Shot My Homemade .50 BMG for the 1St Time Today!!
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.From: Gunfreak ([email protected]) Subject: Shot my homemade .50 BMG for the 1st time today!! BOOOOOMM!!! Newsgroups: rec.guns Date: 1996/07/30 Today I fired my homemade .50 BMG single shot rifle for the first time. WOW! I've never been around a gun this big before and what a noise it makes!! The gun isn't completely finished, but it was done enough to fire so I couldn't wait any longer. Here's how the firing went: I put the gun on the ground and laid a spare car wheel (with tire) on top of it. I put a small log under the barrel, aiming it a large tree (aimed the barrel, not the log. By the way, sorry you tree-huggers out there-the patient died!). After putting on ears and eyes, I turned away from the monster and reached behind me to hit the temporary trigger with my hand. BOOOOOOOMMMMMM!!!! WHAT A HUGE NOISE!!! A lot of sawdust was left at the base of the tree. (Didn't check the other side, it was in water.) The blast echoed through the Florida swamp and, poetically, the sound of police sirens could be heard in the distance just afterward. (A coincidence). The gun survived fine, and so did I. In case you're wondering why I took all the precautions on firing this gun for the first time, I'll tell you; I designed it myself. While I am a Mechanical Engineer, I don't have any "formal training" in gun design (which, it seems, can only come from working for a BIG gun company because there's NO real data out there for the little guy!) Anyway, this cartridge produces a lot of pressure, and puts that pressure over a large area (the cartridge base). That equals a hell of a lot of force on the bolt and receiver. I guess I calculated correctly, because nothing blew up. I even got the headspace right. My gun is basically like a Maddi-Griffith (from what I've seen in the magazines) and is a shell holder type action as well. It uses an AN-M2 barrel (I think) which is the lighter, aircraft type. The receiver is machined from 8620 steel, case hardened to a depth of .015 and heat treated to RC 53-58. The bolt is made of 4340, heat treated to RC 46. The firing pin is two-piece, part Ti 6AL-4V, and part 4140. Various action parts are heat treated 01 tool steel. I made everything but the barrel, and the total cost including heat treating (but not including time, of course) was about $275.00, $175 of that for the barrel. This gun will win no beauty contests but MAN what a blast! Can't wait to shoot it without the car wheel!! Mark Serbu From: [email protected] To: [email protected] Date: Mon, 21 Jun 1999 00:58:03 EDT Subject: Blowing up 50 BMG's In a message dated 6/21/99 12:04:03 AM Eastern Daylight Time, [email protected] writes: > [email protected] wrote: > > Hmmmm...awfully small diameter on the bolt in that design. If anyone gets > > the plans for these I'd be happy to do a stress analysis on the critical > > parts. As someone who's built a few .50s from scratch (you have seen my > > web page, right? www.serbu.com) and had one blow up, I have a great respect for this cartridge. > Mark,> 1. What happened on the one that blew up? It was my own design (please see the web page: the BFG-50) , which, unfortunately, was a cock-on-closing shell holder. All in this learned group should realize that this is a recipe for disaster, which I knew as well. To counter the danger I included a passive firing pin block in the bolt. The firing pin couldn't reach the primer unless the bolt was in battery, fully locked, and the trigger pulled. Unfortunately I didn't include the Murphy factor enough in my design. The firing pin block got jammed up with sand and it's return spring (which wasn't very powerful because it affected trigger pull) wasn't strong enough to block the firing pin when it (the firing pin) broke. The firing pin was hollow and had the spring in the center of it. There was a disc on it which engaged the sear. When the shooter shoved a round into battery, the firing pin broke just forward of the disc, and ran right over the block, igniting the cartridge out of battery. Big bang. He was in the hospital for several months and could have died from blood loss if he hadn't had his wits about him. He retained all his body parts but has much diminished use of his right hand. Very bad scene. Worst day of my life. Ironically, if I had just made the firing pin block safety work off of camming the bolt closed, instead of trying to be extra safe and not have it disengage until the trigger was pulled the accident might not have happened. Why? With the block camming off closing the action, the spring which "powers" the firing pin block can be made MUCH stronger since you're not using trigger force to return it. Also, the block can move (interfere) much more. With the trigger actuated block, it only moved about .050". Now (on my improved personal gun) the block moves .125". I've also improved the firing pin by making it solid, and having the spring act on the disc. No way for a runaway firing pin now, and even if Mr. Murphy intervened and it did, the firing pin block is bigger, had a more powerful spring, and intrudes more. > 2. Which designs have you built? Only my own. I'm working on a semi-auto now. Gas-operated, rotating bolt. Much more conventional design. Very neat gun. You can see a CAD image of it on the web page. > 3. Any preferences for design, among > the ones you've built / are familiar > with? Aside from building one from scratch, if I were just trying to build one as quickly and as cheaply as possible, I'd go with the Maadi-Griffin design. They say you can build it for $450. Or contact State Arms...they sell complete guns for around $1200. I think they sell the action for much cheaper, and you can possibly get one which will accept the M2 machine gun barrels, which are available from Oyster Bay Enterprises for around $140 each, brand new. Someone else sells a fairly conventional Mauser-type action plan set...think it's called the Big Fifty from Gun Metal Designs, if I remember correctly. See the FCSA web page. Lots of stuff there. Mark Serbu www.serbu.com From: [email protected] To: [email protected] Cc: [email protected] Date: Tue, 22 Jun 1999 01:47:24 EDT Subject: The cheapest .50 BMG rifle? m I couldn't help myself. I went and designed a cheap, single-shot gun. This thing is cool! Simple construction (well, for a machinist it's simple). The barrel screws into a small barrel extension which is welded to a tube, forming a receiver. The tube and extension are to be heat treated together. Another tube screws onto (or may be welded) to the front of the extension and encircles the barrel. The bolt is made from two pieces. The front part (with the lugs) is machined and then welded to a tube. Okay, a third piece, the bolt handle. The whole bolt assembly is to be heat treated together. It's a standard 2-lug bolt with a plunger-type ejector. I don't know what to call the extractor type, but my Savage 110 uses it. The lower receiver is the generic one I designed and had built for another gun. It uses all AR-15 parts. If you're a cheap ass, this is the .50 BMG rifle for you! Check out the CAD image of the gun at: http://www.serbu.com/cheap50.jpg and see a picture of the lower at: http://www.serbu.com/genlow2s.jpg I'm trying to interest a guy with a gun company in building this thing. It should be able to sell for under $1000 easily. If that fails, I'll think about offering plans. Mark Serbu From: [email protected] To: [email protected] Date: Tue, 22 Jun 1999 17:51:59 EDT Subject: Re: The cheapest .50 BMG rifle? In a message dated 6/22/99 2:43:05 PM Eastern Daylight Time, [email protected] writes: > Mark, you may find this hard to believe, but I'd designed the same rifle > last week, and was mulling details in my head when I read your post. Copycat! :-) > I'd planned to use 4140 HW tube for the rec. & bbl ext. What's HW? Heavy Wall? I'm making the receiver (basically just a barrel extension) out of 4140 bar stock, and welding it to 4130 DOM tubing. Much easier to make those in two pieces and weld 'em together...very hard to try to bore in that far! > No need to weld the rec tube to the ext, there's no firing stress on that joint. Doing it for manufacturability, not stress. >Going to use > the bolt handle to pin the bolt head to the bolt body sort of ala Sav 110. > The FP locks it in.