"Be aware that there are open rear sights called “express sights” that have folding leaves for ranges past 100 yards. These are a superb waste of money, as dangerous game is almost never shot from beyond 100 yards, and usually much closer, and the useless little leaves have a habit of folding flat whenever they feel like it. An open rear sight is properly set to hit dead on at 100 yards, or 50 yards, and then left strictly alone."

BULLSHIT! Stronger words should follow. I have shot 2 1/2", 5-shot group at 200yards with my 200 meter leaf on my 14 bore rifle. It's a muzzleloader, patterned loosely after an 1950-era English Sporting Rifle. Using must the lighter 140gr. 2F GOEX charge, the 200 meter leaf is zero'd at 200yards.
With the 300 yard leaf, no one in the club who has attempted it, has missed the 300 meter plate using that leaf.
The leaves on my sight remain in the position I place them, until I either open them up, or fold them down. Same happens with the leaf on my bro's Joseph Lang rifle made in 1983.