Originally Posted by Trickymissfit
there are several options to consider:
* the age old 6mm Ackley Improved. Just a good round in everyway. You can take this reamer and make a 6mm/6.5x55 Ackley. Probably just as fast as the 57mm case. Of course you could do a .243 Ackley that would have the .35" 6mm neck length. You can't make cases out of .243 brass as the over all length will be about .110" longer due to the better neck design. Another option is the 6/250AI, but with the longer neck design. This one can be done with .243 brass, and will push a 105 grain bullet to 3000fps.
* you should order in the finish reamer and reamer to cut the die blanks (which ever you decide). Many do this with a roughing reamer. I wouldn't let the tight necked chamber bother me much. They're usually a .264 chamber size, and a .262 die size or a .266 chamber and a .264 die diameter. Still remember that you have to order the reamer anyway, so you can also spec a no turn chamber.
Might ask a reamer manufacturer if they have prints for the 6mm Arch. It's very close to the 6mm/55 Ackley (he actually never did this one, but did his own 6.5 case design)
I'm building a 25 inch medium taper barrel hunting rifle using a Winchester Model 70 intermediate length action, so I have 3.0 inches in the magazine well. Being a hunting rifle as opposed to a varminter or target rifle, I want it to feed smoothly. That is why I am steering away from the 40 degree Ackley Improved chambers with their wide shoulders to 26-30 degree shoulders that are not as far forward and not quite as wide. I like the long neck and 26 degree shoulder angle of the 6mm Remington case, but it has too much taper. I am planning a 1 in 10 twist and will be starting off using the 95 grain Berger Hunting VLD. If that doesn't do well, then I will try 90 grain or even the 100 Sierra GameKings. The .243 case would be ok in a short action, but in an intermediate length action using only 90-100 grain projectiles, it just doesn't have enough case capacity, even in AI format. I expect about 3200 fps with the 100 grain projectile out of 25 inch barrel.
I've just received the print for the 240 Page Super Pooper, and despite the stupid name, it seems to be about perfect. The neck/shoulder junction is 20 thou' below the neck/shoulder junction of the 6mm Remington, so I won't have any headspacing or fireforming difficulties using 6mm Rem brass. The shoulder diameter is 0.45 which is similar to the 6mm AI but not as far forward and with the 28 degree shoulder it should feed better. So it will have almost as much capacity as the 6mm AI but should avoid the potential feeding issues. The neck diameter is .277 at the top of the neck and gradually tapers out towards the bottom on the print so I won't have to turn the necks (I hope), even though the 6mm Remington is supposed to have a .276 inch neck diameter at the top and bottom of the neck.
I asked for a finish reamer and was told that they can supply me with a "high speed steel" reamer. Is a "high speed steel" reamer a finish reamer?