I think if you're going to build a sheep rifle and be as light as possible, then go with a short action. The 6.5 Creedmoor is only 100-150fps behind the 6.5-284, but works perfect in a short action. The 6.5x47 and .260 work well too, but the Creedmoor has more support and options for brass and is a smaller package than the .260 Rem. Even a short action with a mag bolt face would be great, then you could run the 6.5 PRC or 6.5 SAUM. An extended mag box would really benefit the short mags and then you'd have 100-150fps advantage over the 6.5-284.
If I was building a rifle from the ground up and was not limited to a standard bolt face and wanted a long action, then I would consider again the 6.5 PRC or SAUM or the old .264 Win Mag. Even with a short 22-24" barrel you'll be as fast or faster than a 26" 6.5x284.
The .284 Win or .280 Ackley ,as mentioned above, are great choices too for a mid sized long action cartridge. The 7SAUM or 7WSM are both contenders as well, and would even work on a short action with an extended mag box or with the 150gr class of bullets. You have a lot of options. I think the .300 WSM, though an excellent cartridge, is not needed for sheep and would not be worth the recoil in a light weight rifle.
If I was building a rifle from the ground up and was not limited to a standard bolt face and wanted a long action, then I would consider again the 6.5 PRC or SAUM or the old .264 Win Mag. Even with a short 22-24" barrel you'll be as fast or faster than a 26" 6.5x284.
The .284 Win or .280 Ackley ,as mentioned above, are great choices too for a mid sized long action cartridge. The 7SAUM or 7WSM are both contenders as well, and would even work on a short action with an extended mag box or with the 150gr class of bullets. You have a lot of options. I think the .300 WSM, though an excellent cartridge, is not needed for sheep and would not be worth the recoil in a light weight rifle.