a 338 ultra mag necked up to a 270.would it be a viable wildcat?or not? what do you think?

There is already a 270-338 Winchester Magnum, and a 270 Weatherby Magnum, and a 270-300 Weatherby Magnum.

I can't see any advantage to coming out with a .270-338 Remington Ultra Mag...

Go one caliber higher and you have the 7mm Remington Ultra Mag...

"Just as viable as the 7mm RUM."
That's a lot of powder space for a .277" or .284" hole, either. But if that's what you like, then that's just what you'll get.
If you mean will anyone buy it, I'm not sure many will buy a 7mm RUM. The barrels can't just keep getting longer, and for hunting purposes, there are some pretty damn flat-shooting cartridges already, like the STW, with a 160gr at 3250 fps. Not sure how much more you can get out of a hunt-worthy gun.
Just my 2 cents.

Actually, my dad has two STWs(don't ask why, he likes redundancy), and both of them get over 3400 fps with 95+ grains of IMR5010 and a 160gr partition. As far as I'm concerned its almost worthless unless you just really want something dead quickly. He shot a blacktail in the neck with it last year, and I was picking vertebrae out of the things hams when we cut it up. They do fall quickly, but what a mess when you open them up.

Personally and this doesn't have anythign to do with the original question but it does address the post b4 mine.

As to 160 bullets going 3400 out of a STW--I'd be darned careful with that-either he has a long long tube of your chrono is off-or he is flirting big time with disaster!!

Just my unsolicited thoughts...

"GET TO THE HILL"

Dog

Well, the ones I chrono'd at 3500 were pushing it. Primers fell out, there was brass flow, etc(although the bolt was never sticky). But the ones at 3400 had none of these pressure signs. As for my chrono, I actually wondered about that, so I borrowed a buddies unit that is identical to mine, and a buddy of mine who has a Beta chrony. All three were within a couple feet per second when lined up back to back. I know others who get similar balistics out of the STW. I'm sure were over max pressure, but 70,000 psi never hurt a gun or brass in good condition.