One thing to consider about the 10 gauges scope mount is that the factory drilled holes are forward of the chamber, making it somewhat of a forward "scout scope mount" position. Depending on the shotgun scopes eye relief it may or may not work.
The range of the 12 and 10 gauges depends on the combination of load, choke, gun, target, and the shooters confidense in the pattern. It's pretty much subjective since there's isn't a standardized target and peoples cut off, 3 pellet in the brain or spine, 6 pellets in the brain or spine, or 10 pellets in the head and neck, etc etc etc. It seems every turkey hunter has a different target and pellet cutoff. One guy might consider a gun to be good to 40 yards another might consider it 55 yards.
I wish someone would come out with a standard target and standard pellet cutoff, that weather you agreed with it or not, had something to compare turkey guns with.
Also, just like any other brand, take remington for instance, one 870 could have a range of 45 yards max no matter what choke, load, etc, while another 870 is good to 50+ yards easy with almost any load and a few different chokes.
So I'd get the shotgun, 12 or 10, that fits your needs best and then worry about the pattern when you go to the range with a bag full of different loads and a few different chokes to try. Then find a couple that pattern good and determine it's max range depending on how comfortible you are with the patterns shot at different distances.
Also, and this is not 3" mag verses 3.5" mag talk, try some 3" mag loads in your shotgun along with the 3.5". Alot of shotguns pattern 3" mag loads better than the 3.5". More shot payload does not mean denser patterns, alot of times it mean more dispersal (bigger patterns) or worse, holes, unevenness with dense and sparse spots, etc. You want to find that "sweet spot" where you get as much payload down the barrel into the densest most even pattern you can. That may be a 3.5" mag load with one choke/barrel combination, or a 3" load in another choke/barrel combination.
I personally believe that finding the "best load for any 12 gauge 3.5" shotgun is never ending. With the number of loads, both 3" and 3.5", diversity of chokes, pellet materials, etc I don't think you could ever try every possible combination. The 10 gauge, depending on the area, is much more limited to extremely limited in variaty of ammo and chokes.
Good luck with your choice. This past year was my first year turkey hunting with a NEF 12 gauge (used to use a Remington 870 super mag in the past) and loved the little NEF's light weight, easy carry qualities. You'll enjoy hunting turkeys with either the 10 or 12, so you can't go wrong.
later,
scruffy
