The Faux Machine Gun

Robert Roswick is a grey-haired tinkerer born, raised and living in Bismarck, North Dakota.

The Ask

Hark,

I get this call, "Bob, this is Dan. Mike says you can build us a couple of machine guns for our T6 Texans."

I respond, "I think I can, but I would have to want to."

"I'm looking for someone to help me out."

In the words of my idol, Louie Zachmeier: "Never do the same job twice and only do what you find interesting."

I respond, "I'm interested."

Research

Next step is to research what kind of gun is in a T6 Texan. It is a trainer, like a P-51 with training wheels. So it doesn't say "Iceman" or "Maverick" on the side of the cockpit. It should say "Bedwetter" or "Mollycoddle". Given the only purpose for a gun on a trainer is practice, its armament is limited, probably small caliber. I did learn the South Africans used these planes and I understand they used the guns for real so it is not just a trainer. A quick trip to Wikipedia and other countries used them in a ground support role.

I find the gun I think is in this machine.

The Nest

Next step is to see the gun's nest in the wing of the airplane. Dan has the covers off. There is a bin for a belt of ammo. Also he opened the bay for the gun. I find a fore and aft cradle. The fore cradle is 2.2 inches above the aft cradle. The barrel is narrow and the receiver is fat so I assume the difference between the height of the barrel shroud and the bottom of the receiver is 2.2 inches. Each cradle has a 2 inch circular depression around the mount. The depression is 0.05" deep.

The fore cradle has a 1" or so hole that I come to find is tapered. Well it is tapered in Dan's aircraft and only in Dan's aircraft. In Mikes aircraft there is no taper - interesting. I believe this to be from wear. In MIke's aircraft there is a cable. We think this is to charge the machine gun before firing. The aft cradle has a squared off hole that looks like it accepts a locking pin.

This may make sense. If the large front pin is for for handling the recoil then only having a rear lock would make the gun very fast and easy to swap out. Now that I know pin dimensions and offsets I can model this out in CAD. I need the practice in learning Fusion 360 so we will see what we can do in that package.

Seeing how the only thing we see is the barrel, then it is only a barrel that I need to make. This simplifies things tremendously.

I design an expanding nut for the fore cradle so when you tighten it up, it wedges itself into the tapered hole. Aluminum is gummy. It will stick in the hole. This will work well.

I'm off to Pahlke Steel to get some pipe that looks like a 30 caliper barrel and another piece of tubing that will serve as a shroud. I make a ferrule to mount the barrel in the shroud. Correct that, I found a ferrule in my scrap bin that actually fit. It wasn't standard dimensions, so dumb luck to the rescue.

Within a day I've got a credible barrel. I think Dan will like this.

The Build

Next is to design and make some mounts. This is fairly straightforward if you have the right raw aluminum stock. Well I didn't have the right stock but I can design around that.

Using V-Carve, I design the mounts. V-Carve Pro 9.5 is nice and easy way to design it and has a great post processor to generate commands for the computer.

Next is the expansion nut. No design or CNC here. I just pick a taper (15 degrees) and make a tapered cavity then a tapped tapered nut. I need a saw to cut into the expansion nut, after all it needs to expand. I don't have the tooling to cut the tabs so off to see Jim-The-Tool-Man. He sets me straight and ...

Now that I have brackets that can grab the barrel, I need some pedestals to place them on. The barrel comes out the leading edge of the wing. I assume it is inline with the cord of the wing but it actually may point up or down, I really don't know. What I do know is the barrel needs to exit the middle of the leading edge so this is what I shoot for. Making pedestals is what is known as the easy part.

Or so you would think. This is were I start to break tools!

The Trial Fitting

With pedestals done, its time for a trial fitting.

and this:

And...

I don't know about you but I'm feelin' it.

The Shroud

Next is the hard part. That barrel needs a shroud that is fenestrated, ya know, full of holes. in a machine gun this is the part that does not get so hot so you can grab it. It is now apparent, I need bigger equipment. Well that may require another visit with Louie. We talk about solutions and although we have many ideas it will take the big stuff to get this done. I've got this giant rotary table under the bench that was being converted to computer control by my mentor Ed. A project that ultimately failed. Let's check it out.

Well lets start out with how do you get that bad boy from the floor to the table. In the words of my brother-in-law, Mick, "When I turned 60 I realized I was down to the strength of two men." So I put it on the table. P.S. does anyone know how to fix a herniated disc?

Notice the hand-wheel is missing. Nice thing about a shop is I can make one. I found an old piece of scrap that already had two holes in it. I guess those holes will have to do. Using my father-in-law (Robert Leo) rules I have to use it. He relished doing projects without going to the hardware store for anything. So I draw up the piece of scrap with the two holes and other imperfections in it in CAD then using those two holes I design my handle.

Then I gives it to my CNC machine...

Then with great pleasure ...

(Now that is the look of satisfaction ...British Satisfaction I must say!)

Now I can use the rotary table.

You can see the shroud mounted in the chuck above. With shroud in place I start milling holes. Sixty in all, times two barrels. Great, I have 120 chances to screw up. Luckily I couldn't do it. This "not screwing up" only happens after your old man passes away.

I end up with this:

The challenge is done. So in the pride of the moment I send it to John, My machinist wannabe freind. He is actually much closer to the mark than I am. He responds to the photo with, "Ok. We're amazed. What's the other end look like?"

So I send him this:

That black thing is my greasy thumb lining it all up for me. John responds, "Jeezus!! Scared the crap outta me. "

The barrels need to be blued then it is time for the mount. The next trick is cutting a hole in the leading edge of the airplane. Dan has a plan that will work. He has some heavy rubber to seal off the hole. He made some adjustments and we have this:

And this:

I'd say our little trainer has lost its innocence. I think we need to reconsider some names for the cockpit now. Dan is a dentist so I was thinking "Fang" in the front and "Mongrel" in the back. What say you? On second thought, if he gives his charming wife a ride, the consequences are too horrible to contemplate, I might want to rethink the moniker for the rear seat.

Das ist alles. (That is all.)

Bob

Post Script

I delivered serial number two to Mikes bird. This install was a bit easier. We discovered neither cradle had been modified nor were they worn. This was a revelation. I thought the fore hole was tapered, it's not. It is just worn on Dan's plane. I also thought the aft mount was unusual. It was. Dans cradle had been milled away to accommodate a wing skin. Mike also has the original cover for the leading edge. Although it was a guess, I was dead center on the leading edge hole. Another application of dumb luck. We both agreed that rubber was in short supply during the war years so it was likely the gasket used to seal the gun in the hole was made of leather.

Comments

Pat Durick on May 22, 2020:

Fantastic and very, very interesting. The range of your talents is unbelievable. Thanks for sharing the experience.