RAW .25 cal, JSB Heavy (33.95 grain) BC, Measured (Polygon Barrel)

Update: I took the day off today to go speak at the Wisconsin State Capital regarding a proposal to allow ‘high powered air rifles’ during gun-deer season (went extremely well-but that’s a tangent), and I had some time to recheck my BC’s this morning, now that the temp was warmer (30F) and calm. I followed the same procedure discussed above (~1’ from muzzle, then 49 yards back from there). I checked the BC from both the JSB heavies (Mk I) and the H&N Barracudas through my Polygon barrel. The results are as follows:

JSB Heavies:


H&N Barracuda:



It was really something to see the Baraacudas start off 40 fps faster and then drop to slower speeds compared to the JSB’s.

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:

"AirSupply"Hey Sean. If you want to true or check your bc in a real world sort of way try this. 
Shoot at 100y. According to strelok at that distance (assuming you are zero’d at 50y) you will need 13.35 moa elevation or 3.9Mrad. 
Temperature will have an Impact on the figures. 
I calculated this with your figures and a temp of 26C
Its a good way to check. 
Michael

Click to expand...

Thanks Michael-good suggestion. Right now, my longest distance at my range is 75 yards, but I did set that up the other day, after zeroing in at 50 yards. The adjustment seemed to be pretty close (pellet was slightly high, but I ran out of time to really test it out). I plan to repeat and check at 75 yards, and then set my 100 yard range back up again to check it at the longer distance, as you suggested. Great suggestion.

Here's what I posted over in the RAW Report thread from that shooting session:
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Got home tonight, converted back to .25 cal, sighted in at 50 yards, then determined how many mils up/down for various ranges. Turned out it was as follows:

JSB Mk I, Heavies, 885 fps MV
45F temps
4 mph SSW (mostly calm though)
Zeroed at 50 yards:
@ 75 yds: +1.6 mil
@ 35 yds: -0.6 mil
@ 70 yds: +1.3 mil

One shot at 35 yards knocked over the spinner target, but with -0.6 mil dialed in, here was the shot:



While checking holdover at 75 yards, I shot the following 5 shot group-crazy!



Edit: here’s a photo without the measurements to show the group better:



That was at 75 yards!

Here is a group at 50 yards from sighting in:


Thrilled with that performance...

Sean