Magnum rounds in short barrels

[wildfowl4444]

What kind of velocity loss would you expect when shooting magnum rounds in short barrels. I am talking about the 7mm Rem Mag and .338 Win Mag in 20" barrels.

[rem 700]

Not much power difference. Just accuracy difference.

[L. Cooper]

I think accuracy may be irrelevant in the shorter barrels, but velocity loss will be enough in a 20 inch that the whole advantage of the magnum will be lost. Certainly the 7mm will lose from 30 to 50 fps per inch off of a 24 inch barrell. Take off 200 fps from the 7mm Rem Mag, and you have the .280 Rem.

Not so sure about the .338. May do better because of the larger barrel diameter, but it will still lose velocity. My take is that if you are going to shoot a magnum, you need 24 inches (or more) to make it worth the effort.

[Tater]

I'd agree with the first 2 responses: why get a magnum if you're not going to try to get the most from it?

[Jack]

There should be no accuracy difference-many of the world benchrest records are set with 20" benchrest barrels.
I would guess that a 7MM Rem Mag in a 20" barrel would give you a very loud, expensive, hard kicking 280 Rem, 7mm-08 or 7x57, in terms of performance.
The 338 Win has a less extreme expansion ratio (ratio of powder charge to bore diameter) than many cartridges labeled magnum. Consequently, the loss in velocity per inch of barrel cut off might not be quite as bad. Still, I suspect you would get performance similar to a 338-06- and again, louder, harder kicking, and more expensive to feed.
I agree that magnum cartridges need longer barrels to burn all that powder- 24" or 26" makes sense. To me, at least, a 20" barreled magnum doesn't.