I just measured A swedish mauser mag its 3.230". I dont have the 284 case demensions handy. Im guessing its about the same or longer.
I load my swede rounds over 3".

One of the med. Muaser actions would be a great candidate for a 6.5x.284 because you can load to 3.0 or a bit more. To me, the M700 or sim. w/ a 2.9 mag box is just too short for opt. perf. of a 6.5x.284.