How much does BC Vary from identical guns, under identical conditions?

[SMH77]

I see where the OP is going with this, and I really think that yea there could be a difference, but is that difference worth all that work....I doubt it. If you get down to it, every projectile will leave the barrel differently....that barrel has changed from shot to shot. A little lead here, a little heat there, moisture, BP, temps....all that is going to make each shot different....just IMHO not different enough to worry about. Now if there is a unique feature to one barrel...or gun for that matter....to another then yes you can see changes that we can make out. This is why we see things like my wombat 2000 loves JSB heavy, but billy bobs Wombat 2000 shoots FTT's so much better and just sucks with JSB heavy pellets. And this goes to no two guns are alike.....a perfect replication from one to the next, not happening, but it usually is close enough.



Really the only way I know of to really test this is to grab a perfect pellet(s) after they have been fired from a gun, then do all your calculations....and do you know what you will find, each pellet is going to be different....so you will average.....you will also come across the one that is 2% off the specs from all the others.

Agreed-and that is why I was surprised to see the numbers come out exactly the same! And, as I just mentioned in another reply, it turns out great that our two guns are shooting exactly the same because I'm teaching him via phone conversations so having the same ballistics make it that much easier for me to tell him what he's doing right or wrong with his aiming.

 

[stoti]

Sean, have you read any of Eric Stecker or Brian Litz? Both have a wealth of information to share about this kind of stuff. Some of it can be like going to the dentist to read but there is a ton of valuable information that really makes you think about all of the variables involved.... internal & external ballistics, aerodynamics and so many other variables that most never even think about. It’s pertains more to powder burners but the math is all the same. You should check it out if you haven’t, especially Litz. I’m out, have a good one! Stoti

 

[ackuric]

Barrel tightness / choke may absolutely influence form factor and even slightly section density as more or less lead can be stripped...but both figures are very, very marginal when compared to the big picture, is it worth taking into consideration when accounting for ALL things? Sure, why not! When it comes down to it you have dozens if not hundred(s) of tiny variables to consider from trigger pull to pellet hitting target, the more of them you figure out and understand, the better shooter it makes you. JMO!

 

[JamesD.]

I appreciate all your work, and everyone else's. It's made my life much easier

 

[JW652]

Thank you for these very interesting tests. It makes me wonder if the care and quality control that goes into each RAW product might contribute to the consistency. Equations aside, the proof of this consistency is reflected in the tiny group size you obtained - which is what most of us care about.

I do know that Martin takes great care in production quality. Besides tolerance measurements of parts, each regulator is adjusted and pressure tested and each barrel is inspected and pellets are pushed through to detect obvious internal inconsistencies. Together, his tests should help to establish velocity consistency between rifles.

I believe that John Whiscombe was the first rifle maker to incorporate slow twist (1:30) poly barrels and minimal choke (.0005) in his barrels. He is a firm believer that waisted pellets are drag stabilized (don't flame me - I know that the term is not scientifically correct - but still a good description) not twist stabilized, and that the axis of a moving pellet can be thrown off (pellet canted) when entering the choke. He also paid for the tooling of his barrels so he could control internal consistency. It would seem that Martin has independently come to much the same conclusions in his current products, which may help explain your findings.

Perhaps a meaningful test of your results would be to use the same procedures with two, high quality, unregulated pcps with standard rifling. If you chronographed the extremes of the shot curve (to test the effects of velocity change in the same barrel) you might detect BC differences that could be attributed to choke and rifling.

Besides velocity and atmospheric conditions, the only other variables I can think of are pellet consistency and distance. Obviously, pellet shape affects BC ( CPs still exhibit reasonable BCs even under the current shoddy Crosman QC) and pellets of the same weight from different makers seem to confirm this. It is probably no accident that two of JSBs best performers are 10.2 / .177s and 14.3 / .22s that retain much the same shape of their Crosman counterparts. It would also seem likely that BC changes would be more pronounced at long distance where velocity and shape induced changes might become more obvious. Most of us know from experience that pellets or rifles that group well at 25 yards can show high extreme spreads in both velocity and group size at 50.

Enough of the long winded musings. Your carefully controlled and documented testing has really got a lot of us thinking and are greatly appreciated. One of the many fun aspects of airgunning is learning how and why they work as they do and how to improve performance. So, many thanks for sharing these findings. They are always informative and we look forward to the next installment!