There is 4150fps but with 50gr, 4092 with 55gr and 3710 with 70gr(barrel length is not mentioned), larger cases than 243 are too big and velocity is not much higher, great example is to compare Saeed's data for 22-243 middlestead and 22-6mm remington here http://www.accuratereloading.com/reload.html
It sure looks like a "short magnum" case. ie:338 Win,308 Norma, 7 Rem Mag,264 Win Mag etc.Approximately 2.50" long? Just judging by the picture on the website.The velocity advantage over the 240 Weatherby,which has nearly the same case volume as the '06, looks like about what you get with a "short magnum" case,over the '06 case.Looks like the 257 Arnold uses the same case.It only gains 100 fps over the 257 Weatherby which is about what you could expect with the little bit of extra powder capacity you'd get by shortening the neck and tossing the radius shoulders of the Wtby case,which is all the "short magnum" case really does.Anyway....just my opinion by looking at the pictures and velocities. Anyone else have a theory to share on this case?
It is a necked down .264 Win Mag case. Guy I know has the .257 Arnold, it's a real smoker. Has a rifle built by Arnold and it is as fast and accurate as Arnold claims. Have to neck turn as they have relatively tight necks.
