So I'm thinking of designing my own .308 semi auto from scratch and maybe building it
I'm studying mechanical engineering and I really like guns. I do not like the fact that semi-autos directed at the civilian hunters (especially concerning us europeans) are constructed using large amounts of pins, springs and large numbers of small parts. Therefore I'm thinking of designing my own rifle with a traditional layout, an exterior similar to a traditional hunting rifle and an interior similar to an m1a or a SCAR rifle. Sure I hope I'll be able to make myself a working prototype, but that's far, far away from where I am now.
I'm not even halfway through my education but I'm not aiming at setting up a factory or anything like that. If I can make myself a good CAD-model to run some ADAMS simulations on and maybe make a mockup (with all the moving parts) I'll be very happy and satisfied with the project. This is not part of a class I'm attending, I'm just trying to have fun with the wonderful access I have to equipment and know-how at my university.
Currently I've decided for a general layout that could be described as an ar-10 upper/lower receiver with an ar 18 BCG, but the BCG is run by an m14-style pipe containing the mainspring, located above the barrel. I guess you could say that it's like an AK but with a short stroke piston, or like an m1a but with a modern BCG design, but the old style op-rod. My notebook contains some other layouts and sketches but right now I'm really into this design and so far it's the simplest one.
Anyway, I've got a concrete question for those of you who have more experience and knowledge;
For the first digital prototype, do you think I'd have a good starting point if I use a gas piston assembly with the same dimensions as an m14 gas piston assembly, and a BCG/op rod+ bolt carrier + bold with the same mass as the op rod and bolt of the m1a, and a spring with similar specs as the m1a mainspring?
The only difference between the m1a and my gun that I can see at this very basic and humble level, as far as mechanical dynamics go, is the forces counteracting the rearward movement of the op rod due to the unlocking of the bolt. This would of course lead to a timing difference that in turn could produce forces acting rearward on the bolt face from the pressure in the chamber.
All responses and any input is appreciated.
I will subscribe to your project with intrest, building a gun from scratch is an intriguing idea that far exceeds my skills.
Thoughts on your design.
Most Auto-loading hunting rifles are marginally accurate at best. I have a friend who has argued with a Winchester 100 in .308 for years, and has never beaten it into submission. The nature of the beast.
The apparent reason, or a major contributing factor is all the moving junk hanging around and attached to the barrel, adversly effecting harmonics. This also effects most military auto-loaders making the majority of THEM marginally accurate, ergo; the AK, Dragunov, FAL, HK, M-14, Garand.
The exception is the AR platform, which is direct impingment. Many people believe that the lack of moving parts directly contributes to the inherent accuracy of the platform.
Direct impingment might not be right for a hunting rifle( or maybe it is), but perhaps you could incorporate into your design :
A) a super sturdy op-rod and spring unit that is isolated as much as practical from the barrel, or
B) some sort of recoil /op-rod system that ran in the butt, isolated from the barrel.
Anyway, IMHO, focusing on isolating and freefloating the barrel would be a primary consideration if I was designing a new auto-loading hunting rifle, and personaly, I would make the operating system as invisable as possible(like under the barrel, out of sight) if at all possible, giving the gun as clean a look as possible.
Making it unitized by design would be an improvement on all the mods that go into rifles like the M-14 to accurize them.
Accuracy is king now days.
In the past, Auto-loading hunting rifles were apparently designed with accuracy by volume an acceptable outcome
Just my opinion, I could be wrong, but you asked!
I agree on every point! For a long time I was drawing a rifle with the gas beneath the barrel but you end up with such a mess trying to get the power from the piston to the bolt carrier while keeping the parts count down and creating a rifle that's easy to disassemble.. The "other" design had a BCG that could be disconnected from the "op rod", which was located beneath the barrel, but the "quick detach" mechanism was all made up in my head and I don't know how well it would work in reality.
I suppose that if I'm making an imaginary rifle I should go with the more daring design, but in part I want to challenge the somewhat too conservative ideal that the barrel must be visible from the above. If you think of it, it makes sense to keep the op rod or gas piston above the barrel so that it doesn't have to go around the magazine by being split in two or arc-shaped ( like the Garand). If the gas system was covered with a nice piece of wood ( fine wood or laminate) like the Garand, Enfield and many other rifles I'm sure I could like it. And other imaginary users could as well.
On the other hand, The receiver can have a much slimmer profile height-wise if the gas system doesn't occupy any space above the bolt. On the other hand yet again the gun gets pretty wide if you're having both a double stack magazine and a split op rod that runs on the outside of the magazine. The Browning bar (sporting rifle, not mg) solves this by having the piston hit a block, and then the block is connected to two long, thin plates that run along the foreend and between the magazine and the receiver walls. These plates have a slot cut in them that allows them to transfer force to the bolt carrier.
The bolt carrier itself has a bunch of plates and pins and springs in it that I think you could rationalize away. It's a really nice rifle designed by a more competent engineer than myself (I'm not even an engineer yet), I love shooting it and I'm sure FN/Browning has very optimized production and all that, it's just a question of taste. How many different size screwdrivers are you supposed to need to get the bolt out of your gun?
Don't expect speedy updates with simulation videos or pics of me with the first deer kill, this might very well end up being too hard for me as well, but I'll learn a lot and have fun finding my limit.
If you want to see a very unique gas system do some research on the Czech VZ52 Rifle.
I'm going to give you a suggestion - design and build a single shot falling block self cocking action for rifle with an extractor and ejector. The Haenel action as built in German shops is one example that works well for .22 LR and has an interesting hammer. In fact, your first action should be even more simple, with an external cocking hammer.
That will fill your design plate.
One more suggestion for starting with a clean sheet of paper. The design starts at the chamber, the locking lugs, and the barrel outer diameter around the chamber. Then draw the barrel to the length required. After that, start detailing the firing pin and the mechanism that will actuate the firing pin.
This is an iterative process, and the less experience you have, the more erasers you're going to burn through. You have to understand the function of every part, and be able to draw a balanced free body diagram of every part to analyze this mechanism. If you rely on 'puter simulations alone, you'll get the wrong answer, especially with your level of analytical experience, which is virturally none at this point. Learn to walk before you run, the path will be straighter and smoother, and shorter in the end. Trying to copy piston areas and bolt weights is foolish, this is a wholly different pursuit that requires engineering, and you can either do that with smart analysis plus testing, or boat loads of testing with lots of expensive iterations and blind alleys.
Thank you, your reply is very much what I was looking for.
We've been drawing and balancing free body diagrams every day for about a year now. I just passed an exam doing it and I've got another one in two weeks. I'm not saying that I'm ready to build whatever using free body diagrams, but that I want to practice using free body diagrams designing a rifle. More precisely what I would like to understand is the relation between the "obvious" vectors witch to me are a) the work conducted by the piston as it accelerates the BCG and b) the work by the mainspring as it accelerates the BCG in the opposite direction and the to me not so "obvious" vectors as the work conducted by the chamber gases upon the bolt face and thus first causing friction as the bolt face rotates and then further accelerating the BCG rearwards.
The goal is not so much to get a new rifle but to understand the thermydynamical and mechanical aspects of the design of a self-loading rifle. I do not mind finding out that building a .308 semi auto is way too difficult to me, that is the purpose of this exercise. I want to know where the holes in my knowledge are so that I know which courses to pick for the upcoming years.
In fact, the force/work vectors I can think of while writing this with my moms notepad in my lap are
a) piston upon op rod
b) mainspring upon bcg
c) bolt friction before unlocked
d) chamber pressure on bolt after unlock
e) friction upon bcg from magazine
f) cocking of hammer
g) friction due to the forces acting on a body and not a particle, causing the bcg to want to rotate and be forced to the walls of the receiver
a) can probably be worked out by analysing .308 pressure diagrams and the dimensions of the gas piston assembly in relation to the distance to the chamber/muzzle.
b) easy peasy if I can find a velocity that is close to initial velocity. Should be able to find with a) and maybe high speed filming of m14.
c) chamber pressure causes rearward force changing as an estimated known function of time using pressure diagrams that are cartridge specific. I'm assuming that these can be found somehow.
d) One of the really uknown unknowns to me, because this force is acting on the bolt (partly via cartridge) as the pressure is low compared to the initial pressure, and it is also escaping both to the rear and the front of the barrel. This raises another question that I have thought about for years; to what degree are locked semi automatic rifles delayed blowback rifles unlocked by piston as opposed to "piston driven"? Answer or link to answer much appreciated.
e) Also puzzling, because many rifles handle a spring force from 20 to zero remaining cartridges no problem. This is one of many things that suggest that these actions have a large margin of error.
f) not problematic to calculate to an adequately accurate degree
g) very scary as it is a result of all of the above. I haven't looked into this but the friction might be a lot and it might be a little.
a,c and d require use of pressure data that I think is availible online. Assuming it is I'm still going to have to have a pretty productive e-mail conversation with my thermodynamics professor.
All in all I hope that using the results of the above will get me close to understanding the mechanism. I might be missing out on obvious and not so obvious aspects of this, which is the reason why I'm doing this.
I do appreciate your advise on starting with a single shot, but if I'm doing this I really would like to look at some form of auto-loading mechanism. I would like to look at quantifying the forces involved in a moving bolt mechanism and therefore If i'm starting off with something simpler I'd much rather look at a blowback-operated pistol caliber carbine with a moving firing pin firing using a hammer, from a closed bolt. This would pretty much give me the same problem as the rotating bolt rifle but it would allow me to skip some of the more difficult aspects of it.
Have you thought about making this rifle striker fired (ala glock) instead of utilizing a hammer?
A gas operated striker fired .308 would most certainly be unique.
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Both you, runsbellows and patriotarmory223 have given me suggestions on complicated features that would be cool to have. I'm not trying to do innovative construction, I'm trying to understand old constructions and incorporate them in a new composition/layout. I don't own a glock and don't shoot handguns, so it's a lot harded for me to visualize it. I own and have used more hammer fired guns than striker fired guns. I simply am not nearly skilled or experienced enough to try and innovate like that, that's not where I am.