designing a rifle and would like some input

Sorry if I missed this, but what caliber are you considering at this point? I'd assume you're going to use one already in existence rather than build something new, right?

dcarch, I think he said it was going to be a 7.62x51mm NATO/.308 that could be converted to 5.56

One thing we still didn't discuss is wether if it's going to be striker fired or hammer fired.

Oops. Sorry I forgot you said it was going to be an ar-15 trigger group.

I say go hammer fired in a conventional setup, with an AR-15 trigger group.

As far as ergonomics it would be nice to save a lot of stuff from the ar 15 like the mag release, safety, and the barrel being in line with the stock unlike the AK, Thompson see, etc. It would be cool to see features like bottom ejecting with a charging handle mounted on the left side of the receiver. And being bottom ejecting, it wouldn't be terribly hard to switch over to the right side for left handed shooters.

Good thread. I wanted to dump a few thoughts and comments here.

On the flapper bolt, I was almost sure these got a bad rap with the G43, but after seeing that .338 lapua semi-auto in G&A shooting sub MOA, it sounds promising (at least with CNC precision).

On the idea of a 556 tube gun, how about designing a one-piece tube bolt that has a raceway cut and rotates on a reciever mounted pin? i.e. instead of a bolt that rotates inside the carrier, have a bolt that rotates inside the tubular reciever. To combat friction, a washer could be mated to the back of the bolt while a coil spring pushes the bolt forward agains the pin. Of course, careful use of cutaways would have to be cut so the hammer wouldn't be obstructed. So perhaps this could be a blowback rifle with rotating bolt delay...with no gas system other than that directed a the bolt face.

Another strange idea I had: rotational locking...of the piston instead of the bolt. You could use directed gas to rotate a piston, or just push a cam/raceway oprod in a linear direction. My point is the unlocking would be done either near the gas block or front trunion, or hell, even inside the carrier. It throws aside the notion that the bolt itself should rotate to lock/unlock.

How about a spin on the ZM/Para TTR concept (minus the DI)? Just replace the gas key with an extended cylinder that could also act as a carrier guide. So essentially a piston AR upper with half a DI gas tube that injects gas into a female piston counterpart (i.e. an extended gas key with no gas flow to the bolt). Then work out the action spring to your tastes.

Benelli ARGOS gas system is pretty neat, presumably reliable, and seldom used. I like the idea of an annular piston (if anyone has a good pic of one I'd like to see it)

Maybe the latter is better. SCAR style.

I think you got the gist of my forementioned ideas. To clarify on the tube gun idea, 'screw delayed' isn't too bad description for what I called blowback rotating bolt delayed. That 'groove' is called a raceway IIRC, and that 'knob' is what what I refered to as a pin.

And as for the ZM-ish spin-off, you're spot on.

http://www.lannertactical.com/AR15-Straight-Pull-Side-Charging-Rifle.html

ar-15 bolt action upper

VZ58, DP28, FG42... stay still, darn it! Just kidding, I can tell you're still in the spaghetti-throwing portion of the design phase, where lots of ideas get thrown around really fast. My skorparev went from recoil-operated tilt-barrel, to tilting telescoped bolt, to gas operated tilted bolt for a really long time, to (briefly) VZ58-style tilting locking piece, and finally to the Degtyarov system with a long-stroke barrel centric square-profile piston (for now). A word of advice; you will chase fewer rabbits down blind alleys if you keep track of why you should choose one design over others (strength, ease of manufacture, safety, reliability) and firm up your desired feature-set as best you can.

Feature set as I understand it so far:
-Locked breech (any particular reason delayed blowbacks are off the table?)
-Gas operated (long stroke fixed piston, from the sound of it. Any reason recoil-op systems are off the table?)
-Rotating bolt head (repurposed Savage bolt head)
-Mag fed (max size 308-ish envelope)
-Grip-centered weight distribution (be sure you aren't compromising ergos with the side mag just to satisfy this condition, when you can probably achieve it through other means)
-Low profile upper (which is why I assume you desire the piston alongside the barrel, but remember you'll still have to lift the sights up unless your stock is dropped a bunch)
-Lightweight construction (might I suggest aluminum G3 mags or paratrooper FAL mags)
-Multi-caliber capability (bolt/barrel are swappable)
-Multi-magazine capability (magwell is swappable)
-Overall modularity (modularity is highly over-valued on a one-off custom gun, unless you have industry/military backing to take advantage of it. AR's get re-configured because there's lots of parts available and it is therefore easy; each change on your custom gun will be a monumental effort to design and build extremely expensive parts)
-Accuracy (sounds like you want the thing DMR-rated, as a minimum, so let's say 1-2 MOA?)

Try to focus on why one operating system vs. another helps you or hurts you in satisfying this feature set, and try your best to not mess with the features your are going for without good reason. Design is hard enough when you aren't designing to a moving target; you'll just end up churning forever and accomplishing nothing.

TCB

alfon99 said:

One thing we still didn't discuss is wether if it's going to be striker fired or hammer fired.

Click to expand...

There's also merit in a linear hammer like the VZ-58 or Norinco Type 97.

"probably wont consider delayed blowback is what it takes to flute a chamber"
Why, you use a fluted reamer, of course

Here's a short recoil action I just found about today; the 14.5mm KPV heavy machine gun. Obviously yours would be smaller D), but it's a rotating bolt short recoil op gun with gas assist (so some flexibility for loads/barrels/cartridge power levels)

"the 1941 johnsons were as accurate as garands firing the same ammo"
Yeah, but is that really saying a lot? Neither were 1MOA guns, which is what you'd want to at least aim (pun) for if you hope for an accurized design. Accurization wasn't really the chief consideration during the rifle trials (a consideration, yes, but not a primary one for the job it was meant for)

"how would you tune individual barrel assemblies to function in the same rifle without serious reliability issues?"
You primarily operate the gun by muzzle-boost rather than straight recoil. That way the booster can be modulated to work with a wider array of rounds, but I still don't think you'd have luck getting a 9mm to operate a 308 capable gun (I'd just use a straight blowback bolt for the little pistol-cal rounds like they do AR uppers). The trouble here is that the ATF will at some point declare your booster a silencer (and only The Shadow knows where that distinction lies...)

"as for the side mag.. it has some pretty big benefits, longer barrel in a shorter package, longer recoil track over a bullpup, conventional trigger packs could still possibly be used without linkage, much easier to operate and reload when prone, and if the magwell and the plate on the opposite side with the ejection port are swappable then its ambidextrous with only needing to cut out two sides of the receiver, and not three (one for the magwell plus two for the ejection port).. ergonomics while carrying are the only issue"
Couple thoughts on the mag setup;
-the pros you list are true for top-mounted magazine setups, but none of the downsides. A Calico/Bizon helical mag up top would keep plenty of ammo on hand (even long rifle rounds, unlike the PS90 mag), and the mag bodies would be the same for all calibers (the internal fins would be the same, and only the helix would vary), or if you are feeling ballzey, a protruding BREN or Madsen like magazine has certain benefits (and costs) as well. Namely they make drum/pan/belt conversions easier (as do bottom magwells)
-Another option would be a grip-well; only instead of attempting to wrap you palm around a 308 box mag D) you keep your thumb on the strong side and only wrap you palm around the front of the grip. Radical departure, but would get extremely-long mags in the same package as a bullpup while having slightly better balance and (maybe) ergonomics of mag changes. Find a way to load them from the top like stripper clips and you'd have all your benefits and none of the costs.
-If a side magazine is what you end up running with, consider making the gun automatically ambidextrous. When the mag is latched on one side, the latch automatically shifts a linkage that causes one of two extractors to act as an ejector (see the ARX160 by BRNO), and if you're really clever, pops the charging handle out the other side of the gun (like the ARX160 does with manual manipulation). The gun could freely be fired either way with no other action by the user, which remedies all issues except for the awkward carry. On that note, I'd suggest a 'carry magazine' which only holds a few rounds and is flush with the (also flush) magwell opening, and doubles as a dust cover. Not an awesome solution, but a solution. Either that, or an L-shaped magazine that drops down alongside the weapon to at least minimize the snagging annoyance. If the receiver is wide enough, the bend of the L would be flush with the sides.
-Be sure to make the magwell flush with the gun; good for carry, and you'll have extra width to work with seeing as you have guide rods/pistons located to the sides of the barrel. Your receiver will be fat and short, as opposed to the AR's tall and skinny; take advantage of that where you can.

"if you used some kind of an op-rod on the bolt carrier to allow the bolt carrier to also be used as a striker in the FG-42 fashion, you could also eliminate all linkages in the trigger mechanism and your trigger would be no more stiff or long than firing something from an open bolt"
That's because you'd be firing from an open bolt. The open bolt regs aren't there (just) because the ATF is a big meanie, it's because what you describe is laughably simple to convert to full auto; just remove the sear and it's "go" time, same as any other open bolt. And that's what the Bureau seeks to prevent by requiring a firing system independent of the firearm action, so that if the FCG is removed, you get hammer/striker follow and no ignition instead of fun times in the Federal P.

Besides, you can do better than that, anyway. Without the huge running start a full-length open bolt has, you'd need a very strong spring to kick the carrier the last little bit of its travel fast enough to ignite a primer. 90% of the charm of open bolts is they can be cocked easily, and you'd lose that. You'd still get the heavy/awful trigger and 'open bolt lurch' and long lock time, but at least an out of battery slamfire would be less likely. You get all the benefits and little cost besides a small bit of extra complexity if you go with a hammer or striker setup (which you wouldn't even be designing if you use an existing one)