It is a tweener but due to the length of it's case 2.68 inches; it is considered a long action cartridge. Current factory .220 Swift ammo overall length is shorter than factory .308 ammo but it's case based off the 6 mm Lee navy is .4965 inches longer.
The .225 Winchester was a short action almost comparable cartridge. I believe Buffalo Arms is the only place you can buy production ammo. They discontinued making rifles in 1971 and I believe they no longer make cases for the ammo.