how about the 6.8 SPC necked to .257?
 

it all ready works really well necking it up to .308, so why not go down a small amount? Anyone have any guesses as to what it might do?

Should be about like a 25/35 I would guess.
I would at least have one of the few quarterbore AR's around.

It's been done:

25 Remington...factory chambering in 1906---equivalent and ballisticall identical to the 25-35

Factory ammo was discontinued in 1950

And then it was re-done:
http://www.68spc.com/history.php
".....After a lot of work on the 7.62, they started with the 30 Rem cartridge as a basis for a totally new cartridge and tried the basic combination with a variety of bullets including: 25 cal, 6mm, 6.5mm, 6.8mm(270) and 30 cal."
This is an exerpt of the origins of the 6.8SPC

If you want a harder hit with just a barrel swap..look into 6x45
No differrent mags or anything from a standard AR...just a different barrel.
This was the cartridge that the M249 SAW was designed to shoot.
60 grain bullets at 22-250 velocity and 100 grain bullets at 3000ft/sec
EDIT: Woops 100 grain bullet at 2700ft/sec...should have checked...sorry
The largest I shoot is 80's at 2900ft/sec

Not quite a quarter bore....but the 25TCU is the same thing just in 25 cal.

6x45? must be a different cartridge than I was imagining. I'll have to do some research but I thought it was just the .223 neck up AKA the 6mm TCU? if that is so I find those numbers hard to believe. Of course I could be basing this on complete mis information. The more I think about it the more likely it seems as the SAW was intended for a unique cartridge that was not based on the .223.

Guess I will do some research.

More to the point, I am currently playing with a .256 win mag and a rimless version running very high pressures (based on .223 cases) and I was looking for something to put in an AR upper in .257 caliber. I am liking the looks of the 30 Herret rimless tactical (6.8 spc necked to 30) as it is an easy fit in the AR (obviously) and it will drop down and do the whisper end of things, and reach a little higher as well. I was just thinking on maybe taking it the other direction, especially since I currently swage my own projectiles in .257

http://en.wikipedia.org/wiki/6_mm_SAW

check the link the 6mmTCU is totally different from the SAW cartridge

and read the links inside the link i posted above also the 6x45 can barely reach 3200fps with a 60gr bullet and just a tad over 2600fps with a 100gr bullets so its about equivalent to a 223 with a 55gr and 90gr bullets