would be interesting to check with a chrony if the last shots would be faster then the first one.not only because of the bands getting warmer, but of the loss of friction from the weight of the bolts in the magazine.
Compleet crazy, but so much FUN !!All the best.
have you ever considered using pearl for the handle. probably not real pearls though.
Did you make this because of what you saw at that knife and gun show in nuremburg? I remember those swedish guys made some kind of harpoon gun.