jbmauser and leeper, you are both incorrect about loosing the gas shield if you change to the pre 64 collar. The pre 64 collar retains the shield/bolt stop in the short action calibers like the 308 Win, but it is not such a loose rickety affair, the pre 64 stop is fixed onto the collar in a way that it is very solid and does not wobble around like the new one, neither would the bolt be coming back any further than usual since it still retains metal between the lug and action bolt stop, really no comparison between the old and the new.
Hope this helps.
bb