Depending on the model...you can get the Encore down to just over 6 pounds naked although the Pro Hunters are heavier because of the longer barrel. As a single shot has no action, you can go with the long action calibres with no increase in weight. That's why I feel the 7STW would be perfect as it has ample ballistics with a sizeable projectile (better on windy days) and from all I've seen, it's super accurate, even with factory loads. There's no advantage to going to a short mag in a single shot as there is no action to save weight on. A .300 winchester mag will give identical performance to the .300 short mag. IMHO, you have way more calbres to choose from with a single shot as you don't have an action to contend with. I wouldn't go with a 270WSM in a single shot...only a bolt.

I'd go with the short mags in the bolt just because of the weight savings in the action and these calibres do seem to perform well out of 22 and 24 inch barrels. I don't think you'll match the performance of these short mags with any of the standard short calibres so why not go with the increased velocity and flatter trajectory.

If weight is of the ultimate concern, there are far more custom bolts available than there are singles and there are even loads of off the shelf bolts that make great sheep rifles. Personally, I just like the compact nature of a single but for most people, they are far more comfortable with a bolt.