The 270 Allen Mag is based on the 7mm RUM case. Since I designed and built several of these rifles I have since become aware of some slight headspace variations in the 7mm RUM brass.
As such, I not recommend using 300 RUM brass as the parent case. This allows for a more positive shoulder to provide a gas seal should the original headspace measurement of the parent case be a bit short.
Basically with the 300 RUM case, you can set the correct headspace for the fireforming loads. With the 7mm RUM you are pretty much at the mercy of the factory spec ammo.
Also, using the Federal 300 RUM cases has also produced good results if you can find this bulk brass.
The 270 Allen Mag is best suited to the Rem 700 RUM receivers but any long action receiver can work as long as it has a magnum bolt face. I do recommend RUM receivers if a repeating rifle is desired.
Savage receivers work great but you will be limited to single shot rifles as the fat shoulder does not agree with the Savage magazine system, even in RUM chambered receivers.
I recommend Lilja barrels for all of my custom rifles
. Any top quality barrel will work though. I did have Dan Lilja tool up to build 1-8 twist 3 groove barrels specifically for my 270 Allen Mag.
THis fast 1-8 twist will easily handle the big 169.5 gr ULD RBBT and there is also a 185 gr ULD RBBT in the works from Wildcat Bullets for this round as well as the new to be released 277 Allen Mag.
If you will be shooting conventional bullet weights, the standard 1-10 twist is great. In 30" barrels the 270 AM has been able to drive the 140 gr Accubond to 3625 fps!!! THis is a hot load but you may have read the report of a mountain goat recently hammered with this combo. Seemed to work great!!!
If you want the extreme range performance, the 1-8 twist is the best way to go as it will allow you to use the heavy 169.5 gr ULD RBBT as well as the new 185 gr ULD RBBT when it comes out. You can also use conventional bullets in this twist as well such as all the Ballistic Tips and the 140 gr Accubonds. Thin jacketed bullets may not be able to handle this fast twist.
Let me know if you have any other questions and I will give you all the information I have.