Monday, January 17, 2011

The .50 receiver, bolt, and some other miscellania.

Well, I finally got around to taking a few pictures of some new parts I had fabricated for the .50 BMG rifle I am building.

As I previously mentioned, I wasn't happy with a handful of the parts I had made, either for reasons of aesthetics, or due to them being really heavy. Heavy is good, up to a point. Heavy is really good for rifles like this, as the increase in mass translates into a rifle that is easier to shoot due to reduced felt recoil. After a certain point, though, it becomes too hard to transport the rifle. Such was the case of the rifle as it stood near completion a month or two ago. Hence my decision to change up a few things now, while I am still working on the rifle and the major components have not gone off for heat treatment and hardening. Some of the parts that were rebuilt will not see heat treatment, but will be blued or duracoated. Some of the parts outlined in this entry have been coated in PermaBlue cold blue to give me an idea as to their final appearance and to outline any areas where grinding and sanding have been done that still have etching marks standing out. Similar to the black spray paint system I used on the trigger assembly, I apply a coating and then look carefully for any traces of sanding marks, welding pits, or any thing else that might keep the surface from appearing 100%.


As mentioned before, I made a new bolt head from 4140 steel, 1.5" diameter, 3" long overall. I turned down the first .75" to a 1" outer diameter, then cut the lugs on the opposite end on the milling machine before turning down the .75" long 1" diameter shaft directly behind the bolt lugs. After everything was milled and turned down, I cut threads on the rear part that was turned down to 1". Then I took the 2024 T3 aluminum round stock, 1.5" diameter and center bored it to .5". I then bored one end out to .90", .75" deep and cut threads inside it, 12 teeth per inch, to match the 1" threaded part on the bolt head. Once this was done, I threaded the assembly together until the shoulders locked together tightly. With the bolt together in one piece, I chucked the whole thing up in the lathe and took a very light facing cut to true the outer edges of the 4140 and the aluminum stock, to a total outer diameter of 1.475". This made sure that they were as one piece. Next up, I cut the threads 12 teeth per inch for a length of 1.25" from the edge back. I then cut identical threads on the rear of the bolt shaft. The area between the two threaded sections on the aluminum bolt body was turned down to 1.370" With the bolt threaded and ready, I took two pieces of 2" OD DOM tubing, .3125" wall thickness, with an inner diameter of 1.375" and an overall length of 2" each. I threaded both pieces 12 teeth per inch to a thread depth of .05 and length of thread depth of 1.25". One piece was then cut to 1.3" long, the other left at 2" length. I took a .002" facing cut to ensure they would still fit snugly inside the receiver, but would turn smoothly with no binding or scraping. After facing, the DOM pieces were sanded and smoothed, then a bit of cold blue was applied and some Remington gun oil.
 With everything in line, the bolt sleeves were threaded onto the bolt and the bolt was set aside.



Anyway, here's the bolt assembly.




I still need to drill and tap some retaining screws that will serve to keep the sleeves perfectly in place on the bolt assembly.

Next up, I had some 1" 1018 cold roll square stock laying around I wanted to use. I liked the scope mount on the Serbu BFG 50, and though I think those are merely drilled and bent sheet metal, I figured using drilled solid stock would not hurt anything. I also wanted a decent bipod mount, and since I haven't found a bipod for sale on the market at a reasonable price that looked worth half a damn, I figured building my own would be the way to go. The bipods I have seen that look worthwhile have all been in the 200-300 dollar range, which is almost more than I have invested in this rifle so far. I have not yet build the bipod, but it will be very heavy duty and will mount to the front piece of bar stock, and when collapsed will fold up and bolt to the rear piece of bar stock.
There wasn't a whole lot to making these pieces. I did not have anything that would cut a 1.125" radius for the square stock to fit on the 2.25" tube, so I improvised. I did have a 1" two flute endmill that could be chucked up in the lathe, and the 1" bar stock fit in the tool holder on my new lathe, so I sat down with a calculator and tried to figure out roughly what angle I could turn the bar stock to in order to make the round end of the endmill cut a conic section on the bottom of the bar stock. Doing this correctly would yield a good 'nuff fit for these pieces. Once these pieces were cut, rough sanded flat on the top and sides, I drilled .5" holes on 1" centers. I then welded these three pieces in place on the receiver, then tightened the receiver assembly in the milling machine vise and milled the tops of the square stock (or bottoms, as the case may be) flat and perfectly parallel to the receiver. This was really only important on the top piece of stock, as this is where the scope will mount when I machine a weaver rail that will bolt to it. I milled the bottom ones flat because I already had the mill set and figured it wouldn't hurt. As it stands, I only had to mill about .05 off each piece to get it perfectly parallel.
Everything had been MIG welded together, so there was a lot of cleanup to do. (Plans are in the works to purchase a TIG welder, and using the oxy-acetylene torch would've generated too much heat. So, MIG had to do the job.), so I put a 100 grit sanding disk on my angle grinder and sat about sanding down all the welds. Then the arduous task of hand sanding the whole thing and removing the last bit of the grinding marks began. The smoother the welded areas transitioned into the receiver tube, the better the whole thing would look. Thankfully, it went fairly well........
Although my arms now look like Popeye's after all that sanding.





Yes, there is a touch of rust in the holes. I should've cleaned the holes and oiled them before taking pics. So sue me. I threw some cold blue on the receiver and had to wash it off with water, thus the light rust.
Somewhere along the way I also cut the old trigger housing I made off of the old receiver, ground it flat where it would weld to the receiver, and welded it to the new receiver.


Next up was the muzzle brake. I had some small sections of 4140 2" round left over, and a 1.25" long piece of 2024 T3 aluminum left over.  I figured I could come up with something workable from all these scraps, and so I did.
I found a 5" long piece of 4140 2" diameter, faced off both ends, turned it to 1.5" outer diameter, and then center bored it to .5625. I then bored out one end 1" deep to a 1" bore and cut .05" deep threads, 12 threads per inch. This would enable it to screw directly on to the threads I had cut on the end of the barrel oh so many months ago.
I then clamped the assembly in the mill vise and, using a .5" cobalt end mill, milled .5" holes all the way through the sides of the brakes. I then rotated the brake about 15 degrees and took another plunge cut all the way through. I then rotated the brake 30 degrees back the opposite direction and took yet another plunge cut all the way through to the other side. This gave me a decent set of ports on the brake that should be sufficient in capturing and redirecting enough of the pressure blast leaving the barrel when fired. I then chucked the brake in the lathe and with a piece of 100 grit sandpaper, smoothed out the brake and then after cleaning it I applied some cold blue. A couple of the pictures I will post over the next post or two will probably show the brake "in the white", as in it will be machined but not sanded.
The small section of aluminum I had was center bored to 1.1", threaded to 12 teeth per inch, and knurled. It makes for a decent lock nut to keep the brake from rotating.




It's hard to tell, but in the last picture, you can see a dimpled area on the barrel retaining nut on the end of the receiver. This dimple allows me to tighten the barrel nut using a special build spanner wrench.

That's about all I have to post about for today. I start classes again tomorrow and will be likely too busy to work on this rifle every day, but I should have at least two days a week to dedicate to working on it. However, not much remains. I have to build the striker assembly as the old one will not work with the new bolt, a bipod, and finalize a buttstock assembly of some sort.
I did a little creative manipulation of the old buttstock, and liked it a little bit more than I once did, but was still unhappy with the overall look. I will cover that later. However, here is what it looked like with the old buttstock.....



Yes, that bolt handle is actually, well, a bolt. I drilled and tapped the rear piece of DOM tubing to .375"x16 tpi (yes, I actually cut some threads that weren't 12 tpi. Of course, that's because the tap was already 16 tpi.) I had to have something to rotate the bolt with and check clearances with, so until I can machine a bolt handle, the bolt works. No way in hell is it permanent, though.

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