to critical stage of construction, but I don't think it is.
 

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Yes indeed before you start your build write a letter to ATF and get their interpretation.

Most likely this gun you are building is covered under the GCA and not NFA. It would not be considered a destructive device. But you need ATF to say that.

Since it is hand cranked fired, replicates a pre 1898 design and is under .50 caliber there should be no problem building it. You are allowed to build a gun for personal use and not for resale. But most importantly don't take my word for it, my word is not worth the paper its written on. You need a letter from ATF saying O.K.

You do not want to build any device and send it to ATF, that's called evidence. We can talk about his next Saturday.
 

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Discussion Starter · #4 ·
Thanks for the concern on the ATF stuff. I didn't explain earlier but it will be ok this way. My company holds an 07 FFL firearms manufacturer license and 02 SOT to manufacture NFA items, so as long as we file the forms, we can (and do, that's part of my day job) make pretty much anything (although there are limits for Destructive Devices on an 02) The suggestion from a friend at ATF was instead of a letter that takes forever to get answers to, the track for actual items from a manufacturer is faster so in this case, make something, file the form we think applies and send it to ATF-FTB because if they differ, all we have to do is file an updated form (versus an unlicensed person sending something in, which is not a good idea absent an ATF request to do so).

I planned it under .50 cal mostly to use existing barrel blanks, but it also makes the FTBs job a bit easier if they just have to determine one thing instead of both GCA and NFA all at once. Also helps with the DD limits of the 02SOT.

I also think the primitive ignition is probably going to be easier all around than trying to squeeze a centerfire system in that block as the mass and springs needed to set off a primer just doesn't scale nicely.

Btw, it's this gun I'm modelling, not the more often discussed revolving cannon:

 

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Nice start! I've always liked that sliding wedge kind of breech and thought about using my Seacoast 1" rifled blank to make a 3" Hotchkiss mountain gun, but it didn't scale very well.


I looked through the documents I have on the Hotchkiss guns and neither the 1.69" or 3" drawings show how the trunnions are held on.


I can't wait to see how this project turns out!
 

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Discussion Starter · #7 ·
The said:
Nice start! I've always liked that sliding wedge kind of breech and thought about using my Seacoast 1" rifled blank to make a 3" Hotchkiss mountain gun, but it didn't scale very well.

I looked through the documents I have on the Hotchkiss guns and neither the 1.69" or 3" drawings show how the trunnions are held on.
Thanks. It's briefly described but not illustrated in the sources I have. Must have been usual enough back in the day that readers knew what was meant or just didn't care that much since the breech was the new high tech innovative thing.

The Seacoast sleeve looks like a good candidate although I can't remember if there's enough diameter to do the breech integral. I think not as you'd need 3.5"+ diameter. Doing a separate breech ring helps with some of the setups though.

This small is problematic on the other side trying to get everything to fit without making fragile watch parts. I may need to make small scale artillerymen to load the thing.

My apologies as well, I should have made the ATF stuff clearer up front. I do appreciate everyone trying to make sure things are up and up and keep me out of trouble.
 

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Shred, best wishes from Mike and I on this new project, can't wait to see the result! We agree with The Jeff 100%; the sliding wedge kind of breech mech. has always been our favorite, except for the Fr. 75. You have picked the Mountain gun as your type of arty. We would pick that one too if we had to make a field gun, but call us weird if you must, because as you and most other folks around here already know, our favorite Hotchkiss is the rapid fire deck gun pictured below. Love those 47mm guns!

Tracy

Cruiser mounted, quick firing, anti torpedo boat gun:

CANON HOTCHKISS A TIR RAPIDE DE 47m/m L/40 SUR AFFUT AUTOMATIQUE
MODELE 1897
click to enlarge
 

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Shred, I did some brief googling and it looks like the trunnions could have been shrunk on as well.


https://books.google.com/books?id=eStJAQAAMAAJ&lpg=PA448&ots=AVajRDzFAD&dq=%22hotchkiss%20gun%22%20%22trunnion%20ring%22&pg=PA448#v=onepage&q=%22hotchkiss%20gun%22%20%22trunnion%20ring%22&f=false


Mountain guns of smaller calibres are made of a solid forging, only the trunnion-ring of wrought iron being shrunk on

I also found some references where the trunnion band was screwed on, but it looks to me that at least some mountain guns had their trunnions shrunk on. That'd probably be the easiest way to get the trunnions to line up right on your model. I can just imagine trying to cut threads so everything lines up when torqued down.
 

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Discussion Starter · #10 ·
Thanks everybody. Progress has slowed due to the influx of family and holidays and catching up with real work, but I did get time to put in the stop screw, rear-sight bore (temporary stand-in bolts and sight) plus test an extractor concept which turned up a minor (fixable) geometry problem with the block.

A kind collector of original HMGs tells me the trunnions were indeed threaded on and trimmed to fit. I'm not sure if I'm going to that level of detail on this one or not.
 

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shred,


Just noticed your mini Hotchkiss breech and block.
Very nice looking assembly. what is the status of barrel and
trunnions ? Have you solved the exstraction system?


boremax


(collector and shooter HMG's)
 

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Discussion Starter · #12 ·
Still working on it, thanks. It's a slow-moving project since it necessarily has to take a back seat to family and holidays and real work and all.

The barrel will be made from a rifled blank and the trunnions either shrunk or threaded depending how ambitious I feel when I get there.

I modified the block to correct the geometry error I introduced in the extractor track and did some more extractor experiments which seem to be working reasonably well with dummy ammunition.

The next step is the handle and lock which is mostly one of sorting out how to miniaturize the key parts of the mechanism while keeping the look and function correct.

While the physics of scaling-down can get annoying at times, it's fun discovering the reason for some feature or other on the full size guns is the way it is.
 
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