.778" Bore 110 Pounder Armstrong Gun

The 110 Pounder Armstrong Gun was a British breechloading cannon developed in the 1850's. They were safer to use than muzzleloaders and they were used in both ships and costal defense. They fired a 7" diameter shell that weighed up to 110 pounds out to 3,500 yards, but the materials of the day weren't quite up to the task so they had to be fired with relatively weak 11 pound powder charges. This was a problem since ships were beginning to switch to iron armor around this time. The guns were quickly taken out of service in the late 1860's, although they did see action by shelling forts in Japan and the Maori in New Zealand. The HMS Warrior is probably the best known ship to be armed with these weapons. She had 8 on broadside carriages and 2 on pivot carriages on her deck.



Period photos of 110 pounder Armstrongs are hard to come by, here's the best one I know of. The gun is mounted on a pivot carriage so it can be pointed in any direction. I haven't been able to find a picture of the gun mounted on a broadside rear chock carriage which is what I intend to model.



Here's a drawing of how the gun operates. To load the gun, the breech screw is loosened a few turns and the large "vent piece" is taken out of the top of the gun. The shell and powder charge are loaded through the hollow breech screw into the chamber and the vent piece is put back in place. The breech screw is screwed up tight against the vent piece and a primer is placed in the stem of the vent piece. Fire goes down the stem and makes a 90º turn into the rear of the chamber igniting the powder thus firing the gun. It's not a particularly strong design and it isn't used today. The vent piece weighs over 100 pounds and had to be lifted out manually. Later Armstrongs experimented with cutting the slot horizontally to make it easier to load.



I decided to make my model in 1/9 scale so it will be in the same scale as my 1" Dahlgren gun of the same period. Here's the 3" diameter chunk of 1018 steel I started with. It's 14" long.



I chucked the 3" diameter piece of steel in my lathe and started turning the various diameters and lengths of the barrel.



I made a trunnion band and separate trunnions that were pressed into place. The trunnions ended up in the correct location when they bottomed out against the 1/2" drill bit.



Then I drilled and bored out the trunnion band until it was about a .002" shrink fit on the barrel.



I heated the trunnion band with a torch and dropped it onto the barrel where it clamped down. Then I put the barrel back in the lathe and finished shaping the trunnion band.



Next I used a long 49/64" drill bit to drill out the bore.



The 49/64" drill bit left the bore 0.766" in diameter and I want .778", so I welded an adjustable hand reamer to a piece of pipe and reamed out the last 12 thousandths leaving me a nice finish.



I flipped the barrel around and bored out the area for the breech screw to a minor diameter of 1.365" and threaded it with 11 TPI double start threads. This is the first time I've done double start threads and basically they're two 5.5 TPI threads 180º out of phase. I didn't have any trouble making them.



Then I made the hollow breech screw out of 4140. It has the same 11 TPI double start threads. 4140 might not be necessary and it certainly won't offer any benefit since the barrel threads are 1018, but my thinking was it might help prevent recoil from mushrooming over the end of the screw since there's not much area supporting the vent piece.



Now! The fun part. The original Armstrongs were rifled guns so I decided to rifle my model as well. The originals had 76 grooves, but I don't think I can make grooves that small. So I decided to use a piece of 1/16" thick O1 tool steel and make a "tail dragger" style rifling head and put in 18 grooves. This should give me equal land and groove widths.



You can see the set screw is sharpened to a point so when it is advanced into the holder it will force the cutter upwards. By making small rotational adjustments to the setscrew, I will get small depth of cut increases. It ended up taking about two revolutions of the setscrew to cut a groove to a depth of .020".



To make the rifling spiral, I tack welded a 1/4" rod around a 5/8" rod at a 1:26 twist rate. Then I made a plate with a 5/8" hole in it for the main rod and filed a 1/4" keyway in it. I welded a long piece of wire to the set screw so it will go through the cannon barrel and out the rear of the lathe's spindle so I can adjust the wedge.



Here's the setup. The plate with the keyway is locked in a tool holder and stays in one place for the whole rifling operation. When I pull the bar with the cutter head on it, the 1/4" rod riding in the keyway forces the cutter to rotate at a 1:26 twist rate. Then I can adjust the depth of cut with the wire welded to the set screw.



I have 18 grooves to do and I need to accurately index to the next one. I wrapped a piece of paper around my chuck to find its circumference and then I divided the length into 18 parts. Then I taped it onto the chuck and used a test indicator as a pointer. I put the lathe into its lowest gear so the chuck won't move. After each groove is cut to depth, I rotate the chuck to the next location.



The first few grooves! They're a little uneven, but I was surprised my system even worked! The rod coming out of the barrel is the one that is welded onto the setscrew and is used to adjust the cutter depth.



Halfway done!



Finished at last! It took about 20 hours to do the 18 grooves. It was a ton of work, but I'm really proud of how it came out. Not many people can say they've rifled a barrel from scratch.



The cutters had to be sharpened at least once a groove so I went through three of them. The second one broke after two cuts since I forgot to temper it after hardening. I ended up taking about 35 cuts per groove, so 630 cuts total.



Here's a video showing how the rifling machine works.



Next I worked on the handle.



Lots of filing and sanding.



Originally the handles could rotate on the breech screw ~180º so you could use the inertia from the weighted ends to drive the screw tighter via two dogs. But that's a little too much detail and delicate parts, so I just pressed the handle onto the screw.



Here is the vent piece. The fuse goes down the hole in the stem, then makes a 90º turn into the back of the chamber.



I had to buy a milling machine to cut the slot for the vent piece! It's a nice sized 7x27 Powermatic Millrite that's in good shape. It's big and heavy enough to do some decent milling with, but not so big that it's impossible to move around. It's basically a 3/4 size Bridgeport. This tool will really open up the possibilities in my shop.



Next I flipped the barrel around and finished profiling the outside, and I also cut the chamber for the cartridge.



Originally the Armstrongs used cloth powder bags and a tin cup to obturate the bore during firing, but that seems really finicky. I decided to make a cartridge by soldering a flat piece of copper to some 3/4" pipe to hold the powder. Pressure should push the pipe against the chamber walls to seal the breech during firing. The bullets are aluminum and have a thin skirt at the back. The idea is pressure will force the skirt into the grooves and rotate them as they go down the barrel.



The last operation! I flipped the barrel around and tapered the chase down to the muzzle.



Painted!



On to the rear chock carriage. I started by cutting the notches at the rear of the carriage.



The carriage sides taper at a 5º angle to clear the larger breech diameter so I made a quick jig out of a 2x4 so I could cut the slot for the axle at the correct angle.



I made the axle with my four jaw chuck.



Then the wheels.



The sides are cut to shape and I made the key bolts that hold the trunnion caps down. The longer bolt goes through the axle and holds it on, and the shorter one goes to a metal strap to reinforce the axle under recoil.



A test fit.



A quick bending jig to make the trunnion caps. I used the vice to squeeze the metal into the die.



Here's how the trunnion caps fit.



Here's the axle stays that support the axle under recoil.



Here's a hole drilling trick. If you need to drill a hole that accurately points in a certain direction, center punch both locations and put a live center on one. That way you can drill at any angle and have the holes match.



A lot of British cannons from this time period used a "Smith's Elevating Screw" for fine elevation adjustments and a quoin for coarse elevation. I've also seen these referred to as oscillating screws. The basic idea is the elevation rod has a set of gear teeth cut in the top, and a handle with a stud can be used to rotate the elevation rod. When you run out of room, you can pivot the handle upwards, swing it in the opposite direction, lower the stud into the gear and pivot the screw again.



In this case the stud isn't engaged with the teeth so the handle is free to rotate.



The stud is engaged with the teeth so the handle rotates the elevation screw.



The finished carriage!



The finished carriage from the front.



The finished carriage from the top.



























Here's some clips of the cannon down at the gun range!



I later determined that the hollow base bullet design wasn't expanding into the rifling adequately, as my group size was over 2' in diameter. So I made a chamber reamer from O1 tool steel to taper the rifling at about a 3º included angle and made some more bullets with a larger diameter driving band at the rear. This way the bullet engraves itself into the rifling as it is pushed down the bore. My groups shrank to 4.5" diameter with this change.