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Sturgeon's House


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  1. You send your nerdiest engineer to go do some theoretical calculations while you stay in the workshop and mount the barrel in a press drill and put a small hole in the barrel. If you can't get the rifle to function properly, then you put the barrel back into the press drill and use a larger drill bit. Repeat until the rifle works properly. You'll be done before the nerdy engineer. I too look forward to a theoretical answer, but in practice, fluid dynamics are very complex and I'm sure that most gas operated firearms developed during the 20th century had a lot of empirical iterative development to get the working right. Engineers are not scientists - everything is done within the scope of a development process were results are more important than a scientific lab report. It is often the case that it is more rational to find the answer by experimentation rather than theory. Of course it helps if you have some theory to start off with when you pick your first drill bit, or when you design the gas piston and cylinder that you intend to screw on to the barrel, but my bet would be that the engineer with the press drill finds the correct answer before the engineer with the calculator.
  2. And also, I thought I was quite the wannabe who keeps diving deeping into knowledge about military equipment that I'll never work with, but the Japanese can hardly own a knife without asking permission for it first. They are so irrelevant to gun culture, and gun culture should be so irrelevant to them. Yet here we are. There is Japanese fan fiction about Obobob.
  3. What the fuck? I thought I was pretty obscure who keeps checking this thread for content, and the obscure guns in it, but there is actually some guy out there who is making anime schoolgirl artwork with these weird prototypes. What the fuck? This world is so strange. By the way, I mostly lurk in this thread, but I'm really grateful to you guys who keep filling it with content. Especially the русский stuff.
  4. So my mind wandered off the other day and I started thinking about bolt carriers, recoil springs and caliber conversions. I feel kind of ignorant for not getting this completely straight, but I'm wondering if I'm missing something. Let's say you'd have an AR-style rifle chambered in .308, and you'd convert it to .223 with a swap of the bolt head and the upper receiver. Let's ignore the magazine issue for this discussion. I'd imagine that the optimal bolt carrier velocity is the same regardless of cartridge (within some reasonable limit). Thus it should be perfectly possible to compensate for the new cartridge only by changing the gas port location or size, and leaving the same bolt carrier mass, the same bolt head mass, the same recoil spring and buffer in there. For some reason I've always had it in my head that a larger cartridge requires a heavier bolt carrier, but I just realized that that's not right. A larger cartridge requires more space on the bolt face, more space in the receiver, and a sturdier lockup. This tends to lead to a heavier bolt carrier group, but there is no need for a heavier bolt carrier per se. Is my understanding correct? Of course there is less volume to work with when running a gas system on a .223 versus having a larger cartridge, but it should be perfectly possible to fiddle with the gas port size and location to compensate. I could also imagine the larger surface area of the larger cartridges to increase friction during primary extraction, but the difference between different calibers should be negligible compared to the difference between dirty ammo and slightly oily ammo. The Saiga rifles use the same bolt carrier and virtually the same bolt for all of the difference cartridges. The Knights Armament SR-25 uses the same springs and buffer as the M16a2 (although they have a heavier carrier and had some issues) The DPMS genII small frame .308 rifles use the same buffers and springs as the 5.56 rifles. Bonus: Check out this thread from arfcom on bolt carrier velocity https://www.ar15.com/forums/general/for_AR_fans_and_engineering_nerds__I_calculated_the_bolt_carrier_velocity_profile_for_an_AR_10/5-1150725/
  5. If you look at the difference in manufacturing costs, I think Sturgeon does have a good point. The tubing or operating rods or whatever needed for the gas system are rarely very expensive parts. I'd guess that the difference in costs associated to the bolt head and barrel extension is larger than the entire cost of small parts for the gas system. I could be wrong. Also, there's the development hurdle of designing a new rifle. If you're trying to invent a roller delayed blowback, as far as I understand, you'd have to experiment with different angles on pieces that are made from hardened steel. I might be overestimating the amount of work to do that, but my spontaneous estimation is that that would be way more time consuming than modifying an SVT-40 (AR-18) gas system by moving the gas port up and down the barrel and changing its size. All you really need to do that is a serious drill and a way to plug your old hole...
  6. Well the FAMAS isn't a bad design. Given that they were going to make their own production line from the ground up, I wouldn't say that it's an irrationally expensive design either. On the modern US commercial firearms market there has been some efforts to make semi-locked or locked bolts for pistol caliber carbines and submachineguns. This lowers felt recoil, ameliorates performance with suppressors and with some larger pistol calibers. SIG has a gas operated, rotating bolt design, but I wouldn't say that it's completely clear that the SIG design is leaps and bounds better than the CMMG radial delayed blowback.
  7. There are a bunch of semi automatic hunting rifles that have solved this problem. The reason for placing the gas system below the barrel is that it fits nicely into the handguard. This makes the rifle keep a traditional and slim profile. In general, they are much lighter than for example the M1 Garand, but they have a lot of small parts and are kind of messy to disassemble. This is afforded by the designers because they have access to modern manufacturing methods, and their customers rarely have to take the rifle apart in a hurry, or in the dark, or in the rain. One positive side effect of this layout is that it moves the recoiling mass down. All of these rifles are sold primarily with traditional stocks, but having the recoiling mass below the barrel somewhat compensates for the lack of an in-line stock. I've spent a lot of time thinking of how I'd design my dream hunting rifle. I've ended up deciding that it's worth it after all to put the gas system below the barrel. It really creates a whole host of problems. I despise the cuckoo clock engineering on rifles like the modern Browning Bar (not the lmg), but it's hard to move the impingement force from the gas port to the bolt carrier in a satisfactory way. You also don't want a heavy and bulky op rod like the Garand design. And I want the bolt carrier to be easier to get out of the rifle than in the Browning Bar or H&K SLB 2000 (or worse yet - the Remington semi autos). The nicest design in this category is definitely the Benelli R1. It has a wonky gas piston system it and hides the recoil spring in the stock, like a lot of semi automatic shotguns. It is the only rifle of this kind that has an acceptable takedown procedure. It's not perfect, but for a rifle that can be chambered in .338 Win Mag it is very nimble and nice to handle. As a comparison, google the youtube video with a guy that takes the bolt carrier out of his FNAR. "FNAR disassembly". Compare that to how you get the bolt out of an AR-15, or an AK, or any other rifle designed without a strong traditionalist aesthetic requirement.
  8. I still admire Amerika for generating so much disposable income among its citizens that they feel like they can afford sinking so much of it into such futile projects.
  9. I'm sorry, my sarcasm wasn't coming through. This spring is (according to the designer) what keeps the bolt from rotating in the bolt carrier, before it has moved all the way forward to the chamber. https://ibb.co/K5Xs4zV <a href="https://ibb.co/K5Xs4zV"><img src="https://i.ibb.co/mRGyZFh/APED.jpg" alt="APED" border="0"></a> I don't think they'll ever get anywhere with this.
  10. Don't get me wrong. I'm an ignorant fool. I'm not an entrepreneur. It takes courage to go into the unknown and take a chance. I still think this anti-pre-engagement mechanism is worthy of some appreciative attention. I know you all read or write for tfb but here's the link. No time link cuz the whole interview is worth watching. I really wonder how he funded this thing. I really love how he's passionate, but how will he ever compete with a guy like serbu? https://www.thefirearmblog.com/blog/2020/11/18/the-50-bmg-ak-project-as-told-by-brandon-the-ak-guy/
  11. Well, as long as you have a set of premises to keep your definition of "normal circumstances", your formula holds up very well as a general pointer. I still think the factor "3" is high. There are so many different kinds of tried and tested receiver shortening "crazy BS" design elements availible to a designer that perhaps the formula could be further specified to: If: R2>3*(C2-C1)+R1 Then: designer is lazy What do you mean by bolt extension distance? I'm guessing the difference in how far the bolt lugs protrude from the bolt carrier as the bolt is unlocked.
  12. I could buy the first two increments, but I don't think you should count having the spring in the forend as "crazy BS", and if it's in the forend (or there's just enough space in the carrier to stuff it in there anyway) you don't have to extend the receiver to fit the spring. Also you counted the bolt length twice. And you could have the hammer extend over the rear end of the magazine (or any bolt hold open device), which would save you from having to extend the firing pin all the way from the chamber to behind the rear of the magazine. I think the SU-16 has that configuration, but I'm not sure. Yeah ok I'll quit the semantical nitpicking. I appreciate the attempt at generalizing. I wonder how well this stacks against the HK roller locked series, or a comparison of Garand/M14/Mini 14. I've got some CAD models of long guns as well, but I'll keep them to myself for now. I have a PCC design that wouldn't require all that much of a workshop to put together, compared to a locked rotating bolt, gas operated rifle.
  13. Hey Sturgeon, you can't just drop an authoritative statement like that formula and not back it up. Please show us how you came to the number three. I'm genuinely curious. Having this extremely specialized hobby as well, I have a notepad at home with calculations made to the same end. Writing from on top of the loo at the office, I would have guessed at 2 rather than 3. Also, you must need a lot of premises to be valid for it to be true, mainly that the receiver was designed for optimization with regards to overall length. There are a lot of CAD models of the scar availible online. Check out grabcad. I've got a model on my home pc that is quite accurate, and I think I got it from there. Way easier to measure than counting pixels. Finally, while I enjoy the high level posting on this forum, I think you're being quite harsh on the OP. Who cares if it's the trunnion or the barrel extension.
  14. Yeah that looks like quite the distance. Is it meant to reduce the angle the cartridge has to tilt to get from the double stack to the center position?
  15. On bolt arrangement: I've been thinking I should go with a six lug bolt, with the lugs arranged like the seven lug bolt on an AR-15. This is inspired by the recessed lug that some manufacturer (I've forgotten which one) uses on the lug opposed to the extractor. As far as I can calculate, this setup is plenty strong. Bolt carrier should look kind of like a .308 AR carrier cut off behind the firing pin and instead of a gas key it has a protrusion where the op rod/piston hits it. I have some vain dreams of spending a ton of money to have a prototype of this rifle built. As I'm based in Europe, I don't have a cottage industry of AR-15 manufacturers that I could pay to make me a barrel extension with a broach. Because of this, I'd have to CNC mill/lathe both the bolt and barrel extension (and then maybe heat treat and remill, I don't know, not really an expert on metallurgy). Right now I'm leaning towards making the housing for the bolt catch be less obtrusive, by moving it to the side (with a little arm that reaches in towards the spot where the follower actuates the bolt catch) so that the hammer doesn't have to reach too far to hit the firing pin. I think the bolt carrier/hammer interaction during the recoil stroke will work out. The hammer will be pushed way deep down by the bolt carrier, but I guess you could let it follow the BC by a small bit before it's caught by the disconnector and later held by trigger. This will reduce the distance the hammer will have to travel upon firing to make it to the firing pin. Of course, I haven't calculated the hammer travel speed, so I don't know how big of an effect it would have on lock time, which in itself is not that important. All in all, this would mean keeping the AR style carrier/firing pin. If I can't make a trigger group/hammer that seems plausible enough, without compromising receiver overall length, I'll have to go with a different arrangement for the carrier. The linear hammer thing is interesting. Civvie semi autos usually have the bolt catch by one of the front corners of the magazine. I suspect they do that to keep the receiver short. They also often have a bolt carrier that is shorter than the long action ejection port, but with a telescoping dust cover on the carrier.
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