Gunsmithing Custom 50 BMG

[gsuty17]

Hello there, this is my first real post so please bare with me. I am working on designing and building a bolt action, magazine fed 50 BMG. I have run into a few, not necessarily <span style="font-style: italic"> </span> problems <span style="font-style: italic"> </span> , but things that have held me up. I was hoping one of you could help me, specifically somebody here with any knowledge of 50's.

First, I am not sure what to design my firing spring to. I don't have any primers to test and CCI doesn't have any information as to what impact energy they design to for their primers. They basically said they duplicate the mil-spec primers. Does anybody know what it takes to reliably ignite a CCI #35 primer? If not, can somebody give me some accurate dimensions of your bmg firing spring pin? I can reverse engineer things if I have to.

Second, what firing pin diameter is common? I know the McMillon 50 uses a .093" pin, is this common? Seems large, but I don't have any prior 50 experience.

Last, does anybody have a 5 round barrett magazine that i can get accurate measurements of? I am designing my bottom metal around this magazine, but don't have one. Nor do I want to drop the $130 just yet if I don't have to.

Thanks for any and all help!

 

[The Mechanic]

Re: Custom 50 BMG

No one jumped into the deep end on this yet. So I will give it a go.
How many rifles have you built from scratch so far? I have to ask as you are venturing into the hand grenade an inch from your face zone here. Your drawing skills look good but this is a very dangerous undertaking. You know that right?

 

[gsuty17]

Re: Custom 50 BMG

Thanks for being brave and speaking up Mechanic. I understand what I'm getting into and was hesitant to go listing my qualifications initially at the risk of being arrogant or a "no it all"....because I don't. BUT I am a mechanical engineer for a big name rifle company. I have not physically built any rifles from scratch, but I have done everything but. I have done a complete fea analysis of the bolt and action, in addition to all the hand calcs. I am 100% confident in the safety of this rifle, although I do wonder how one goes about finding proof loads for a 50???

I am a little surprised my screenshot didn't get ripped apart yet.

So... anybody have any measurements? Thoughts? Comments? Any help or feedback would be appreciated.

 

[jwSubMOA]

Re: Custom 50 BMG

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BigG</div><div class="ubbcode-body">Thanks for being brave and speaking up Mechanic. I understand what I'm getting into and was hesitant to go listing my qualifications initially at the risk of being arrogant or a "no it all"....because I don't. BUT I am a mechanical engineer for a big name rifle company. I have not physically built any rifles from scratch, but I have done everything but. I have done a complete fea analysis of the bolt and action, in addition to all the hand calcs. I am 100% confident in the safety of this rifle, although I do wonder how one goes about finding proof loads for a 50???

I am a little surprised my screenshot didn't get ripped apart yet.

So... anybody have any measurements? Thoughts? Comments? Any help or feedback would be appreciated. </div></div>



Howdy!I designed and built a 50bmg rifle from scratch a couple of years ago,look under the photo gallery under "My self designed/self built 50bmg".I would be glad to help you if I can.My firing pin tip is .125" and I have had no problems.I used a chrome silicon die spring on mine.I will have to look up the spring rate for you but the dimensions are 2" long by 5/8" dia by 11/32 ID.It's pretty tough and using the die spring allows me easy access to spares and different rates for testing.The reason you are having trouble getting data on the 50bmg is there are no SAMMI specs for it.Mine is very over built as I do not wish to die.Your qualifications inspire confidence as to your ability to design your action safely but I must ask,are you a machinist?If you are I hope you are a good one.If you are not are you planning on having a machine shop build it?I highly advise you seek out a machinist/shop that is VERY familiar with firearms custom work.I used SAE 4142 for my reciever and bolt and had them fluidized bed heat treated after machining to avoid distortion.I youwant to ask any questions just PM me,welcome to the Hide.

 

[gsuty17]

Re: Custom 50 BMG

jwSubMOA, thanks for the info. Until I make a living as a machinist I certainly won't be calling myself one. I can run lathes/mills very well. I am also borderline OCD, so if I make a mistake it's just going to cost me time and another, albeit huge, chunk of 4140. I plan on sending it out to S&W to have them austemper it. I couldn't find your 50 build though, what page of the gallery is it? I probably just missed it, I am hardly an efficient forum searcher.

 

[jwSubMOA]

Re: Custom 50 BMG

Here is that post on mine http://www.snipershide.com/forum/ubbthreads.php?ubb=showgallery&Number=1843484#comments

 

[LRI]

Re: Custom 50 BMG

Mech,

First, nice puter model. Second, I'm not an ME nor have I ever played one on TV. ( )

Since your working in the trade I'm making a few assumptions:

1. Your aware of the required amount of surface area on the lug faces to tolerate the cartridge.

2. You've paid attention to the primary extraction cam surface for yanking the cartridge out of the chamber.

3. Gas checks are oriented in such a way that if the gun "sneezes" a case your not going to eat the powder charge in the face.

4. If this is a twin lug, 90* turn bolt action I (pray?) hope your not using anything resembling a Sako extractor. (very dangerous-I'll explain why later if you like)


Now, regarding your questions:

I've never built a 50. I'll state it plainly up front. I have however worked on other "boomer class" magnum cartridges in prototype actions of my own design. (The Dakota Scmitar in 338 LM and the M-400 prototype AR-15 "ish" looking semi auto sniper rifle chambered in 408 CheyTach)

If it were me I'd start with a high chromium/silicon wire for my spring. It'd be around .065" for wire diameter probably. in Hatcher's notebook he talks about 45lbs as the magic number for "all fire" on primers. I'm not sure if this applies to 50's as well since the cups are drawn from a slightly heavier gauge material. I think it would be a good place to start though.

As far as your striker OD I don't think a guy has to go bezerk on the size. For all purposes the rifle doesn't know nor does it care what the cartridge is in terms of ignition. The only reason I can think that would play into this is the mass needed to sufficiently get the fires lit during ignition. Obviously a larger OD striker is going to carry more mass than a skinny one. Powered by an appropriate spring over the length of fall typical for a modern bolt action (around .200" to .235") it should get things moving as it'll develop more impact energy (assuming the spring is up to the task.)

A cardinal rule in action making seems to be pretty common to all receivers. .055" is the magic number for striker protrusion. I (dare I say) want to disagree with this a little. This figure goes back to WW1 and the 03 springfield. As we can all imagine those rifles were manufactured in volumes hard to appreciate. Critical dimensions like headspace were a sliding scale at best. .055" of striker protrusion ensured the gun went bang. Since the cartridges were a "fire and forget" there wasn't the risk of ruptures during reloading, etc.

Dry fire on a fired case once and it'll quickly be discovered that the primer turns into a very different shape after a few strikes. That being said go one further and measure the inclusion from a single strike. It's only around .025". So I have to ask why have the extra .03" sticking out the bolt face? Why not cut it back a bit to around .035"?

If the striker were to actually bottom out as it does dry firing would this improve the accuracy of the gun?

It might. . .

Last and then I'll shut up.

When John Browning designed the 50 he basically took a 30-06 and blew it up on an overhead projector. Compare the two and they are virtually big/little twins. It's a powerful cartridge no doubt, but it's also quite inefficient, expensive to load, abusive on the shooter, and can be tempermental with accuracy. It's also pretty cool.

Hope this helped a bit and good luck with your project. If you'd like to chat a bit, just visit my site and gimme a call.

All the best,

Chad

 

[gsuty17]

Re: Custom 50 BMG

Wow, thanks for the help gentlemen. First of all jwSubMOA your rifle is sweet, I really like the design, without a doubt unique. Also Chad, your rifles look first class and your website is sweet.

As for my design, through friends in the industry, I got somebody at CCI to fess up to needing at least 100 inch-ounces to reliably ignite a primer. I should have no problem completing my firing pin spring design as well as the related components. I also recieved great information from Mark Serbu and a few others online who i don't know by name. Once again, a testament to the quality character of those in the shooting industries.

My design is anything but creative, I went with what i know and that is 90 degree turn bolt action. I have designed my receiver for a SF of 2.99. My extraction cam surface is a 1.75" pitch helical surface camming out .150". I have the bolt designed so that if, for whatever reason, I do "sneeze" a case, the pressure vents through the magazine while the bolt remains closed.

I don't know how to include attachments, if I even can? But I was going to include my stress calculations, not being able to do so I'll just tell you how I went about creating my safety factor. Basically I designed to a chamber pressure of 65,000 psi action on a bolt face diameter of .680". This creates a bolt thrust of 23,600 lbs. Using that value I designed my locking lugs to withstand that amount of shear and then some. After backing out of what my lugs ended up at my SF was 2.99 with a maximum bolt stress of 54,957 psi and a maximum bolt flex of .002".

As for firing pin protrusion I am certainly open to your advice. I'll have to do some experimentation before I settle on a solid value. Interesting history though about John Browning.

All anybody ever "needs" is food, water, and shelter. Everything else is just gravy. A bolt fed 50 falls into the latter of the two catagories. Are there many superior rounds, yes. But ultimately, I'm building one because I don't have one and i want a huge freakin' rifle! Kinda falls into the realm of "ehh, why not?" for me.

Thanks again guys! I'll keep you posted with any new progress once I finish polishing this turd.