The idea of a rail on the bottom of the receiver for the grip and mag well is interresting, but it would need to be hollow in the center of the grip section and especially the magazine cut out, kind of like an AK side rail... only engaging the attached piece in a few areas.

If the bolt will be intended to cover high power rifle rounds all the way down to pistol rounds, the largest of sizes must be taken into account. Thus, the AR-10 might be a better starting point, and design from there as you can always go smaller...

(XCR, AK, AR-10, AA Grendel, AR-15, Kel-Tec)
 

And here's one for the WSSM

The Bushmaster ACR has the gas piston system attached to the barrel.

Check out this new video showing more details

http://www.youtube.com/watch?v=C8OBr0-0d9k

they show how the barrel comes off etc...

blurrededge wrote: Wrench build into the barrel and gas piston unit, I like it.

 I thought that was a great solution to the problem, and one that our modular build should follow in one form or another.

Look at the last diagram in the pictures at the beginning of this thread and that is exactly what Stoner did.

I think he called it the M96W (turn it upside down and it reads M96W)

Nothing I can find on the net but I just took the AK/PKM Armorers course at LMO and we went over the Stoner Weapons System for a while, along with a discussion on the AR developement.

For my thoughts on the the AR lower should work well. Dedicated belt feed could eject out the mag well, or like the Shrike dual feed the links fall out over the ejection port. Another thing off the Shrike I like is the gas piston is mounted on the side making it easy to feed from the top belt fed. This frees up the option of a top mag feed for a larger caliber that will not fit in the mag well. One of the original AR designs also used a side mounted gas system.

Sub guns you can use a mag well block to accept smaller mags.

I like the AR18 bolt system for a few reasons, it is easier to fab the reciever, the guide rods are ala M3 Grease Gun and lock into the front of the receiver and allow for a folding stock.

If it is true modular on the AR lower then all you have to do is swap uppers that are complete, less parts to fall off the lower.

I have been thinking of a way to adapt the MG42 quick change barrel to an AR compatible upper. This would only be needed on a LMG anyway. You can also use a floating fore arm to make an easy change barrel with the basic AR set up like this bolt gun:

Unscrew the fore arm and swap in a new barrel. Since you are using a barrel extention that is pre head spaced to the bolt assy, it would be quick and easy. Also lets you go to suppressed pretty easy with a full length suppresor that replaces the fore arm by using a slip nut at the receiver and keep the gas system contained under a shroud.

Dunno just thinking out loud and thats my $2.50 worth for now.

Maybe we should have two discussions (two threads) because you just brought up not only valid points on the modular build, but a subject which we should also be talking about for builds, and thats just building a new upper receiver for the AR15 lower.

just working on an upper, leaves you with half the work already done right when you start the build.  Plus...   It's not an FFL issue..  You can transfer the upper (sell) if you wish later without worrying about the ATF.

The AR platform has a lot going for it, and there is no real reason why we could not expand on that idea a bit more, and we may end up with more people interested in building.

Building a complete weapon (the modular weapon) for someone just starting with building is asking a bit much, but getting them interested in a bolt action AR15 design thats easier to build and/or sell parts for. may be enough to get new builders a start, so they can work towards the scratch builds.

OK so sounds sort of like a PKM, 1919 set up. Instead of stripping a round, pull it out to the rear and drop to it into position at the rear of the stroke to feed.

Mainly need to remember that this is out side of the box type stuff. We can convince ourselves that some thing is not possible, but if you don't know it isnt you might be surprised what you can make work. Every thing is possible to some one who doesn't know any better.

If stroke is a problem how about 7.62x25 using cloth belts? Instead of trying to make the stroke longer make the bullet shorter. I say rob and modify existing systems and parts. Use the best parts of all systems and make that work together.

Look at this it proves anything is possile

I always LOL at those 'belt-fed' photos! 

Honestly, I love where this discussion is going thus far!

I think a barrel wrench ratchet is a little bit too 'complex' for my blood, but nothing is impossible!

I'm a ***HUGE*** fan of the M96W, and the AR-180 (Original), so this has seriously boosted my interest in this project!

I ESPECIALLY find it interesting to note how the AR-180's internal receiver rail and the slots in the bolt carrier fit together!  It's quite a bit like MG-47ES bolt rail setup, using a simple U-channel bracket type rail.

I know a great many of you were mentioning how you really like the function of a side-mounted gas piston & gas tube setup.  At first glance, I do as well, but I also wanted to note how that DOES have a few limitations as well.

By keeping the gas piston in a larger, heavier-duty format, and off to one side, that limits the angular space allowed for different functions.

I was thinking if I build such a gun, I might try to incorporate a Gas Piston assembly that utilizes a split operating rod (Like the B.A.R. for example).  This way, the gun can function with a belt-fed mechanism on top, right-side ejection, and at the same time can be magazine fed from the bottom (without having to switch out the belt feed mechanism on top).  As you see on the SHRIKE, that's clearly possible to do the other way by having the piston assembly connect on the left-hand side, but then you're pretty 'cramped' for space where the belt comes in from the left side -- the gas piston has to fit in just beneath it.

All options have their pros & cons.

As Gary mentioned, using a short-stroke piston may not be ideal for a belt-fed operated weapon where the long-stroke piston reigns supreme.  This is because the short-stroke weapons and also direct impingement are designed around short-term or 'impulse' forces being imparted onto the bolt.  Expanding gasses only need to force the bolt carrier far enough rearward that it unlocks the bolt, and at a sufficient velocity to overcome the recoil spring & hammer spring - thereby allowing the bolt to travel fully rearward under momentum alone (as the impulse has ended rather instantaneously).

With a belt-fed mechanism there are more (stronger) forces that resist the rearward motion of the bolt/carrier throughout the entire length of the bolt carrier's rearward stroke.  The impulse not only has to be strong enough, but it has to have a longer gas dwelling time.  First it has to push the carrier back enough to unlock the bolt, and at the same time it's meeting resistance from the Hammer Spring, Recoil Spring, all the Cams & Springs in the belt-feed mechanism, as well as the resisting force of gravity pulling the weight of the belt downward throughout the entire length of the stroke.  It isn't until the bolt carrier has almost completely reached the end of its travel that excess gasses are allowed to bleed off, and the recoil spring can then stop the carrier, and force it forward.

That is also one of the reasons why belt-fed guns tend to feel as though they have more 'recoil' or 'kick' into your shoulder.  It's not because they're a more powerful gun, but it's because much more of the energy from the fired shot must be redirected into cycling the mechanism, and there is a higher degree of inefficiency so a lot of energy transfers from the bolt carrier, into the receiver, and into the shooter's shoulder at the end of each bolt stroke.

I LOVE the AR-15 becuase it is so easily tunable due to its very light cyclic impulses!  What I'm trying to think is having a dual recoil spring assembly on my own belt-fed gun so that once the bolt carrier is nearing its last 1.5" to 2" of bolt carrier stroke, a second spring connects and begins to slow down the rearward bolt carrier motion faster!

The more efficiently you can transfer that excess cycling impulse energy into SPRINGS, the less 'Felt Recoil' you will experience through transferring it into your shoulder!

Tgunner, 7.62x25mm has already been done in a belt-fed gun using RPD belts from what I heard.

Blurred, thanks for posting the bolt pics!  That's really giving me some nice ideas! 

continuing with this thought process, in the past I considered designing a system (spring etc) that would absorb and store the rearward energy in  a spring and sear setup so that energy can be released again at the proper time to assist in stripping the round from the belt and moving the next round/links into position.

The whole purpose was a booster to assist in cycling..  (most of the energy is wasted and not used for any real purpose). Such a system may allow for a shorter receiver as well as recoil dampening, and in a full auto, the rate of fire may even be tunable or adjustable with the system.

now such as system would be in addition to the normal recoil spring, and not just a bigger spring.  it would be built as a more energy efficient energy transfer, rather than the current system of absorbing the impact and then using a spring to push it back forward.

That was the idea...  but I never tried to design such a system (mental excises only)

Sounds to me like a hydraulic buffer system, nitrogen charged?

those leak

maybe the way to look updating the system is the way the compound bow works with a cam.  what if a cam was introduced into the system?

I think the KRISS(sp?) SMG uses something a lot like this and have little to no recoil.

Thats the way a PKM pulls the belt in. Side of the bolt carrier cams against the feed pawl.

The KRISS bolt cams down at the rear and the recoil spring is more or less vertical so energy is absorbed and displaced perpendicular to the barrel. A great way to displace felt recoil, but not sure how to use this unless turned the cam sideways and the feed pawl ran off of it that way. This would be out side of the box factory.

http://www.kriss-tdi.com/media/KRISS-TDI_Datasheet_SBRSO.pdf

Above is a link to the KRISS brochure that breaks it out. Also, once again I think it was Small Arms Review had an article on these a few months back.

A lot is going to depend on cartridge choice and where the gas is pulled off and how large the piston surface is, if a piston system is some ones choice.