Whats the shortest you could get a away with?

18" that, of course is dependent on your range needs

I've been looking at the ruger mk77 compact with a 16" barrel.
Looks like a really nice truck gun, paired with a leupold vx2
ultralite silver 3-9x would make for a really nice package.

The ballistics are very good on the 123gr ammo too. I was getting
around 1240 fps at 1000y out of the calculator.

Weigh your pupose of the firearm. If ya shoot inside 500yds you can go 16". Past 500 increase accordingly. With barrel length you only give up velocity. Mine is a 20" as I have the opportunity to shoot to 600.

Legally, 16". But I did some load work-up on another forum for a rifle group buy with an 18" barrel. I got 2701 fps with an 18" barrel and 139 Scenar. Normally, that load runs about 2780 to 2820 fps in a 24-26" barrel. It shot good. If you want a short barrel 260, it will work fine. You just loose a little velocity, but gain a lot by having a handy rifle.

Just FYI, rule of thumb is about 15 to 20 fps per inch gained or lost with barrel length.

I have a rifle just like the one Gravytrain mentioned. That Ruger Compact in 260 has become a real favorite of mine. The 16 1/2 inch barrel is Ok, though I'd prefer 18 inches or maybe 20 inches (for added velocity). It's a super handy little rifle and perfect for carrying around and for use in a tight blind. I had great success last year with the 120 grain Nosler Ballistic Tip over a near max load of Reloader 17. This year I'll try the 100 grain Nosler BT over a very warm load of IMR 4064. Everything I've shot, large and small pigs, deer, and coyote, all fell straight down. Nothing ran more than 3 steps, and the shots ranged from 40 yards to 400 yards. The scope is a Burris FFII 4.5x14, and I'm happy with it too. The rifle seems to be every bit as effective as my 270 with 130 grain Noslers at 3000+ fps. The ONLY problem with my Ruger Compact 260 is that every time the Grandson comes to hunt with me, he grabs that Ruger and I have to go back to my heavy old Sako. He's got a perfectly nice 308, but he wants my 260. Darn that kid.