could one replicate the ballistics of a 6MM ARC, using a really short fat cartridge - the OAL is only about 6mm longer than 5.7x28.

this means that you could use a horizontal magazine like the P90, allowing for easy prone shooting and very long 39 round magazines

option one would be to use a neckless approach with a very tall, flat shoulder. this would bring the powder capacity up to standard. 

 The other idea would be a totally CT approach.  i don't know what the OAL of textron's 6.8 was, but with a 6mm chambering and a very fat case, you could bring the OAL down to 46mm.

any thoughts?

would you go with the short fat neckless cartridge or a CT approach, and would it work in a P90 type gun?

Are you proposing something with a shorter case than the 243 WSSM?  What base diameter are you considering?  Even the WSSM's were limited to pretty stubby bullets, so the case would need to shortened significantly to utilize the 6mm ARC's high-bc bullets.  It would be interesting to see how such a thing could be made to feed.   DTech in the US claims they have figured it out.

243 WSSM, 243 Win, 22 WSSM, 22-250, 223

nincomp said:

It would be interesting to see how such a thing could be made to feed.   DTech in the US claims they have figured it out.

1. I gather that the WSSMs do not feed reliably?
2. They could work better if they did not have to fit into an Ar-15 length action?
3. Any info/link for DTech?

http://www.dtechuppers.com/

The other short-fat AR15 cartridge was the 30 Remington AR which was based on a shortened 284 Winchester case.  

At one point you realize there is little sense in trying to shoehorn a cartridge to magazine sized for 5.56x45 when making a new spec magwell and magazine is rather trivail problem 

I wish somebody would do that for Grendel ...

The WSSMs have feeding issues in bolt guns. Manufacturers had to go to single-stack magazines, and you still get reports of funky feeding.

Mr. T (MrT4) said:

At one point you realize there is little sense in trying to shoehorn a cartridge to magazine sized for 5.56x45 when making a new spec magwell and magazine is rather trivail problem

According to magazine manufacturers, designing a reliable magazine is not really such a "rather trivial problem"!

According to many gun designers developing a reliable magazine is one of the most complex problems they encounter

That's why many prefer to use existing magazines that works instead of developing their own

Back then USSR put a huge effort in developing the AK magazines that were inexpensive, reliable, sturdy and reasonably light. As a result, USSR had an excellent plastic assault rifle magazine well before anybody else (the so-called "bakelite" AKM magazine became the standard issue item in mid-1960s)

Maybe , probably , but has been done since mag fed guns were invented, the question is not really of the magazine but the decision to go for a cartridge that needs a larger or smaller one.

End of the day shoehorning different cartridges into existing magazines mostly doesn't work too well.