Thursday, May 19, 2011

Bill Holmes .50 BMG rifle update

After my last update on the projects I've been tinkering with, I decided to go ahead and make a new receiver for the .50 BMG rifle. I had the material sitting here, and it didn't take long to do the second receiver, so I assumed the third receiver would be just as fast.
I was wrong.
It went much faster.
I started out by gluing another template I printed out onto a 3' long section of tubing. After doing this, I got the receiver setup in the milling machine vise, then made sure the vise was square with the table.

Next, I cut the loading slot ends with a 1.25" hole saw after the center holes were located and drilled.

Now, with the holes cut for the loading slot, I center-punched and drilled the bolt handle slot on both ends and on the bottom where the handle rests when a round is chambered.

Now, the fun part, milling out the loading slot area between the two holes I cut with a hole saw. After this, I slotted out the bolt handle area between the holes I had previously drilled.

Now, I transplanted the inner tapered piece from the old receiver. This piece secures the barrel assembly inside the receiver tube. This consisted of milling out the welds on the old receiver, pressing the piece of steel out, then taking some measurements on the new receiver, welding it in place, then sanding and polishing the welds on the new receiver.

Next problem I had was with the old bolt head. I had tried my hand at machining an extractor, and although I got it working, I wasn't really happy with the way it looked. I had drilled the hole out for the extractor pin in the wrong place, and even though that only meant drilling a new hole to fix it, I still had an extra pin hole left over. I saw no reason to reuse the bolt head when I knew I could make a better one.
I had a length of 4140 prehard 1.5" diameter round stock sitting here, so I figured I would make a new one. Doing this allowed me to make a simpler extractor arrangement and allowed me to use a AR firing pin if I so chose. Another upside to this was that this material was already hardened to 28-35 Rockwell hardness. This is still too soft for use with a .50 BMG, but it's darn close. I'll still send it out to be hardened, just to be safe, but it machines VERY nice and doesn't get dinged or scratched up as easily as annealed 4140 like the old bolt did.
After digging around the internet, I saw that a few other people who had built similar rifles had made their bolts "capture" the cartridge by milling out a hole big enough for the body of the case, but slotted the inside of the head so that the cartridge slides in from the side of the bolt. This was pretty simple to do, and I saw no reason not to do the same on my rifle.
After cutting the main profile on the lathe, I put the bolt head in the mill and cut the bolt lugs before chucking it back in the lathe and boring the end .20" deep and .68" in diameter. I then took a high speed steel cutting bit and ground it so there was a tooth on the end about .1" protruding. This let me cut out a slot at the bottom of the bolt head to hold the case. Once that was done, I took the bolt over to the mill again and slotted one of the lugs and a small area around it so I could push a case in place.

The not-quite-sanded-or-polished bolt head with an AR pin protruding.

A .50 BMG case slides right in place with a satisfying "click*, and does not come out unless I want it to.

Here's the backside of the bolt head, drilled out for an AR firing pin.

AR firing pin in place, and with the pin all the way down, the pin protrudes into the primer .085", which is exactly how deep I wanted it.

Next, I needed a new bolt body. I liked the aluminum one I had made before, but with the new AR fire control group I made, I could open the bolt and pull it back, but I could not close it; it would catch on the AR hammer. A new aluminum bolt body would have been great, and lightweight, but I was afraid that the AR hammer would wear down the aluminum quickly after being cycled several times. After thinking about it, I just decided to use a piece of 2" OD cold roll steel, and bore it out for the new bolt head.
Here's the bolt body after some sanding...

And the freshly bored hole for the bolt head. Notice the holes in the side. Those are for retaining pins to hold the bolt head in place. (I actually plan to use allen head machine screws, but I'm not there yet. Right now pins are all I have on hand.)

Here's the bolt head pinned in place on the bolt body with a case inserted in the bolt head.

I really like the way the new bolt turned out so far. I've spent a good 75 bucks so far on materials on the other bolts and receivers just to find something I liked. I don't mind breaking a few eggs on this project, as long as the finished product looks right to me.
I still have to make a spring loaded firing pin extension for the bolt body, as the hammer will strike it at the back end of the bolt. This pin will transfer the energy of the hammer to the AR firing pin. With the hammer all the way to the rear of the bolt, I should be able to cycle the action back and forth without any binding of the bolt on the hammer.
I did manage to get the fire control group assembly mounted on the rifle, but forgot to take pictures of it. I have also come up with a simple buttstock solution that should look pretty good once finished, but that will probably not get started on until Monday. I still have to finish the firing pin assembly, drill and tap the bolt body for the bolt handle, make a new scope mount, and make the bipod. Thankfully, I have plans for all these things, and more importantly I have the materials sitting there to make them. I hope to have the necessary parts sent out for hardening by the end of the month, and have the rifle ready to test-fire within a week or so of that.

I should have finished this thing long ago, but I've wasted a lot of time and money re-doing things I wasn't happy with. I also took 16 hours of classes this last semester, so I stayed pretty busy with that. I have a couple of summer classes to take, but they should not take up too much of my time.

I will probably get some more work done on the rifle this weekend, but I figure most of the important stuff with have to wait for Monday.


  1. That bolt head is a work of art. Seriously. I really hope everyone reading your posts realize how difficult proiducing that kind of work is. Not long until the boom now.

  2. Six,

    As always, thank you for your kind words.
    There is a lot of work involved in building almost anything, but seeing the finished product makes it all worthwhile.

  3. I would like to see more pictures of the bolt assy. nice work so far