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We used old steel sprinkler heads with 3/4” 6” nipples. Slide a firecracker down the pipe so the fuse pokes out the nozzle. Roll a marble down the pipe, light the fuse and whammo!
I was more interested in the little dark circular marks and what causes those marks, rather than the marbles themselves, as I have them on a handful from various makers. Initially I thought maybe caused by dirt marks from rejects tossed out, but dirty rollers, as suggested above, makes more sense to me now.
Of the two images I posted showing mine having the circular marks, the first is a Rainbo. The second, I have no idea. It's half brown with the other half no color, as shown, but certainly not a Pelt. More likely a Vitro, or perhaps foreign. I'll dig it out and get a couple more pics taken.