Certainly, any high performance 6mm chambering will deliver a properly constructed light bullet well past 400 yards with sufficient velocity to result in explosive terminal performance, which leaves no question as to the instant result of a decently centered hit. However, such guns tend to generate so much recoil that we cannot see the impact unless the target is near the limit of our ability to make such hits. This means that on close-in targets we miss seeing the impact, which is what this game is all about. Moreover, such guns generate so much barrel heating that use is limited – when the shooting is good, we must frequently stop, to allow the barrel to cool!
For these reasons, most varminters opt for high performance 22s. The Swift is certainly up to the task, as is the 22-250. However, these numbers also generate sufficient recoil so that the shooter may not see impacts on close-in targets and barrel heating is still an issue. With best handloads, the 223 may be the best choice for most applications owing to reduced recoil (one can see hits at significantly closer ranges) and significantly less barrel heating (one can fire about twice as many quick shots without overheating the barrel). The problem is, the 223 simply cannot deliver explosive performance at 400 yards.
Modern
Alternative
I believe consideration of the above can give one a reasonable
understanding of why the 204 Ruger came to be. With best bullets,
it shoots usefully flatter than the 223 and delivers similar performance
at long range with similar barrel heating and usually somewhat less
recoil. Unfortunately, the 204 simply cannot deliver explosive performance
at 400 yards.