Keep in mind that the Cobray and MAC design places the mag behind the trigger. A redesign would be needed for an AR-type lower trigger group.
Also the Cobray/MAC bolts have become rather spendy and hard to find in recent years. A 7.62x25 mm version using a Czech M-24 mag seems like the thing to do.
I believe that the square tubing used on these was an oddball size but no big deal if you can find a standard Cobray/MAC upper.
The mag is behind the trigger, but the sear and hammer are behind the mag. There is a cubersome transfer bar arrangement in a MAC semi. You could easily enough make a traditional mag in front of the trigger gun. While they are not as cheap as they used to be, they do turn up on gunbroker, and things like firing pins and recoil springs would be easy to get. Another advantage would be that the telescoping bolt design would put the bolt over the mag well when closed with a few inches of the barrel inside the receiver. That would mean you would have less barrel sticking out front, so a legal 16" rifle would look shorter. Food for thought.
I have a Cobray M11-9 bolt and recoil assembly. Maybe I should try a mock up.
I like the CZ 7.62x25 idea.
Orion's hammer did some calculations on a 9x19/7.62x25 bolts and used the bullets' energy as the recoil force, makes sense to me. The payload has the same bore diameter as the shell's head, and both sides have the same pressures exerted on them, so both will have the same force put to them, right?
Originally Posted by kernelkrink
Anyways, so with the 12 let's assume you're gonna be throwing a 1 oz slug at 1,500 FPS or so, you want a bolt velocity of about 10-15 FPS Let's use 15 as it'll result in a lighter bolt, and easier calculations... You'll need a 100oz bolt to get the 15FPS bolt velocity, 100/16=6.25 So you'll need a 6 1/4 lb bolt in a closed bolt blowback shotgun firing slugs.
Damn, all I needed was another project... Maybe even reduce the bolt weight by using a striker setup with a 1lb bolt and a 4lb firing striker to get some of the benefit of the whole open bolt inertial advantage that you guys were talking about earlier.
Don't forget the spring tension too.
Originally Posted by 
Oh yeah... And the force to recock the hammer, the ruger mkII relies pretty much solely on that...
Originally Posted by Markp
I think I slept thru all this in class are you guys rocket science guys or something. this much info is giving me a headack. I am impressed with this whole topic!!!
impressed as well
Originally Posted by carnut