twist rates, high bc pellets

What on earth does he mean long twist rate? I do not know how to translate 500mm twist into English,can anyone else understand what he means?

SendinSlugs,
One revolution per 19.685 inches... or roughly a 20 inch per twist. 

Thanks addertooth,I could not interpret that from metric into english. 

That has me thinking,I recently checked my Maximus.22 twist rate and it seems to be a 1-22" twist which I couldn't believe that I checked many many times. Seems to be a departure from the standard 1-16 twist on most .22 barrels and if this information is correct I should see stable JSB 25gr while in flight. I think I will order some and try. My max might not shoot it fast enough tho I've modded the gun to shoot a bit faster than stock but don't know if it's fast enough for these.ive ported the TP ,swapped hammer spring,switched the value retaining screws,which there are only 2 of so I can accept a 2400psi fill,I won't try any higher than that.its pushing 28ft lb with JSB heavies so hopefully I won't be wasting $10 on pellets.