Shawn68,
The 6.5mm RUM is about as large a powder capacity as you can use and still be somewhat practical in the results. My 6.5mm Allen Mag is based on a blown out, improved 338 RUM case so it is shorter by 90 thou then what a 6.5-300 RUM would be but its capacity is basically the same.
I went with the shorter 338 RUM case so that it would give me the same capacity but in a shorter case allowing me a better OAL for a repeating round.
Our current 6.5mm AM has a 30" Lilja 1-8 twist and it is my wifes long range rifle. We are using a relatively mild load to get a legit 3540 fps with the 142 gr ULD RBBT. This is only a temporary load as I have some prototype 168 gr ULD RBBTs coming from Richard Graves of Wildcat Bullets to test in this big 6.5mm.
As others have said, it is also my opinion that anything lighter then 120 gr of bullet weight is really a waste of time in this type of round. The 120 gr X bullets are a pretty good choice as they are much longer then a conventional 120 gr 6.5 bullet and will obviously like the velocity potential.
I still feel that for long range consistancy, nothing helps these large capacity small bore rounds like alot of bullet weight.
Simply put, with todays powders and bullets, this is a practical chambering, just do not use it for a varmint rifle or you will be rebarreling it every season!!! /ubbthreads/images/graemlins/grin.gif
As a big game rifle, they will last many seasons easily.
Good Shooting!!
Kirby Allen(50)