Thursday, January 13, 2011

Update on the .50 BMG

Well, a couple weeks ago, I was working on the .50 BMG and I was actually ready to assemble the rifle and do the final test fitting on everything before sending the bolt, chamber, and a couple other parts out for hardening. I had put the receiver in my milling machine and was making some cuts for clearance and a few adjustments, and after a couple hours of machining, I realized that I had made a HUGE mistake. Originally, Holmes outlines in his manual that when you cut the slot for the bolt handle, you cut it all the way to the end of the receiver and actually cut through the end of the receiver. I did not want to do this, as it looked sloppy as hell and the receiver would actually separate a little bit, keeping the bolt from having a close fit inside the receiver. Well, when I was milling away on the receiver, I managed to cut clean through to the end of the receiver and did not realize it until I was done.


So, scratch one receiver.
A quick jaunt over to Aircraft Spruce&Specialty's website procured me some more 4130 round tubing to build anew receiver out of. It was at this point I realized that I really didn't like a few things about the old receiver I had built. To be a bit picky, I did not like how out of proportion the whole thing was. The receiver was fairly long while there was only a small amount of barrel protruding from the receiver. I also found I was not fond of the holes I had drilled in the end, between them being a tad off kilter and also a bit overdone, drawing a bit too much attention to it. The old receiver started out as a 36" long piece that I cut down to a little over 28". This left about 12" of barrel protruding. I decided to simply buy a 24" piece of 4130 tubing instead, and make the rifle the same overall length as it had been before. To do this, I had to machine the barrel about 5.5" further down than the threads already were and cut new threads for the barrel nut to thread on to.

I did this on my old lathe before I got rid of it, obviously.

Shortly after I got done with this, the new receiver tube came in. I went into my CAD system and drew up the modified receiver and printed it out. I changed up a few things, including drawing the slot for the bolt handle so it could be milled out beforehand. I did not want to screw up another tube, so I made sure this was outlined first and foremost. I also wanted to cut a set of 1.5" diameter arcs at either end of the cartridge slot. These pics are of the new tube and the old design I had printed out. The new design was close to the same thing but with the revisions to the cartridge slot.

I got a can of 3m spray glue and adhered the printout to the receiver tube, then mounted the whole thing in the mill and cut the bolt slot correctly this time. A 1.5" hole saw gave me the correct arc for the cartridge slots, then I finished the slot with a standard end mill.
At this point, I had an idea for a few changes to the rifle design, such as the bolt itself. The bolt I machined was really, really heavy and a bit ungainly, and I really wasn't happy with the overall look and design. I saw another Homes rifle variation that used 4340 sections for the rear of the bolt, the front of the bolt, and the bolt lugs, while the center section was made of 2024 T3 aluminum. The whole thing was threaded together. This kept the overall weight of the bolt down by a couple of pounds or so. Since my bolt was machined from 4140, I figured it sounded like a good idea to me as the rifle bolt was extremely heavy. I had a chunk of 4140 2.25" OD round left over, so I turned it down to a 2" minor OD and then bored it out to 1.40" inside to a depth of 1" and threaded it to 12 teeth per inch.

I then ordered a 12" section of 2024 T3 aluminum round bar and a 2" round section of 4130 in order to make a new tapered barrel sleeve for the new receiver.
I then parted off a 6" section of the aluminum round bar and bored it to .5", then cut threads on the aluminum bar stock to thread into the 4140 section.

I then center bored the whole 4140 assembly for a firing pin and when I went to cut the lugs out of the opposite end of the 4140, I wound up ruining the threads on the inside of the new bolt head.
Of course, I only got a bit pissed off until I came up with a better idea, and set about making it happen.
I had an apparently good section of 4140 still there that was long enough to use, so I turned down the outside of it to 1" and threaded it to 12 tpi, center bored it for a firing pin, and cut new lugs on the mill. This worked out pretty well this time. Now, instead of having the aluminum center shaft thread into the bolt head, the 1.5" aluminum shaft is bored out and threaded to 1" 12tpi so it can thread on to the threaded area protruding from the 4140. The outside of the aluminum shaft was still threaded from before, so I parted off this area, threaded it onto the bolt head, and then I chucked it up in the lathe and turned 12tpi threads across the joint between the two pieces. This worked out very well, as the two pieces are completely seamless where they are joined together and threaded inside and outside.
After this was done, I cut similar threads on the rear of the aluminum round rod on the outside only, then I turned the main shaft down to 1.375" between the two threaded sections. I took two pieces of DOM tubing I had, 2" OD .375" thick, bored them to 1.4" ID, and threaded them to 12 tpi. This allowed me to thread them on to the bolt assembly to use as bushings in between the bolt and the receiver. I did turn about .004" off of their overall outer diameter in order to get the bolt to reciprocate smoothly inside the receiver.
A quick pic of the new bolt inside the new receiver:

I know this sounds odd and may be hard to picture, but the camera on my iphone crapped out on me the other day, and I don't really have any other options for a digital camera at this point. As soon as I do I will post some pics of the new bolt assembly.

The hour grows late here, so I will have to finish this update at a later time.
I hope these updates are as entertaining for those who read my blog as they are for me when I actually do the building.


  1. I'm loving this build and I really appreciate you posting about it. I'll probably never do a rifle build but all the techniques and tools you talk about will transfer nicely to other things.

    Thanks and keep it up!

  2. FWIW I posted about you today. Awesome build, simply awesome.

  3. Thanks for the linky love, Six.
    I'm certainly glad that someone is enjoying it enough to comment.
    The build goes on. Should have another update this evening.