Here are some initial test results with two pellets, JSB 15.9 gr. Exacts and JSB 18.1gr. Heavies.... With my Disco Double I have the ability to change the velocity over a wide range very easily by varying the amount the retractable bolt pulls back to clear the barrel port.... My testing is limited to 20 ft. indoors until the snow clears in a couple of months, so I tested for shift in the POI at various velocities.... I did the tests with the 7/16" OD Crosman barrel free floated, and again with it in tension, with the shroud free floated, and then with the shroud supported about 60% of the length out from the breech by a barrel band with an O-ring.... Group were 5 shots, and sizes were overall tighter with the barrel in tension, and slighty tighter again when supported by the O-ring barrel band, but to be truthful they are all just small ragged holes, so much further distances are required to evaluate the group sizes....

However, it was amazing to watch the group wander horizontally and vertically across the paper as I changed the velocity.... With the Exacts, the group moved 0.26" to the left from 638 fps to 871 fps, and then 0.11" back to the right at 1048 fps.... Once tensioned, the total movement left to right shrank to just 0.04".... The Heavies had 0.18" of horizontal movement with velocity, but it also was cut to just 0.04" when tensioned.... The vertical movements of the groups was about half the horizontal movement with both pellets, but it was reduced to about a third of the movement when tensioned, less than 0.04" with both pellets.... Here are the results....



The vertical axis is the distance the group moved from it's average position in inches, and the horizontal axis is the velocity.... The upper graph is the vertical movement of the group, and the lower graph is the horizontal movement.... The solid lines are with the barrel tensioned, and the dashed lines are without the shroud, just the 7/16" barrel floated on its own.... The red lines are the Exacts, and the blue lines are the Heavies.... Overall, the barrel has only about one quarter of the deflection as velocity changes once tensioned, so just on that basis the experiment is a screaming success as long as the groups were not worse, and that is certainly the case, they are definitely tighter as well.... There is no doubt in my mind that tensioning the barrel in this manner drastically tames the whippy Crosman barrel....

I did some testing with the Heavies at 980 fps, with various tensions on the Bellevilles, both with and without the shroud support by the O-ring barrel band, and there is significant side to side, and some vertical, movement of the groups as I change the tension.... and as the group size changes as well, but not by a lot.... It appears at it's worst to be as good as before, and at best basically a tiny 1/4" hole.... Both with and without the O-ring 1 turn of tension on the Bellevilles (about 300 lbs.) seemed to be a sweet spot, which is why I used it for the POI testing....

Bob