Elvis,

According to the 4-to-1 Rule, the amount of powder space available in the longer-seated version of the 6.5-.284 is about 5.5 grains--an increase of about 10%. This results in a potential velocity increase of about 2.5%, which translates to about 75 fps in the same barrel length, when loaded to the same pressure.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck