actually it's possible to do a belt fed system from the bottom using the modular mag well system.  The Swedish m/1937 (basically an FND BAR) had a belt fed version that worked.  You just need the room for a means of advancing the belt (normally a button on the bolt).

The drawback of the construction was that when you emptied the gun after shooting four loaded cartridges are fell down. If this happens in snow or in thick vegetation the live ammo might be lost and cause harm to children who might find them. Another important drawback of this system is that the last shots in each link will never be fired because there are no more cartridges to lift them.
This was most likely the reason for dropping this modification.

Which brings up the fact that the MAG 58 is said to be a BAR mechanism turned upside down.

Mr. Weaponeer,

We are thinking! I saw an article I want to share with the board, but am at work...

So consider this a BUMP!

TR6guns,

I like your thoughts on the delayed ejection here - I'm not precisely sure how to go about it other than arranging the ejector further back on a rail with an AK style bolt...

Unless you machine your own, new bolt, which will lock up differently, and eject at the different angle things you're pretty limited with the ejection angle if recycling an AK/RPK bolt for this project.

For such a project, also you might want to consider the different challenges involved with using Disintegrating links versus connected links!  The Disintegrating links will need some means of positive ejection so they don't cause malfunctions...  I like the permanently affixed links of the MG-42, but those likewise have the drawbacks of potentially catching/snagging on something while maneuvering.

Other issues to consider is that you're going to need to have separate feed lugs on your bolt as well.  The feed lug to strip rounds from the belt will need to be quite thin - about half the width of a traditional AK feed lug which is used to strip rounds from both the left & right side columns of the double-stack AK magazine.

IIRC, this is how the M-249 bolt is...  On the top there's a thin feed lug for feeding rounds out of the belts, and on the side (visible) there's a rather FAT feeding lug that strips rounds from the standard AR-15/M-16 magazine.

http://www.dsarms.com/FN-M249-SAW-Spare-Parts-Set-----M249KI T/productinfo/M249KIT/

http://www.failsafetechnologies.com/parts.html

OR, for $4000 you can just BUY a M-249 parts kit... 

I like the thoughts & ideas in this thread though!  Anybody have ideas on how to best implement a quick-change barrel feature?  Such a feature would clearly impact the design of the Trunion & Feed Ramp!

Also, what is a good type of Steel to use if you WERE to redesign your own Bolt & Trunion for such a project?  O-1 tool steel?  4140?  Clearly once it is all machined it would need to be heat treated properly too!  These are things I know little about.

Thanks for any advice!    Let's keep this discussion going!

Questions I'd like your opinions/answers on:

1.) What's the best steel to use for machining your own custom Bolt & Trunion?

2.) What, in your opinion, is the BEST quick-change barrel mechanism design for a rifle caliber barrel such as one that would be used in this project?

3.) Anybody have any PHOTOS OR DRAWINGS of such a quick-change barrel / trunion design?

Any photos such as this are GREATLY appreciated!

Thanks!!!    Let's not let this thread lose steam!

I think 4130 or 4140 would be just fine as long as it was heat treated after machining.

Inabadhood wrote: 2.) What, in your opinion, is the BEST quick-change barrel mechanism design for a rifle caliber barrel such as one that would be used in this project?

There are some very good ones, but I would like to retain the ability to use affordable ar15 barrels. which really limits the choices.

The Bren system may work..  a system I used a lot was the M60 quick change barrels. and with a custom made barrel extension it would work.  maybe even simpler like a bayonet attachment style.  AR's don't require a lot of force to be held in place.

Inabadhood wrote: 3.) Anybody have any PHOTOS OR DRAWINGS of such a quick-change barrel / trunion design?

I don't have any drawings  

Inabadhood wrote: Any photos such as this are GREATLY appreciated! Thanks!!!    Let's not let this thread lose steam!

Mongo wrote: First time I clicked on this thread. Here is my Solidworks model of the Stoner 63A trunnion and the QC barrel mechanism. The spring is not shown since it sits under the lever and on top of the stamped receiver that is not shown in this model.

I liked the M60 setup, but now that I have seen the 63A setup I like it even better, and I really cannot see why this could not be made to work with AR barrels (especially considering they were built by the same person).

Now the 63A might be easier to engage by accident in combat (I don't see the details on how it releases), but I still think it's a better setup.

Mongo....  Thanks for contributing!

WOW!!!  EXCELLENT, EXCELLENT MODELING, MONGO!!! 

THAT IS NICE!!!!

Thanks for contributing to this thread - especially the QD barrel & trunion images!!!  That's excellent information!

I do have one question, and perhaps somebody can clear it up for me...

What is the function/purpose of this side-located pin circled in red below???

THANKS FOR THIS POSTING! It's really helping to give me some ideas!

AAAAAAHHHH!!!

I see...  Thanks for clearing that up!

So for my purposes it would be rather pointless to have that at the moment.

It's just something important on the original belt-fed gun though!

Thanks for the great modeling work!  EXCELLENT!

I have always been interested in this weapon.  I would like to see something come from this topic.  Lets make one.  (for each of us of course)

I would base the design on a square tube receiver (one of a common size).

simple square trunions (front and rear much like the AK)

I would like to have a a rail on the bottom of the receiver (basically full length) that would allow you to slide on a grip assembly.

The grip assembly would be modular in nature and allow the adaption of not even having a grip (much like a normal bolt action rifle) this way it would be modular enough for Calif. Sales.

The Mid section of the rail would allow for a modular magazine well assembly, so you could adapt the weapon to common magazine types:

AR15, AK, HK,and larger if needed, as well as to be able to adapt the weapon to a pistol caliber so you can use sten, uzi, thompson, grease gun, suomi, etc.. magazines.

You could then slide on the forearm (molded platic, carbon fiber etc..) as well as mount tactical rails for forward grips and bipods.

The bolt assembly should be modular as well..  kind of a modern bolt carrier and bolt assembly.  This way you could just swap out the bolt to change calibers (when needed).  One bolt should cover 308, 30.06, and 8mm (just like the browning 1919a4)  The bolt for the 7.62x39 is the same as the one for 6.5 grendel, so all you need to do is swap the barrel, and insert the right mag and you have a weapon shooting the other caliber.

Other calibers would require a bolt swap, and pistol caliber may require a carrier assembly swap as well as a mag well swap.

This system allows you to own a complete weapons system...  just buy the accessories you want for the new calibers and other accessories and adapt the rifle to your needs rather than the latge expense of a whole new rifle.

Mongo wrote: If you are going to use a QCB then you need to decide on the type of gas piston system (short stroke or long), barrel mounted or receiver mounted, on the top or bottom of the barrel. The QCB will require a barrel extension for lock up unless you are going for a very strong receiver. The problem will be in the different barrels will have to be kept at extremely tight tolerances along with the trunion so you can swap barrels w/o head space issues. Do you intend on a belt feed unit as well or just magazine fed? Bolt metallurgy can be a real PITA so I would design around an existing bolt. This will also make the barrel extension and extractor stock items. Firing pin could also possibly salvaged from another design.

as far as the gas system, the only one I would use is the long stroke system.  The short stroke is not very forgiving, and the recoil impulse form the short stroke designs is very high. unless everything has been heat treated, it would be torn up or worn out is very short order.

The original plan was to be able to use the same setup for belt fed and mag fed designs.  after some added thought on this I would keep the belt fed unit as it's own dedicated receiver (and a bit heavier).  This would be modular and use most of the parts and design from the rifle, but the gas system would be on the bottom rather than on the top (like the rifle), and the LMG would have heavier barrels to prevent overheating to quickly. 

The rifle would be designed as much as a full floating barrel as possible.  The idea of having the system system part of the barrel (as a unit) has a lot of benefits.  when you switch from rifle to pistol, the gas system is automatically switched with the barrel. it ads barrel weight, and a bit more expensive, but gas systems are not one size fits all.  everything greatly depends on the caliber and the barrel length.

Barrel headspace.  My thoughts were to have all the barrels identical, but not cut the lock/key that determines the headspacing.  You would have to modify, and headspace each one to fit your receiver and your receiver only.  It's a bit of a pain, but due to manufacturing variance from individual builders I don't see a way around it without adding a lot of unnecessary expense.

Personally I like the AR bolts as well as the barrel extensions, but a modified carrier that would be square rather than round.  as well as the AR firing pin.  These two parts  are cheap, easy to find, and of high quality no matter were you buy them.

For a square bolt carrier to fit the AR-15 bolt, the AR-18 bolt carrier might be a good place to start.




There's also Kel-Tek's bolt and carrier from their PLR pistol thing


and the Robinson M96 Carrier


Here's the AR-180's receiver internals as viewed from the back

Hope these help with ideas.