1" Bore IX Dahlgren Gun

The 9" Dahlgren gun was the pinacle of American muzzleloading, smoothbore broadside naval cannon development. I've not come across any report of a 9" Dahlgren bursting in service. The cannon itself weighed around 9,000 pounds and it fired a 9" diameter projectile with 13 pounds of powder. Solid shot weighed 90 pounds, and shell weighed 73. Maximum range was 3,450 yards (1.96 miles!) at 15º elevation.

This is the best period photograph I've found of a 9" Dahlgren on a wooden Marsilly carriage. This one was mounted on the Union's USS Humpback.

Here's another excellent photograph from the Confederate side. Normally these guns were mounted on ships, but in this case a battery of 5 has been set up on land near Gloucester Virginia. Note the missing breeching and elevation screw. Wooden quoins are in use to elevate and depress the muzzle.

Here's a broadside battery of six 9" Dahlgrens on the USS Pawnee.

I don't have access to a milling machine and making a strong trunnion connection has always been a concern. I wouldn't mind welding them on if I were making a Brooke rifle or something that had flared rimbases, but the IX Dahlgren has a sharp corner. So I've been thinking for a while about the easiest way for me to do it. Eventually I decided I'll make a built up gun of sorts. A seamless piece of tubing will form the bore, and 3 hoops will be shrunk onto it. The middle hoop will be drilled through sideways and the trunnions shrunk into it. Then the bore will be reamed out to size and a breech plug shrunk and welded. With that figured out, I made a drawing.

I decided to start with the middle section and drilled a 1-7/32" hole through the center of the piece. Then I turned it sideways in the 4 jaw chuck and drilled and bored a 1.160" hole all the way through. This hole is for the trunnions.

Next I started on the curve in the area of the trunnion. Better get that done now while I can get to it or I'd be getting well acquainted with a file later on.

Next I made the trunnion piece. It is 1.168" in diameter. The surface looks a bit different, but it everywhere seems to measure within half a thousandth or so in my calipers.

And lastly I used a reamer to smooth up the hole I bored to 1.165. Steel expands at .0000064" per degree F per inch of diameter. So .0000064" x 1.165" x 405º = .003". So I need to get the temperature difference between the hoop and trunnion to at least 405º F. I'm planning to put the hoop in my oven for an hour or two as hot as it will go, then blast it with an oxyacetylene torch until it's good and red.

Here's the completed trunnion band with the 1.168" trunnion shrunk into 1.165" hole. I drilled out the center part but didn't take a picture of it.

Next I reamed out the ID of my three hoops from ~1.219" to 1.252". At least on my reamer, turning the nut 1/2 a turn increased the hole diameter by around 4 thousandths. So removing ~4 thousandths per pass, it took 9 passes.

First I started with the the trunnion band. I reamed everything out so the ID of the hoop is only .001" smaller than the OD of my seamless tubing. I stuck a shaft collar on the tubing so the hoop will end up in the right place. Heated to 550º for an hour and it slipped right on. The same with the breech hoop.

With that done I machined the hemispherical bottom. This time I only advanced in 5 thousandths per cut so there was way less filing. The steps by the trunnions were 10 thousandths and required quite a bit more work to smooth out.

Next I shrunk the muzzle hoop on. It got stuck halfway down, so I needed to press it into position with a hydraulic press. I turned a rough plug to slip down the bore to prevent buckling the tubing.

Next I tapered the chase and worked on the muzzle swell. Then I took it out of the lathe and attacked the steps left around the trunnions with a 120 grit flap wheel on a 4.5" angle grinder.

Here's the copper vent liner. The threads are 3/8-16. I drilled a #36 hole for the fuse and tapered the end at 31º so it will match the hole I drill in the barrel. Vent liners are important for this kind of construction so there is an unbroken path from the powder to the outside. Without it, fouling could get blasted between the hoop and seamless tubing resulting in corrosion over time.

9" Dahlgrens had two .2" diameter vents, but only the one on the right was drilled through. The left was "initiated sufficiently to give it direction." After 500 shots the vent enlarged to around .4" and it would then be filled with molten zinc or lead. Then the left hand vent was finished and the gun would be good for another 500 rounds. Tinkering around in CAD I found I needed to move the vent .568" off the centerline, and prop up the trunnion .314" to get the right angle. I drilled .988" down so my copper vent liner ended up halfway in the seamless tubing. The angle on the vent liner matches the drill bit, so it should seal ok.

I taped some parting tools together to get me close to .314". Then I used my super precise Sotch brand depth gauge on the drill bit. A wooden wedge helped keep everything level. I bet real machinists are cringing...

Then I tapped the vent liner with a 3/8-16 tap and screwed in the liner. Then I cut it off with a hacksaw and filed the stub down even with the barrel.

Next I started on the hole for the elevation screw with this dodgy setup. Originally the threads were square threads, but I don't have an appropriate tap. So I drilled a 17/32" hole and tapped it 5/8-11. I'll make a threaded insert on the lathe with square threads in the ID and put 5/8-11 threads on the outside. Then it'll get welded in place.

I made the elevation screw out of a 3/8" piece of 642 Bronze. I've never machined bronze before so I was anxious to give it a try. It was pretty easy to machine and I didn't have any problems. The threads are 12 TPI square threads. The width and depth of the groove is somewhere around .042".

To bore the square threads on the ID of my elevation screw insert, I made a boring bar that holds a little stub of a #55 drill bit. I ground the bit down halfway and put a little relief on the front of it. It cut like a champ, although I was only cutting 12L14.

Here you can see the setup. I like threading ID's by running the lathe in reverse, so that's why I made a left hand boring bar. The insert is in the chuck and the bronze elevation screw threads in easily with no backlash.

With the barrel basically done, I stared on the carriage. During the 1850's and early 60's, the 9" Dahlgren would have been mounted on a wooden Marsilly carriage. I'm not so good at woodworking, so I went with a later era Iron Marsilly carriage. After the Civil War, the military had no money to develop new cannons. So they kept these obsolete Dahlgrens around on improved iron carriages.

I simplified the drawing a bit and slipped them into a purchase order at work of laser cut 11 gauge stainless. Then I welded them together and made the 1.111" diameter by .666" wide brass wheels.

The cap squares are what hold the cannon onto the carriage. Originally they were hinged so you could remove the cannon, but this model is a bit too small for that level of detail. I made some that have a slip fit on the trunnions out of 954 aluminum bronze.

Here they are installed on the carriage. I drilled and tapped holes for a 6-32 screw to hold them on.

On the originals, bronze skids at the back prevented the carriage from chewing up the ship's deck when the gun recoiled. I don't have a milling machine so I mounted a bock of brass in a tool holder and put a router bit in the chuck. I just used the cross slide and compound for this operation.

My completed skids. I ripped the block of brass in a table saw, then cut the two skids off with a chop saw.

Then I soldered them onto the carriage. I've also cut the notch in the back of the cascabel that holds a rope to prevent the cannon from recoiling too far. Also there's a pin that keeps the rope (breeching) from falling out of the notch.

Then it was time for paint!


I also made a breeching for it out of 5/16" rope. I don't know why, but the Ordinance Manuals specify one side should be a spliced eye, and the other side should be bent over and secured with three sets of round seizing.

I made a removable scope mount out of some scrap aluminum. I aimed the cannon at a spot 25 yards away and the scope mount seemed to index perfectly each time I took it off and put it back on.

I made up 10 charges at 100 grains Fg apiece. The balls are 1" diameter steel ball bearings and the sabots are oak.

Down at the range ready to fire!

Here's a frame from a video I took showing the muzzle blast.

Here's a video of it in action!

Here's a video shooting a dead mini fridge.

Here's a few clips from different angles at real time and 5% speed.

Here's a video of me shooting it from my canoe and also from the dock at a floating target.

Shooting the cannon at a dead microwave.