Re: 284 winchester reamer question
I would load the AB like you stated (w/o powder/primer).
Then, measure to the ogive of the AB using something like a Hornady OAL gauge with the insert for your caliber.
Next load a JLK to that same depth as measure with the Hornady OAL gauge.
Finally, confirm that the cartridge OAL to the tip is within your expectations for magazine length limitations.
...or, perhaps I'm oversimplifying?