Depending on what you are considering the defining features of an AR-15 type there was for example a 20 round repeating airgun that was around during the period [0]. Lewis and Clark are believed to have used one.
Although using a fixed magazine, I don't think it would have been too much of a stretch to imagine a detachable one - especially as the air reservoir was.
Energy-wise it's about 10% of a standard 5.56x45mm round, but still apparently quiet effective.
Comments
Depending on what you are considering the defining features of an AR-15 type there was for example a 20 round repeating airgun that was around during the period [0]. Lewis and Clark are believed to have used one.
Although using a fixed magazine, I don't think it would have been too much of a stretch to imagine a detachable one - especially as the air reservoir was.
Energy-wise it's about 10% of a standard 5.56x45mm round, but still apparently quiet effective.
[0] https://en.wikipedia.org/wiki/Girardoni_air_rifle