
The green part is the rear 3/4" of a 12 gauge shotshell hull.
The fabric is a a 3x3 cut from a T shirt.
Hot glue holds the 3" 10d nail to the hull on the outside and the point of the gathered fabric to the inside.
Total weight 164 gr to 182 grains depending on the shotshell brand.
At 600 psi the one in the photo was clocked at 924 fps.
I also got 881, 723, 706 fps with other color/brand hulls.
900 fps is really traveling with 164 grains.
http://en.wikipedia.org/wiki/.38_Special
158 gr (10.2 g) LRN 770 ft/s (230 m/s) 208 ft·lbf (282 J)
BTW This was done with the Trom-Boyn without using the spring operated BV pilot.
I discovered that the spring was not worth the effort after I loosened the BV.