Results in either chamber will be minimal since the 308 case (designed in the '50's) is already pretty much an 'improved' case (developed from the 300 Savage) compared to original 8x57,7x57, 30-06 type cases. It will be difficult to gain much more than 4-6% in capacity by 'improving' and since velocity increases at about 25% of capacity gain, you could be looking at little more than a 1% velocity jump. Then you figure in velocity lose for the short barrel (compared to a rifle barrel) and it doesn't really seem worth the effort and expense.

 

Blackhawk makes some good points. I think he might be alittle generous with an estimate of 4-6% though. There just isn't much body taper to work with. The 40 dregree shoulder will gain some, maybe 1 grain. The biggest gain, will be less case trimming.
Now if a wildcat is your wish. In a 30 cal., the 30-40 Krag AI is a fine choice for the Encore. And I think you will be suprised by it's performance. It will come very close to the 308.

 

I forgot another aspect though, considering the 308 and 7-08 case in the Encore. You can put a longer throat in it, and seat the bullets out. Thus gaining more than "improving". Mostly with the 165gr. and above, more so with 180's. I haven't done it, so how much gain is another ? :wink:

 

Neither will really gain you anything in performance to offset the cost of the dies you'll have to buy. A .30-06 or .280 Rem. will definitely out perform them and with the Encore no reason to stay with the shorter case. You'll find more variation in velocity between two similar barrels of same chambering than you will between those improved versions and the parent case.

GB