That is a very aggressive plan, but I see 3 errors. The first is light, if you cover everything how will you see the target. The second is ventilation, you need some air to circulate especially if you are going to shoot black powder. The 3rd is noise. It might get kind of uncomfortable noise wise on a totally enclosed range. Perhaps full length sides and a partially covered top might be a better plan.  And one other thing to think of is snow build up on the flat roof, that might also be a problem. Hopefully others will chime in and let me know if my views are all wet or not.
Sam

Considering where you're located, I'd definitely consider snow load! I'd at least want a peaked cover on it, and strong enough to contain the weight of whatever snow depths you get. 
Lighting could be as simple as running a 14-2 romex down the inside from a switch at the firing area, to turn on a light above the target. No need for any illumination throughout the cover, just at the target and firing line. I'd add an outlet at the target end for a plug in lamp, for easy maintenance, and also to run a tool if repairs were needed. 
Air is needed, so maybe a small attic exhaust fan at the target end tied to the same switch to pull the smoke away from the shooter, and exhaust it out one side near the target.

Anchored and cross braced, I wouldn't worry too much about snow load on a two foot wide roof, but snow drifts and blown debris might be something to keep in mind along a hundred yards of solid wall. 

I'd try to have a small second exhaust fan closer to the shooting end to clear the air quicker as needed, and figure out that there's a positive draw away from the shooter. Lot's of easily removable access panels, it would not be very easy to crawl through that sized tunnel to do some repair that would come up sooner or later. Maybe figure out if it wouldn't cost too much more to run it at the full four feet wide, at least for a stretch on the target end, to help a bit from damage, especially to get it on paper in the first place. 

Best of luck with the range, and make up a good shooting shed to keep out of the weather and secure stuff that you don't need to haul for every session.

Even if snow and wind do not damage your building, consider how much you are spending and how much it would last?
Regular plywood and 2x4's are not meant for outdoor construction with ground contact. I'd say that within 4 years stuff will warp, break, and rot.

Paint or protect the materials from UV and deterioration.  

Allow for the mid range trajectory height.  

Put the target outside at the far end and forget lighting the full length. 

Good luck, sounds like a spendy project if it is going to last very long.

I'd be more inclined to want to make a simple A frame type framework, and then use metal roofing to cover it. The A frame would be simpler, and also cheaper to build. The metal would not only hold up for many years, but would protect the wood framework from rotting away in a short time. Less maintenance, cheaper build, and longer life span.
If you keep an eye on local CL ads, you might even find used metal building material cheap, to cut down and use on your project.

Thanks for most of the responses. Chickens wouldn't last long around here!  I think I will try an 8 foot long section screwed to my shooting shack to try out the concept.