We didn't shoot enough rounds to see if there was any difference in the projo with or without the extension; in any case they all hit the target at 100 yds so they were both generally OK, but not super accurate. I say this after asking the gunner where he was aiming, and unfortunately he chose various points of aim because I didn't tell him to aim at the bottom of the bullseye like I do. That kind of makes a detailed analysis a waste of time.
Yes we do use a hole saw to make the sabots, but the 1/4" hole in the middle is then plugged by a 1/4 inch bolt, so no gas will get through that way.
Sand-filled PVC projectiles worked perfectly with our 65 mm Krupp; they were very accurate also. They were about two calibers in length. Our failed Hotchkiss 1.64-inch PVC projectiles were much longer in terms of calibers. I think also I should have used a smaller powder charge for the PVC projectiles, to give them a little better chance to survive the launch environment.
