Basically these two rounds were an idea to get more performance in a standard length action. That being one designed for an OAL of 3.400" which is what the Ruger M77 and Win M70 standard length actions will accept.
The parent case is the 300 Dakota case which is 2.550" in length. The case id modified to the same Allen Magnum configuration in body and shoulder specs.
Being roughly 10 grains less in case capacity then the Allen Magnums, the Allen Xpress rounds will be more user friendly as well. In the 257 AX, the 100 to 120 gr bullets will be much more user friendly then in the big Allen Magnum and still offer at least the same performance as the 257 STW will offer but in a true standard length receiver instead of a long action. In the Rem 700 receiver this will not matter at all but it will offer high performance with slightly longer barrel life. While certainly no where near the equal of the Allen Magnums, the Allen Xpress rounds are still on the top of the performance heap on their respective calibers.
I am currently debating on releasing a 270 Allen Xpress as well but need to do some measuring to see if the longer bullet nose will allow them to be chambered into a standard length receiver. We will see. Would offer 270 STW performance.
The head diameter of the 300 Dakota brass is the same as the RUM except that it has a true 404 Jeffery rim diameter. On the Ruger M77 rifles, there is generally enough room to not require any bolt face opening. The extractor does need to be relieved for this though but thats an easy job. Just a little stoning needed there. Other bolts may need to be relieved slightly but when accurizing a receiver this is a 15 second job since the bolt is already dialed in for machining on the lathe.
In my test rifle, the formed Allen Xpress rounds feed very nicely from the unconverted 7mm Rem Mag based Ruger M77 MkII. It will only accept two rounds in the magazine though for a total payload of 3 rounds with one up the pipe.
Still I don;t know to many guys on this board that ever need more then two rounds at most and generally only one so there are plenty for reserve!!!
Here is the first look at the 6.5mm Allen Xpress. On the left is a 264 Win Mag loaded with the 140 gr A-Max. Middle is the 6.5mm Allen Xpress and on the right is the big 270 Allen Mag loaded with the 140 gr Accubond.
If you have any other questions feel free to ask.