As some of you know I've been kicking around a theory for a little while now and I had the chance to do a bit more investigation over the weekend, so I thought I'd post my findings and the conclusions I'm starting to draw from the results.
I should point out that this is theory and as such, it could turn out to be useful information or a complete dead end - I'm approaching it with an open mind, and welcome your thoughts and comments.
Not all JSB Exacts are created equal - there are in fact something in the region of nearly 50 slightly different die's used in the manufacture of Exacts and in addition to that, there are different batches of pellets produced on those 50 dies. If you wander into a shop and buy a random tin of Exacts off the shelf, you could end up with a mega die/batch or a not so mega die/batch. Most of them usually shoot pretty well in most guns, but I've found that some shoot better than others.
So how do you tell the difference between the mega batches and the not so mega batches? (or even the really crap batches for that matter)
My usual method is probably the same as most shooters - I shoot groups at distance 50m and pick the batch that groups the best. But is there a more objective method that eliminates shooter error and environmental factors such as wind drift?
The Theory - Accuracy and repeatability are the same thing.
If you can put a pellet through a bit of paper at 50m and repeat that process so that the following pellet goes through the same hole, that's accuracy!
When you get inaccuracy, a large part of the cause is down to pellet stability in flight and a consequence of poor pellet stability is a reduction in velocity, a reduction on ballistic coefficent and an increased spread in downrange velocity. So could this be measured and predicted by performing downrange velocity testing with a suitable chronograph?.
I've run out of mega pellets, so I'm looking for a new mega batch for the coming season, so I thought I'd buy 3 batches of pellets and put them through their paces over a chrono and see if the theory holds up.
The 3 batches I bought were die 19 (4.51), die 45 (4.51) and for fun, I went for mega batch die 34 (4.52) shot through my TX200.
And these are my chrono readings :-
(Pellets shot straight from the tin)
Muzzle Velocity (fps)
Downrange Velocity (fps @ 50m)
|Shot||Die 19||Die 45||Die 34|
| || || || |
|Shot||Die 19||Die 45||Die 34|
| || || || |
The 4.51's are noticeably looser in the barrel than the 4.52 batch - no big surprise there, but curiously die 19 seemed to give the highest average muzzle velocity, but die 34 (4.52) gave the most consistent velocities with a standard deviation of just 2.4 fps.
If I was picking a batch based purely on how consistent the pellets were at the muzzle, I would have chosen die 34.
Downrange velocities tell a different story though...
Die 19 was mega consistent downrange with a standard deviation of an incredible 1.9 fps and a higher average velocity of 608 fps (BC of 0.027), die 45 was crap - so crap in fact I didn't bother taking more than 10 shots!. die 34 was ok, but quite a big variation with a standard deviation of 21.2 fps and an average velocity of 598 fps (10 fps less than die 19).
I didn't concentrate on groupings, I was concentrating on putting the pellet consistently over the chrono's sensors while not making the expensive mistake of shooting my chrono in a light but variable wind, but I did notice that die 45 grouped like a shotgun!.. I was struggling to put in a sub 40mm group at 50m (the reason I stopped at 10 shots was because I didn't want to hit my chrono!). Die 34 grouped ok, but die 19 was grouping very impressively - often going through the same pellet hole as the previous shot. Like I said, I wasn't concentrating on group sizes, but you do notice when a pellet hits the same hole as the previous shot.
Now you might think that an increase in average velocity of just 10 fps at 50m isn't going to make much difference here or there, but on doing some exterior ballistics calcs, it would seem that the pellet that has the highest average velocity and tightest spread 'should' also be fairly significantly better in the wind as well. Although this is difficult to prove with subjective grouping tests where human error can creep in quite easily.
Next step is to shoot some groups to see if grouping tests give the same result as my chrono tests - i.e. die 19 is mega, die 45 is crap.
Oh I should also say that all 3 batches seemed to land at about the same vertical height ( and none of them hit the chrono screen
The bottom line (for those who don't want to read all the thread)
It's looking promising to use downrange chrono testing as a more reliable method of pellet batch selection. If this proves to be correct, it will give a less subjective testing process for trying out other theories such as - does weighing my pellets give me any benefit? Is that mega air stripper actually any good? Should I lube my pellets? Should I sort pellets with a BIC pen etc etc.. you get the idea.
Oh, just to state the obvious, these tests are likely to be gun and barrel specific - what works in my barrel might not work in yours.
And of course, my theory could very well turn out to be a complete waste of time - it only took me about 45 minutes in total though so no big deal really.