Re: Remington 700 caliber/action fit...
exhogflyer,
An un-modified Remington 700 short action factory mag box will take a max COL of 2.82". You can replace the OEM mag box with a DBM system like Seekins Precision and extend that to 2.94" for 308 head sized cases. Seekins DBM for WSM cases goes 3.14".
So look at the OAL of any case you desire, and if it is over those measurements then you need to go to the long action Rem. 700 or consider modifications to the short action to make it work. But as noted above there are limits.
To answer your question a 6.5x284 could theoretically go in either a long or short action. But if you use a short action, you will not be able to fully use it's case capacity, as the bullets would be seated too deep. Also some 6.5 bullets may not work in a SA 6.5x284 as they are not made to be seated that deeply.
Bob