Roland,
I have shot .38 Special b.p. loads (158 gr bullet 21.5 grs b.p.) at 300 meters and they did shoot well and arrived with enough punch to knock down the steel javelina. I have not tried them at 500 but I would venture to say they will certainly reach that far.
Ballistically, it is fairly close to the 44-40 (1,200 vs 1,300 f.p.s.) and the bullet, although lighter, has pretty close to the same b.c. (ballistic coefficient) and would arrive at 500 at pretty close to the same velocity but with about 25% less authority.
Jack,
Sorry I didn't see your post earlier. The sling was a gift to me several years ago. I have checked it for a source but there is nothing on it.
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