Man-o-man OP you kicked open a door with this one that could take pages to fully examine.
First of all don't eat that about the Buffalo rifles are the only thing worth putting a vernier site on cuz that's horse-sh!t LOL. Fact is, not all lever guns will hit an 800 yrd gong but same goes for a $3500 Sharps...some will & some won't. For an excellent reference to long range shooting with lever guns go to both Marlinowners.com and also LevergunScoundrels, look for any threads mentioning "The Happening at the U-Pick Sagebrush Ranch" yearly shoots that were held until Wind died a couple years ago ( they might still be held as a tribute to Wind by his buddies). These boys shot nothing but lever guns at 400-1000 yards for a week every year, and used every sight from tang verniers to tang Marbles types to Skinner receiver mounted to plain ol' original barrel sights too a great degree of success . If the Quigly pail shot is possible with a Sharps, these guys could surely do it with their lever guns.
Personally I have a love-hate relationship with the Verniers... quite simply they have been used since the 1850's because nobody has come up with a better option that wont hide the target with that big ol' barrel at further distances....BUT...they are not a simple "install and go shoot something at 800 yards" proposition. I have come up with a better "long range" option for one of my Lever-guns but its not going to be a commercial offering because it expensive to build and the howls of derision that it will bring out of the "purist" aboard would be deafening, however the limited amount of testing I have done so far has been very encouraging.
To the OP I will say, don't think I am trying to discourage you from going down the Vernier road as that sure isn't what I'm suggesting as long range shooting with a pumpkin chucker is something every owner should enjoy. It's just that you should be aware of a few idiosyncrasies that can discourage you if your not prepared for them. First off, a vernier can have more parallax (for lack of a better description) than the worst scope you could ever buy but as with a scope with parallax, diligent posturing & repeatable positioning can overcome the worst of it. The fact is that you can lean over a bench with your sand bags and shoot your rifle in at 100-200-300-400 yards and mark your vernier for the appropriate height at each distance. Now stand up and shoot the same distances at the appropriate vernier markings (free hand or with the "sticks" it matters not) and I will bet money that you may be hard pressed to hit 20% of your targets at the different ranges. The problem being is that going from sitting to standing you have changed the eyeball-front sight- target relationship and this throws your vernier sight-in session to the wind.
There is a simple exercise that a neophyte vernier user can do to prove my point and can be done on the kitchen table or down a hall way. Set your rifle on a stack of books the same height as you would with a sandbag and rest the butt on the table. Have your vernier set for a 100 yard distance and snuggle up to the butt stock same as you would for real, now sight the rifle on a spot or something on a wall or out a window same as you would for a real 100 yard shot. Now hold the rifle still and change the vernier setting up for a 300 yard shot...now, remember not to move the rifle and look thru the vernier at the front sight still on the distant target( now your imagining that target is at 300 yards)...if you were to pull the trigger on that 300 yard target would you hit it...not a chance, even though you have a plain 300 yard sight picture you haven't changed the gun elevation...it is still at the 100 yard trajectory setting.
Now it should be evident that unless the plane of your eyeball-front sight-target is absolutely the same at any distance at the sight-in posture, the vernier aperture cannot be placed with any certainty of repeated hits. The other way of saying the same thing is that you can set your vernier for any distance you want and place your face against the stock and get a perfect sight picture at any distant target with no guarantee that the rifle orientation will be correct for the trajectory required.
