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Posted: 3/13/2010 2:37:27 PM EDT
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I have this itch to build a single shot (to start with) shotgun, but in a shade tree way. Think of it as a liberty shotgun (for those unfamiliar with liberty pistols google it...) that is built from parts that are easily found locally and about as precision as a harbor freight Mill vise (ie its not.) My interest has been building over time with the thought of just being able to take a trip to the local home improvement store, grab some steel, lumber, pin stock, springs and build something that will function as a weapon and will not blow up in my face. I want to build it for a few reasons other than to have something silly to work on. As of now its just an idea that I'd like to pursue as a project when I'm bored. I have machining experience, access to heat treating furnaces, Dill press, band saws, belt sanders, etc. No Mill or Lathe to speak of (there is an 18" lathe in the garage, but gears are missing, the main shaft that drives the chuck is bent, and who knows what else. That is a project just by itself.  )
I do need to find a book with basic schematic descriptions of trigger mechanisms, hammer design, etc. Anyone know of a good resource (the older the better, the more hand fitting the better) Thoughts? Help? 
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You'll like this site
http://www.weaponeer.net/forum/default.asp The home build area especially http://www.weaponeer.net/forum/forum_topics.asp?FID=232 |
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Thanks for the link oldgunner.
Somebody built a liberty pistol on there (seems to be building a .45 acp version as well..) I am wrapping my head more around what I need to do in order to have something that is SAFE and functional more than a few times. Seems like I am going to have a strong chamber that I can then add an overbore barrel for a 12ga. More to come once I get more of this drawn up (man I miss my copy of solidworks...). |
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Thanks for the link oldgunner. Somebody built a liberty pistol on there (seems to be building a .45 acp version as well..) I am wrapping my head more around what I need to do in order to have something that is SAFE and functional more than a few times. Seems like I am going to have a strong chamber that I can then add an overbore barrel for a 12ga. More to come once I get more of this drawn up (man I miss my copy of solidworks...). Do you mean liberator pistol? I dont know what type of plumbing you will need exactly to handle the pressure of a 12 gauge, but it wont take a lot. 12ga is a pretty low pressure cartidge relatively. |
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Thanks for the link oldgunner. Somebody built a liberty pistol on there (seems to be building a .45 acp version as well..) I am wrapping my head more around what I need to do in order to have something that is SAFE and functional more than a few times. Seems like I am going to have a strong chamber that I can then add an overbore barrel for a 12ga. More to come once I get more of this drawn up (man I miss my copy of solidworks...). Do you mean liberator pistol? I dont know what type of plumbing you will need exactly to handle the pressure of a 12 gauge, but it wont take a lot. 12ga is a pretty low pressure cartidge relatively. Most shotgun loads are around 14k CUP(IIRC), which is about half of what most pistol cartridges run. A decent piece of iron pipe will hold...at least for a single use. Beyond that, I'd suggest cracking Machinery's Handbook open and doing some hoop stress(IIRC) calculations. You may need to do more math for breech thrust values depending on your action type and locking mechanism. |
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Thanks for the link oldgunner. Somebody built a liberty pistol on there (seems to be building a .45 acp version as well..) I am wrapping my head more around what I need to do in order to have something that is SAFE and functional more than a few times. Seems like I am going to have a strong chamber that I can then add an overbore barrel for a 12ga. More to come once I get more of this drawn up (man I miss my copy of solidworks...). Do you mean liberator pistol? I dont know what type of plumbing you will need exactly to handle the pressure of a 12 gauge, but it wont take a lot. 12ga is a pretty low pressure cartidge relatively. Most shotgun loads are around 14k CUP(IIRC), which is about half of what most pistol cartridges run. A decent piece of iron pipe will hold...at least for a single use. Beyond that, I'd suggest cracking Machinery's Handbook open and doing some hoop stress(IIRC) calculations. You may need to do more math for breech thrust values depending on your action type and locking mechanism. "Decent iron pipe" starts with stock that is seamless, and that means it's not welded. A good place to start is aircraft tubing in 4130 alloy, partly due to its moderate strength and good ductility, but also good availability in different sizes of stock that useful sizes for this project. The weldability is very good for attaching lugs, too. I'll caution you about using simple hoop stress calculations for estimating stresses in a gun barrel breech - the equations are accurate for pressure vessels with very thin walls, and don't fit gun barrel breeches. The thick wall equations should be used in this case. One other consideration is mandatory - sizing with a factor of safety. I recommend a minimum Factor of Safety = 2 for this work, and more is not out of line. The factor of safety is multiplied against the expected pressure to determine the ultimate load condition; let's say the breech pressure is 20000 psi. The ultimate load is 20000(2.0) = 40000 psi, so this figure is compared to the maximum capability of the barrel and breech. A factor of safety has three functions - accounts for variation in workmanship and materials, and variation in predicted loads. A factor of safety is not a trivial consideration. The barrel and other parts should be sized so they will not fail at this pressure level. The parts should also be examined at the limit load condition where the actual expected load is applied. When this load is applied, no parts should yield or deflect affecting the function in any way. A factor of safety of 1.15 is sometimes used for limit loads. If I get some time later today, I'll see if I can work out a simple design and sizing example using 4130 in stock sizes. Since this thread came up I've been sketching some ideas for using a piece of tubing stock for the basis of a breech assembly. One of the cool things about aircraft tubing is that the sizes can be telescoped with about 0.010 inches of clearance (or a little less), and it's easy to match pairs. A 3/4 OD X 0.058 wall tube will accept a 5/8 OD tube inside, and the same holds for all other nominal sizes with 0.058 inch wall thicknesses. |
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I broke out my 27th edition last night and started running numbers.
I also thought about sleeving the design to lessen the possibility of a failure. I've got a metal supply house up the road from me so I should be able to get whatever I want (this is SO not going to be a Cheap project) 
The more I look at this, the less shade tree it becomes and the more my Engineering degree is becoming more useful (Anybody need a MEGR?  ).
I actually get to use my brain for a while... I don't have much in the way of pressure vessel experience beyond what academic stuff I dealt with (so a Machine Analysis class... I concentrated on Thermo a lot more.) Now where did I put that MathCAD CD at.... ++Edit: Birnie's Equation: t= (D/2)(sqrt(S+(1-2u)p)/sqrt(S-(1+u)p))-1) u= Poissons Ratio The S value is allowable tensile stress. (This number seems to be a known value, which I don't know.) I grabbed some quick numbers from matweb about 4000 series steel That should give me Min. wall thickness of the chamber area of the barrel. ++ Edit #2: Yield Strength of Annealed 4130 is 52 ksi per the manual. I assume heat treating is going to be required for it to hold after being subjected to firing. Need to do more reading about triggers, firing pin design, etc. 
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Shotguns made out of 4 parts have been produced. Both as improvised weapons, and as commercial shotguns sold by mail order.
Actually, I think the commercial version had 5 parts. Think stock, nail for a firing pin, ca 1" pipe for a receiver, and ca 3/4" pipe for a barrel. It is a pump action, so to speak. I am willing to bet that they didn't run hoop stress calculations before manufacture. I would not be surprised if a polylmer version were possible. |
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It is not so much that the pipe material can't take (say the hot/cold iron pipe they sell at most big box stores) They are all welded and are probably made of questionable metal in the first place. Using the Aircraft grade stuff will ensure I'm not one of these threads with my hands covered in stitches and bandages.
I'm pretty sure I am going to make a 'Liberator' shotgun that is super simple, bent sheet metal, a little bit of welding, and literally nothing but parts from a hardware store. This would be meant to be fired once and tossed. I also want to build more complicated (and functional) weapons in the future and I figure Shotguns are the simplest with the most forgiving low pressure rounds. Plus I can practice a lot with bird shot  .
My next step (barring getting the time to take a few gunsmithing courses centered around scratch building shotguns) will be picking up a NEF Pardner single shot in 12 ga to tear down and use as a reference. |
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Shotguns made out of 4 parts have been produced. Both as improvised weapons, and as commercial shotguns sold by mail order. Actually, I think the commercial version had 5 parts. Think stock, nail for a firing pin, ca 1" pipe for a receiver, and ca 3/4" pipe for a barrel. It is a pump action, so to speak. I am willing to bet that they didn't run hoop stress calculations before manufacture. I would not be surprised if a polylmer version were possible. I wouldn't take your bet, because the world is full of morons. Plus, there's nothing that stops a person from copying successful designs. If they go to the trouble of conducting adequate testing, and that means subjecting the firearm to extreme conditions and not just overloads, there's nothing wrong with using or even selling their product if its integrity can be backed up. However, there are also plenty of unconscionable or plain ignorant people that believe parts is parts and have no idea about the differences in materials, let alone care once they have your money. Your comment about polymer indicates you have no idea about the requirements for the heart of a firearm. |
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I broke out my 27th edition last night and started running numbers. I also thought about sleeving the design to lessen the possibility of a failure. I've got a metal supply house up the road from me so I should be able to get whatever I want (this is SO not going to be a Cheap project)
The more I look at this, the less shade tree it becomes and the more my Engineering degree is becoming more useful (Anybody need a MEGR? ).
I actually get to use my brain for a while... I don't have much in the way of pressure vessel experience beyond what academic stuff I dealt with (so a Machine Analysis class... I concentrated on Thermo a lot more.) Now where did I put that MathCAD CD at.... ++Edit: Birnie's Equation: t= (D/2)(sqrt(S+(1-2u)p)/sqrt(S-(1+u)p))-1) u= Poissons Ratio The S value is allowable tensile stress. (This number seems to be a known value, which I don't know.) I grabbed some quick numbers from matweb about 4000 series steel That should give me Min. wall thickness of the chamber area of the barrel. ++ Edit #2: Yield Strength of Annealed 4130 is 52 ksi per the manual. I assume heat treating is going to be required for it to hold after being subjected to firing. Need to do more reading about triggers, firing pin design, etc.
I wondered if you would come to the conclusion that you could heat treat the parts. Heat treating to 125 or even 150 ksi condition is not unreasonable, just more complicated and more work. It's not necessary for your project. I generally avoid using commercial data for material properties unless I get backed into a corner as it tends to be "industry average", hence optimistic. IM or email your snail mail address to me and I will send a copy of the last version of MIL-HDBK-5 to you. This handbook contains statistically reduced mechanical properties for all sorts of metals, including all those that are useful for building guns. Frank de Hass published several books about single shot actions that have good sketches of simple actions, both modern and ancient. The German guilds use a really cool Langenham falling block action with the parts contained in the breech block and cocks with operation of the lever. These guns look similar to Sharps Borchardt actions on the outside. If you look around the internet you might be able to find a cross section of a Hagn action. I have a drawing on paper but haven't bothered to scan it yet. |
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I am just making uneducated (in this regard) assumptions about materials.
I got funny looks from 95% of my professors (Some of which had DoD contracts running...) when asking about anything beyond simplistic impact calculations when it came to explosions and such. I drawing up something in Google sketch up (my academic Solidworks copy is gone...  ) I still have mathCAD though .
I found an aircraft supply house that sells seamless 4130 in various ID/OD. I looked at sleeving a 3/4" OD , .620" ID inside a .760" ID , 1" OD to use a receiver slash chamber support (sleeve 3 inches of the barrel section to increase the wall thickness.) This would be for a 20 ga build though. The more I look, the more I want to try a bolt action as well (eventually.) Main thing is not loosing an eye in the process. |
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Thanks
Now I have the learn where they put everything.. This is more a building exercise so 1) I can keep using that engineering brain, since Jobs are scare and I'm not going to retain half the information that was stuffed into my head for long unless I do things to keep them fresh 2) Learn something in the process about firearm design. I picked shotguns because they are the most forgiving as far as pressure goes. I don't think I'd shoulder anything I built until I bench tested it A LOT and checked for stress signs. |
A couple of things after I started drawing this - - I realized an auto unloader would be about as easy to build, - The headspace is adjustable, making the gun easier to build with hand tools and simple methods, - I've worked out two ignition mechanisms, one with a swinging hammer and another with a striker. They both have about the same number of advantages and faults and I need to complete both to decide which one wins. - The aft latching lugs need to be scootched forward a tenth or so in front of the breech to simpify the design and avoid complications. The lugs in the closed position are relocated, the lug on the open barrel is flush with the breech and gets close, but doesn't touch on opening. This drawing is a layout with no refinement, only ideas to get a general idea about how to put the thing together. |
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I've got some drawings on paper that are similar to that... not as much length on the lower portion of the receiver though.
When you look at this concept, the Sten comes to mind. It is basically a Scaled up single shot Sten... so building an autoloading tube shotgun would be just as simple as a Sten is. My main concern with regards to the lugs would be welding onto a thin walled pipe to attach a lug to the barrel at the pivot point. Not that it isn't hard to do for a welder, I just know I can get two pieces of steel to stay together with a mig running flux core or an Arc welder. I could solder it on, but I don't think it would stay together under repeated firing. I would like to do a hammer design that it is cocked by a lever (a super simplified falling block) more to just have more parts to work on and I like the look of Lever actions. I found drawings of the Hagn Action you mentioned.. That is a project down the road when I get a Mill and Lathe to work with. Frank De Haas Single Shot Books are on my find list as well. I have a few images of the Stevens Favorite and a Savage Marksman from his book with scale, so I am tinkering with a making a working trigger mechanism on a block with properly located pins. I did pick on some 20 ga ammunition (I have never seen a 20 ga or a .410 IRL always have shot 12 ga) to use. Materials wise, I found AircraftSpruce.com as a supplier of small quantity seamless 4130 tubing. I was looking at sleeving a .875 OD with .685 ID inside a 1.125 OD with .885 ID. Need more hands on with the inner workings so thats my next real step, getting a function mock up of the action. |
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I figured out a method for attaching lugs with hose clamps. Think about that awhile to see if you can find a solution. The welding is no problem for me, I've welded 4130 tubing down to minimum gages.
I'll work on more details next week and get a pistol grip, trigger bow, and forearm added. There's a better source of tubing in Pa, I'll see if I can find a link. Shapiro Supply in St. Louis also has tubing and other materials. |
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I figured out a method for attaching lugs with hose clamps. Think about that awhile to see if you can find a solution. The welding is no problem for me, I've welded 4130 tubing down to minimum gages. I'll work on more details next week and get a pistol grip, trigger bow, and forearm added. There's a better source of tubing in Pa, I'll see if I can find a link. Shapiro Supply in St. Louis also has tubing and other materials. My initial thought would be to use the barrel part of the clamp and drill a capture hole in the lug. Then radius the contact point of the lug to the radius of the barrel and possibly relieve the barrel so the cylindrical area of the clamp adds stability to the lugs. That would work on the barrel portion and the rear lug in your drawing, but I do not see a solution off hand for the half cut area of the receiver. I am work right now, so a MS paint drawing for a quick concept. 
If the lugs were made from a single piece, a cut could be made through the bottom to allow the band to hold in it place (or a two piece screw together lug as making that sort of cut with specific tools would be rather difficult), again radius the bottom to match the tube radius for stability. I am working on a drill template right now to make a working block for the trigger mechanism. I am basing it off a Stevens Marksman Single shot lever action diagram I have to get started and will most likely have to scale it a bit (not sure how necessary that is.) |
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Just a quick non-update.
Mostly just working on a simple trigger and hammer configuration right now. I have some 1018 (mild steel) I am using (for now) to make the hammer and trigger. Once I get them working and am happy with the shape I will make them out of O1 tool steel, then move onto the receiver area. I have Frank De Haas book on the way so I can read it more in depth. I also got a copy of the Williamsburg Gunsmith (muzzleloading....) and that was a very informative conceptual video. |
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