50 cal plans

50 BMG Rifle Project Since I saw the first 50 BMG rifle hit the market I wanted to build one of my own. A 50 BMG rifle is the ultimate in firepower for a hand carried rifle. At least that’s my opinion, others may have their favorites but this is mine. I looked at some of the plans available but did not want to pay their asking price. I would rather spend that much money on parts for the actual gun rather than pictures of it. I looked at all the different designs that were being built, and drew up my own plans with ideas from many different guns and scaled up measurements from my .270 hunting rifle. I started building a model of my plans to see how it would all fit together, and then came across the Bill Holmes book .50 Caliber Rifle Construction Manual. Paladin Press sells this book for $25, that I didn’t mind spending. After reading his book I decided to put my design on a back burner and give a proven design a try. After sourcing parts, and having trouble finding some parts, I started out with an old shot out M2 machine gun barrel and a section of drill pipe made of 4140 for the receiver tube. I turned the drill pipe down to a little larger than the specs in the book and then got started.

Drill pipe after being turned down to size, and old M2 machine gun barrel before I cut it.

First I cut the barrel down to 30 ½ inches

The chamber area of the barrel was shot out pretty bad, but the rifling was good at the point where it was cut, it’s kind of hard to see in the photo.

Next I mounted the cut section in the lath to face off the fresh cut and turn the diameter down some.

After facing off the cut end, I mounted the barrel between centers and formed the section for barrel extension threads, and the taper that will hold the barrel in the receiver tube.

Next the barrel was turned to size with a step down for the barrel nut threads, and a raised section on the muzzle end for muzzle brake threads.

I mounted the receiver tube on the mill and used a Miracle Point for indexing.

All the major milling is done, I will need some of the other parts completed to determine placement for the milling that is left.

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Here is the barrel nut before I threaded it to match the barrel.

This is a picture of the receiver tube with most of the machining done, the barrel with the barrel nut, tapered mounting sleeve, and the barrel extension screwed on

the bottom, and on the left is the 2 ½ inch round by 2 foot piece of round stock 4140 for the bolt that cost me $45. Good, but expensive metal. 4140 is very nice metal to machine, but keeps me on edge knowing the price to replace it if I screw it up.

Next thing to make is the bolt. I started with a piece of 4140 2 ½“ x 9” inches long. I faced it off and center drilled it, then turned down the outside just enough to true it up, and remove the rough surface. Then I started drilling holes. I stated with a ¼“ bit as deep as it would go, removing, cleaning, and relubricating often. I worked my way up to a 12” long ½“ bit drilling to the depth indicated on the blueprint. Then a 12” long ¼“ bit after that. I next took a 3/32 bit on another lath, and used it to center drill a piece of ¼ inch round stock. Then I turned the round stock down a little under ¼“ so it would fit in the ¼“ hole without dragging. I took a new 3/32” bit and put some flux on the end of it and brazed it into the piece of ¼“ round stock. I used this setup to drill the final size hole of 3/32” out through the oposite end of the piece of 4140. After the hole was drilled completely though, I came back and enlarged the beginning of the hole to 5/8” then steped larged coming back out to 1” then bored to 1 ½”.

Here is a shot down the hole which took a while to drill.

After turning the out side to specs, forming the bolt head, and cutting a recess in the head

for the cartridge, I mounted the bolt in a dividing head on the mill and cut the lugs out.

Using a small endmill I cleaned up in between the bolt lugs by indexing off of the shaft behind the lugs, and making a pass. Then I would turn the bolt in the vise just a slight bit and make another pass. This actually cleaned it up so well that nothing else had to be done to the bolt lug area.

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The next step was to cut slots in the barrel extension for the bolt lugs to engage. This was done on the mill with a dividing head for indexing. I started with a ½ inch end mill to hog out the majority of the metal, then did trimming with a 5/32 endmill because it was the smallest I had.

After this operation came the hand fitting. This took a lot of time, filing, and cursing to get them to fit, but I won in the end.

This is the bolt and barrel extension mated together. My measurements of the bolt head, and barrel extension were right on; it fit snug, but not too tight.

I made the firing pin in a collet lath, which was very simple, and then I turned a piece of 4140 to the diameter and thickness of the cocking piece. I drilled and threaded the center, made a mandrel of just mild steel to mount it on, and mounted the whole setup in the dividing head to cut the legs on it.

Here is the firing pin, bolt, firing pin retainer, and the semi finished cocking piece.

This is a picture of the 12 inch bits that I used to drill the bolt. Also the extended 3/32 bit that I made with ¼ inch round stock.

Here is a picture of the Receiver tube with the barrel, barrel extension, barrel sleeve, and barrel nut all mounted together so I could determine the exact location to drill holes and weld the barrel sleeve in place. One other thing I learned from doing this is that this will be one extremely heavy gun. It will defiantly need the bipod for shooting.

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This is the start of my trigger mechanism. I cut a slice off of a large piece of Timken bearing material, (should be hard enough once heat treated) milled it flat, and then finished it to the right thickness on the surface grinder. I made a copy of the blueprint and wrapped it around the piece of metal, then used a center punch to mark each corner through the paper. Then I removed the paper and connected the dots. It’s kind of hard to see, but they are there.

Here is the trigger and sear after I cut them out on a band saw. Then I cleaned them up and brought the metal down to the lines with a file. Now it’s time to start fitting.

Here is the trigger, sear and trigger housing, assembled and unassembled after lots of fitting and changing the angle of the trigger and sear engagement points. The engagement point as shown in the blueprint would just slip apart when pressure was applied to the sear. Not what I wanted it to do when it was supposed to be holding the cocking piece and firing pin.

Since I had recently determined the exact measurement for the barrel sleeve, I drilled two rows of 6, ¼ inch holes evenly spaced around the receiver tube where the sleeve is supposed to be welded in place. Next with the sleeve in place I used a tig welder to fill the holes up, and weld the sleeve in place.

After grinding the welds even and polishing everything smooth, there were low spots left from welding. I built up more weld on the tube to fill these in and ended up overheating and warping the tube. With the barrel mounted in the tube the bolt would not engage the barrel extension with the bend in the tube. After measuring the run out while chucked in a lath, with the help of one of my mentors, we heated the high point and quenched it with water until we got the warpage down to a minimum, that allowed the bolt to engage the barrel extension once all assembled. Once leveled and polished a few low spots were still left, but after almost ruining the receiver tube once I decided that these low spots were not going to hurt anything.

Finally found a piece of tube the right size for the butt stock, so I could finish cutting the bolt slot in the receiver. Bill Holmes recommends fitting the tube for the butt stock before cutting all the way through the end of the tube so that it doesn’t spring open. I fit the piece of tube to the receiver and made some rivets to hold it in place, and then I finished the cut on the bolt slot on the receiver so I could finish the bolt.

Next I turned a piece of drill rod to the diameter needed for the bolt handle and cut it to a length I wanted. I am making the bolt handle different than the Holmes blueprint mainly because I think this type of handle will look better and be easier to make. I threaded it in to the bolt for now, so I can take it off to work on the bolt. After the bolt is complete, I will weld the handle in place and heat and bend to the proper shape, then thread a ball on the end. Now that I have the handle in place I can determine where the cocking slot and other work needs to be.

Once I marked the position, I cut the cocking slot with a 45-degree angle and cut a matching angle on the cocking piece. Once the cocking piece was fitted to the bolt, I drilled the retaining pin hole through the bolt and retaining sleeve. This wasn’t much fun! Measuring the position was easy with the DRO on the mill, and then I started the hole with a 1/4 inch end mill because it was on an angle. This worked great until I got the retaining sleeve, the bit kept pushing the sleeve out. I had to clamp a large C-clamp to hold the sleeve in place very tightly, and drill slowly. I stopped drilling as soon as it broke through the bottom, so that the pin would not go all the way through, but I still had a way of pushing it out. All I need now is a spring, and to cut the slot in the bottom for the sear to pass and the bolt will be finished.

Here is a picture that shows the cocking piece engaging the bolt in the bolt slot, and a picture of everything completed so far assembled. Should be a good looking rifle, and defiantly will be a heavy one. The Holmes design doesn’t have a carry handle, but I think I will add one of my own design. It defiantly will need one.