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Sturgeon

Designing a Bullpup That Isn't Trash

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All extant bullpups are trash. None of them are designed properly or with even modest levels of creativity. Almost all are some other design crammed up its own ass with resulting penalties in weight, size, balance, and other characteristics.

 

Well that's what it feels like sometimes to easily-frustrated people like me, anyway. It's just as well, as the bullpup concept only has very modest merits to begin with and they are slowly losing market share with military customers to more conventional designs.


Still! I think it's worth going down this rabbit hole all the way. Bullpups are best designed not as adaptations of conventional weapons, but holistically and from the start as such. So, let's do that.

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The bullpup layout itself presents some unique challenges, and is covetous of certain parameters. Where the penalties for having a long receiver are minimal in a conventional rifle (it adds some weight, requires a shorter stock, but doesn't seriously affect balance in a negative way and has some key advantages such as adding overtravel), receiver length is a critical dimension in a bullpup. Longer receivers mean longer lengths of pull, and they push the magazine closer to the pistol grip (which is undesirable). Receiver length is broadly dictated by parameters like bolt group length when unlocked, and overtravel. Therefore, we should try to minimize most of these things. A certain amount of overtravel is necessary for the most reliable functioning, which means designing a bolt group that is as short as possible is even more critical. Since a rotary bolt needs significant translation front-to-back to achieve unlocking, we will not be retaining the bolt design of the F-4 and F-5, nor using a rotary bolt of any kind, for our initial studies. Already the bullpup layout is dictating major elements of the design, before we've even begun!

 

Candidate bolt locking designs include:

Tilt locking

Roller locking (not roller-retarded blowback)

Flap locking

Claw locking

Ball bearing locking

 

The ideal locking mechanism is strong, light, locks at the front, and distributes the force of firing evenly across the bolt face (for best accuracy). Only three of the five methods above offer these characteristics, roller locking and ball bearing locking. Ball bearing locking being little used and somewhat complex to do properly, we will ignore it for now. Flap locking would work fine, but roller locking is simpler and appears to be just as good. Therefore, our first bolt design study will be for roller locking.

 

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Wouldn't it be worth it to try and make the bolt shorter than the length of the cartridge? That would make it easier to get the rear of the magazine closer to the buttplate. You could mount the hammer/striker above the magazine instead of behind it, so that the firing pin doesn't have to extend any distance at all. The bolt carrier could be "telescopic" like an uzi and it could have rollers/ball bearing balls/flaps or whatever that lock into the outside of the barrel, some distance in front of the chamber (or really close to the chamber- the point is that for inertia mass purposes, the carrier should extend forward of the chamber). Or you could have the carrier lock into a steel receiver frame - that's not the point. The point is that it should not be an unmovable premise that the bolt should be COAL+X long (where X is unlocking front-back translation+overtravel), and that it must extend that entire distance behind the base of the cartridges in the magazine.

Another way of doing it could be to use a more or less conventional bolt and carrier, but have some mechanism to pull cartridges out of the magazine without requiring a lot of space behind the magazine. A bent piece of sheet metal that, during the firing cycle, moves back and forth and gets the next cartridge into an intermediate feeding position (kind of like the elevator in a tube fed firearm) that is so far forward that the conventional bolt and carrier can reach it.

I'm thinking that a lot of people have tried making unsucky bullpups, but they all fail, so maybe the unsucky bullpup has to have different mechanics - not just the conventional mechanical components slightly rearranged.

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8 hours ago, Miroslav said:

Wouldn't it be worth it to try and make the bolt shorter than the length of the cartridge? That would make it easier to get the rear of the magazine closer to the buttplate. You could mount the hammer/striker above the magazine instead of behind it, so that the firing pin doesn't have to extend any distance at all. The bolt carrier could be "telescopic" like an uzi and it could have rollers/ball bearing balls/flaps or whatever that lock into the outside of the barrel, some distance in front of the chamber (or really close to the chamber- the point is that for inertia mass purposes, the carrier should extend forward of the chamber). Or you could have the carrier lock into a steel receiver frame - that's not the point. The point is that it should not be an unmovable premise that the bolt should be COAL+X long (where X is unlocking front-back translation+overtravel), and that it must extend that entire distance behind the base of the cartridges in the magazine.

 

Nobody is saying that. The bolt carrier on the F-5 is already shorter than the length of the magazine:

XqJrjUx.png

 

However, the bolt group dictates the receiver length regardless, because it's a linear action. In other words, the bolt has to come back behind the magazine so that the cartridge stack can rise. This means your interior receiver length must be no less than the length of your magazine from front to cartridge base, plus the added length of the bolt catch (if present), plus the overtravel beyond the bolt catch, plus the unlocked BCG length, plus the length of any recoil hardware (e.g., buffer, recoil plate).

 

The way a rifle like the TKB-022 gets around this is by having a nonlinear action that operates more like an artillery piece than a conventional small arm. We might go that route on this project, we'll see.

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I drew up a crude sketch of a rotating bolt design with the lugs milled from a collar on the outside of the barrel. They are roughly one COAL ahead of the base of the chambered cartridge. The bolt looks like a huge AR-15 barrel extension turned over backwards, with a 10 mm thick wall in the rear to house a tiny firing "pin". The tubular bolt has a large cutout for ejection and letting cartridges in from the magazine. The bolt catch doesn't have to be behind the magazine and thus increase the bolt rearward translation. It can act on the oprod instead of the bolt face. With roughly the same distance from the magazine to the grip as the Steyr Aug has, I got a LOP of 9,5 inches. Call it 10 inches if you want a half inch rubber buttpad, it's still less than a collapsed ar-15 stock.

The distance gained by this trick could be used either for increasing the space between the grip and the magazine, allowing for an adjustable and short LOP, or (sacrificing a lot of receiver length) downward ejection like the kel tec rifle. Perhaps all three.

Edit to add: Obviously I'm not contributing much to the thread without an extensive model to back up my pitch of the telescoping rotating bolt, my point is just that you could really, really save space if you took a step or two away from the most common breech locking mechanism arrangements.

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8 hours ago, Miroslav said:

I drew up a crude sketch of a rotating bolt design with the lugs milled from a collar on the outside of the barrel. They are roughly one COAL ahead of the base of the chambered cartridge. The bolt looks like a huge AR-15 barrel extension turned over backwards, with a 10 mm thick wall in the rear to house a tiny firing "pin". The tubular bolt has a large cutout for ejection and letting cartridges in from the magazine. The bolt catch doesn't have to be behind the magazine and thus increase the bolt rearward translation. It can act on the oprod instead of the bolt face. With roughly the same distance from the magazine to the grip as the Steyr Aug has, I got a LOP of 9,5 inches. Call it 10 inches if you want a half inch rubber buttpad, it's still less than a collapsed ar-15 stock.

The distance gained by this trick could be used either for increasing the space between the grip and the magazine, allowing for an adjustable and short LOP, or (sacrificing a lot of receiver length) downward ejection like the kel tec rifle. Perhaps all three.

Edit to add: Obviously I'm not contributing much to the thread without an extensive model to back up my pitch of the telescoping rotating bolt, my point is just that you could really, really save space if you took a step or two away from the most common breech locking mechanism arrangements.

 

Yes that's sort of what we're pursuing with this thread. Last night I had a conversation with @Ulric where we discussed linear and nonlinear actions that could reduce the overall length of the operating group. It's interesting that you bring up an inverted bolt... That's not what I was going to do, but one of the concepts he and I discussed was similar in kind of a backwards way.

 

Like I said before, you can see with the F-5 that I'm under no illusion that the bolt group "has" to be a certain length. I own an RDB, for example, and its bolt group is significantly shorter than the cartridge COAL:

 

keltec-rdb-parts-675x450.jpg

 

So whatever we come up with for the bullpup's bolt, it'll be very short.

 

I just want to make sure that I'm communicating this properly, because it seems like there's a communication gap here.

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TIL rotating barrel extensions are a PITAS, because all your interface hardware has to be pressure bearing. If the rotation occurs between the carrier and bolt, that's not the case, but if it's between the barrel and bolt you need to start getting funky.
I think it's possible but I will be exploring alternatives before progressing further with that design.

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