My lever action rifles use only iron open sights.
It's interesting how some assume that because the sight picture at 100 yards "blots out the target" that they must be useless.
Yet, our forefathers used them quite successfully to decimate the buffalo and harvest big game at this range and greater.
The reality is that you won't be shooting squirrels or gophers at this range with your typical .30-30 or .30-06 moose and deer gun.
If you are shooting same, then you must either have exceptional eyesight or will be using a scope on a bolt action varminter.
For ordinary targets, or big game however, the target is large and quite visible to the naked eye.
Yes, open sights will cover up a large portion of the animal or target, but you can place the front bead in the center of the paper or kill zone without difficulty.
Further, you can sight in so that the actual area targeted sits on top of the bead (A six o'clock hold) to gain more visibility.
But, as they say, the proof is in the results.
My 100 yard targets are usually just an 8 1/2" x 11" sheet of white copy paper which is large enough to see clearly and to center the sights on.
From the bench I could achieve groups that were 3 inches or less in diameter with an accurate rifle, as a younger man.
Even now, I can still group around 4 inches with old eyes and a fuzzy target.
I have one target shot with a then brand new 1894 Marlin in .44 magnum, using hand loaded 270 grain JSP bullets, where the group measures 1" wide by 4" high.
The elevation dispersion likely due to the front barrel band affecting the impact because it was a little tight as the barrel heated up with each shot.
I also recall a 100 yard target shot with a never-issued Polish-made Mosin Nagant carbine, which measured about 2 1/2" in diameter, shot with military ball.
Also, I recall repeatedly shooting 6" groups off-hand at 100 yards, using an AR-15 A2 with military peep sights.
Certainly, if you can group 4" off the bench at 100 yards, then you will get your deer or moose as certainly as any MOA scoped rifle at that range.
In the end, there is no shooting bench in the field, and it comes down to the shooting skills of the hunter, which can only be attained by practice.
Certainly, if you are shooting at 150 yards or beyond, a scope becomes a practical necessity.