Someone above talked about age and hard to see his sites. I have the same problem so went with a 1x on line. A peep with a small aperature works also, much like a higher f stop on a camera lense. Unfortunately like a camera lense it cuts a lot of light. I have been looking for a flintlock, but wow have the prices gone up.
Someone mentioned a pie plate at 50 yards. Read some of the old literature. Lewis and Clark reported killing deer with the air rifle they had a long, at 100 yards and further with their flintlock.
Many people are unfamiliar with how long air rifles have been around. Napoli an used them, and it's reported that a couple in a museum still hold air. Lewis and Clark is in the Winchester museum in Cody, Wyoming.