The telescope would allow one to zoom in their view to about twice as large as normal, similar to the Optifine zoom function. It could be attached to either a musket, blunderbuss, or ballista, and would cause gun to become slightly more accurate, as well as make muskets and ballista zoom in 2x, and blunderbusses 1.5x (since they're shorter range) when the gun is readied (right-clicking). The downside is that damage dealt would be reduced by 1 heart of damage per bullet.
I'm not really sure that works. Not that I'm opposed to a spyglass of some sort, but I think that rifles with scopes is getting uncomfortably close to where a lot of people would rather any hypothetical firearms not go.
(On a side note, seeing a character with a sniper rifle on Mojang's April Fools art made me shake my head. Shame on them for not treating this as a serious issue. There has been a lot of effort to approach this subject in a mature fashion, and I actually found them treating the concept as a joke to be insulting.)