Reply To: RAW .25 cal, JSB Heavy (33.95 grain) BC, Measured (Polygon Barrel)
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Today I decided to retest the BC of my JSB Heavies, Mk I, through my polygon LW barrel after recently taking the hammer spring tension back down to how Martin had it set when he sent it to me (I wanted to see what velocity it was now shooting at, so I figured I’d recheck the BC also). Since the last time I checked the BC’s I have received a tripod adapter that I can put on my chrony and mount the chrony to the tripod-which allows much more precise placement of the chrony for measuring velocities.
Anyway, after taking out the HS preload that I had dialed in during my initial tests, I measured my average velocity at 1 yard to now be 886.0 fps (front sensor was 2 ft from the muzzle, rear sensor was 3 ft / 1 yard). The shots were: 890.9, 883.2, 887.3, 886.1, 883.5, and 885.0 fps (average was right at 886.0 fps, for 59.17 fpe). I then moved the chrony out to 50 yards (at the back sensor) for a delta of 49 yards. The shots were 760.0, 761.7, 762.7, 761.4, 763.5 and 777.0 fps (average was right at 764.4, for 44.08 fpe).
Resulting BC now matches much more closely to what others have been reporting: 0.0517. From some of the reading that I have done (via Yrrah’s posts, and others) it looks like I was placing the chrony too close to the muzzle for initial readings previously. Here’s the calcs: