I had a Rem700VLS in 6mm (same case as .257 Roberts) that was a short action. What I found wasa bullet loaded out to just off the rifling would not fit in the magazine. That tells me factory ammo has a pretty short overall length and, although I don't know about the .257 for sure, could be a hangup as far as shooting tight groups with factory stuffis concerned.If ammo is designed to work in short actions, as well as long actions, then there may be long jump before the bullet engages the rifling. I've never seen that work real well for good accuracy. Just a possibility, but there are a few rounds that are made 'short' so they work in all guns.