If you were building a dedicated lr rifle that in stock form would be a short action would it be wise or possible to use a long action receiver and rebarrel and chamber to the sa cartridge? Reason would be to take advantage of the longer internal or external magazine?

It's been done.

If I had a long action, I'd build a long action cartridge, with zero free-bore. You're immediately gaining more case capacity with the long action cartridge.

If recoil is a concern, put a high end brake on it. You can load a long action down, you can only go so full on a short action.

6.5-06
6.5-284
284 Win
280
280 A.I.

7SAUM is another great round in a long action.

Lots of your 7 WSM and 6.5x284 rifles are built on a long action to use the long magazine box. Also, one of the branches of military built their 308's sniper rifles on a long action Rem 700, and recently were converting a lot of them over to 300 WM.

Yep a 284 Win running 195gr Berger's would be interesting.

Can't get em going fast enough to out perform a 180 gr. in a 7 Rem Mag. A 168 gr. would probably be better in the 284 Win, IMO.

All of my rifles that I built into a 7mm08 have been out of long action receiver. I did this for reload purposes.

I have a 6mm-284 built on an '06-length action, and because of it (and the way it was chambered, I suppose) I can seat quite a bit further out. I didn't have the rifle built, that's the way it came to me. Anyway, I can get quite a bit more out of the rifle the way it's put together.

.284 Win. is a beast. My dad has a custom on a military FN Mauser action with a Shilen barrel. Tack driver.