Case Capacities
6.5 Rem Mag = 69gr
6.5-280AI = 70.5gr
6.5 SAUM = 70-72gr depends on brass used to form
300RCM = 73.5gr (might loose some capacity necking down)
6.5-270WSM = 77.8gr
264 Win Mag = 79.9gr

The reason the 6.5 Gap 4s is built off the SAUM case is because its shorter and will let you load bullets out further and not take up case capacity. With the RCM case at 2.1" your going to have issues unless you built it on an action that can handle 3.0"+ COAL.

I see the 6.5 PRC as an answer to a question no one asked. Even if it has factory ammo it doesn't make a big difference because it wont work on a Rem 700 SA, it wont feed out of most AICS Mags @ 2.88 or @ 2.95 that people use in Comps.