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

[SMH77]

Have you ever wondered how much the 'same model' gun varies under identical conditions? I have-especially when I've tested my own gun extensively for BC performance across a range of environmental conditions, etc.

Today, my buddy was over to shoot and he has the same 'model' gun that I have: we both own RAW HM1000X guns with .25 cal polygon barrels. I told him I'm run his gun through the BC check by grabbing shooting data from his gun, using my standard protocol* of velocity at 1 yard, and 51 yards from the muzzle. After sighting his gun in, recentering, etc (to ensure we didn't shoot my chrony down range-not that it has happened before...) we grabbed a 12 shot string of data from his gun (after he let it settle down to ambient conditions). Once he was done, I decided to grab my gun so I could see how my numbers looked under the same environmental conditions (I thought his gun was shooting a little hotter than mine-and I was correct).

I then walked him through the process of calculating the Ballistic Coefficient of his gun/pellet combination (we were both shooting JSB Heavy Mk I pellets). After doing so, I did the same for my gun to show him the BC would probably be close, but somewhat 'unique' to each gun due to normal manufacturing variations, etc. (as an engineer I expect to see close results-so I was a little shocked at what I ended up seeing...).

Note: when I shot my gun, I clocked the very first shot as well as the remaining 11 shots to determine the BC-usually I don't do that because I know that the first shot will be slightly 'off'-especially after taking the gun from a warm (70F) house to the cooler temps (43F) outdoors. So below, I've calculated the BC of my gun with (top BC number) and without (bottom BC number) the first shot included for comparison.

His gun: BC of 0.0504; Mine: BC of 0.0505 (or 0.0504 with the first shot removed)! Crazy!!!

In summary, I was shocked to see the consistency between two different guns, even with the muzzle velocities being set slightly different from one another.

Do these results surprise you? Hopefully this helps anyone wondering about consistency of setups from one serial number to another.

We then went out and shot a few groups (5 shots at 50 yards, each)...

Mine (wind took one for a ride...):

His:

Sean

I hope you find this info helpful? Is so, please consider taking a second and simply leaving a '+' with a nice comment for me-it let's me know my time and effort is appreciated and keeps me going with this kind of work for the community. I've spent a bunch of money on Chronographs now, lead and time so it's a pretty small way of saying 'thanks for the efforts'. Enough said- Enjoy!

 

[stoti]

The BC shouldn't have anything to do with the differences in guns should it? The BC is based on the projectile and the velocity both close and at distance. No matter the differences in guns, if the velocities are similar and the projectiles, so should the BC. Right??? Great group by the way!

Stoti

 

[SMH77]

Right, you bring up a good point that I should have addressed in my original post-exactly how much variation I would 'normally expect' from gun to gun. So, theoretically, with the same type of barrel from the same manufacturer, set into the same design of action, shooting pellets from the same manufacturer (weight, shape, etc) there shouldn't be much variation in the BC. Based on the fact that BC does change with different approaches to barrel treatment (polygon, rifled, smooth-twist, etc), it's no secret that changes in internal barrel geometry have an impact on the BC of the pellet.

Having said that, I'm all too familiar with the fact that manufacturing includes variation, and those should fall within tolerances. As such, I guess my expectation was to see something like his setup producing BC's of like 0.0505 while mine would have been anywhere from like 0.0501 to say 0.0509. I wouldn't have been surprised to see results like that one bit-I was actually expecting something along those lines. The reason I expected that was because I didn't think the barrels could possibly be that closely matching to one another as they appear to be: I figured (even as good as LW makes barrels) that the variation would still be noticeable in the external ballistics.

I guess that is why I'm shocked that the BC's are essential identical from two independant samples-I just wasn't expecting that. And, BTW, that is why I decided to post this for others as well-I'm sure someone else expects some normal variation in BC's from one gun to the next (even though they're the same model). I just figured this was easy enough to put actual numbers to since I had the opportunity to do so...

Thanks for the compliment on the groups! I look forward to shooting more nice tight groups in the coming days since I'm off for Christmas break and have some time on my hands now...



Sean

 

[stoti]

I guess what I'm confused about is the variation in guns having an impact on the BC? When calculating a BC the only factors that are taken into consideration are the projectile and the velocities at the muzzle and distance. Since there is no factor in the equation for variations in gun, how can the BC's be different between the two? 

Stoti

 

[SMH77]

Great question: the answer lies in the 'internal ballistics' (not exactly the gun, per se, but the barrel) and exactly how the barrel changes the shape of the pellet as it passes from the breach to the muzzle (essentially swaging the pellet). The 'newly shaped' pellet leaves the muzzle and then is subjected to the 'external ballistics' which is essentially the drag of the environment on the pellet-basically all aerodynamics.

For a more 'discrete' example, check out this thread to see how it changes between different barrels.



Sean

 

[SMH77]

I guess I could have demonstrated this another way: simply swapping my buddy's barrel onto my gun and measuring the variability of the BC from the two different barrels. That would have kept all factors the same. I should have been more specific by saying I don't think its the 'powerplant' variation, but the barrel doing the swaging-I would have expected those to be slightly different and not so 'exactly' the same.



Sean

 

[stoti]

I understand there is a variation in guns, but is there a way to enter that variation in your ballistic calculator? If not, then the BC would have to be similar. Get me. I'm not arguing the differences in guns, I'm just saying that most calculators work on sectional density, mass and velocity and don't have a way to enter differences in guns. That's all I was saying. 

Stoti

 

[stoti]

I remember reading that other post and replying back then. It was a good one. 

Stoti

 

[ackuric]

BC is a measurement based on form factor and sectional density of a pellet. There is also a drag function used for different types of projectiles, diablo pellets commonly being GA



The formula for calculating the ballistic coefficient for small and large arms projectiles only is as follows:

 


Where:

• BCProjectile = ballistic coefficient as used in point mass trajectory from the Siacci method (less than 20 degrees).
• m = mass of bullet
• d = measured cross section (diameter) of projectile
• i = Coefficient of form

 

[stoti]

Right, that's exactly what I was getting at. There is no way to take into account variations in barrel or manufacturing. So, if the environmental factors are the same, the velocity and the projectile too, how could the BC be different?

Stoti

 

[SMH77]

My last reply got lost due to my internet connection-grrr....

In essence the projectile that leaves the barrel may not be 'identical' even between polygon barrels, therefore the form factors are different, resulting in slightly different drag functions across the same distances (aerodynamics are very sensitive to geometry changes). That would (potentially) show up in a different BC.

In my testing, I took two unique sets of data-one from each gun-and calculated the BC based on measured velocity differences over the same distance. Therefore any net effects on the drag function would have shown up on differences in the velocity profiles across the same range.

In essence, the 'i' in the formula posted above would be different based on different barrels 'swaging' pellets slightly differently-even though they are both 'polygon' barrels. I guess I would have expected the 'groove' widths or depths (not that they are really 'grooves' in a polygon barrel...) would vary a little from barrel to barrel-and that would be enough to show up in different 'i' values (form factors and resulting drag functions) from the two barrels. In the case of my data above, it suggests an exemplary job of manufacturing consistency by Lothar Walther in making these polygon barrels-at least that's my conclusion so far...

Not sure if I described that one clearly or not...



Sean

 

[stoti]

 Yeah, I understand what you're saying, I guess we just have to assume the pellets are close enough to exactly the same so that any difference in the velocity has to be caused by the gun alone and it's effects on the pellet when doing these calculations? If these minor variations in each of the guns affects the pellet that much then I totally agree for these test to be accurate, you would have to switch barrels and shoot them off the same gun, otherwise the starting velocities could never be exact enough to get an accurate measurement. The imputs I was talking about that can't be entered into the ballistic calculator are inferred from the difference in velocity. That's just doesn't seem accurate enough to me and that's what I was missing. When doing these types of calculations in the past, how much variation in velocity does it take to make a noticeable difference in the BC? Say from like .0501 to .0509 like you mentioned above? I realize I can look that up on my ballistic calculator but I don't have it here. Thanks for sharing with me.

Stoti

 

[JamesD.]

I'm no expert but your example of 0.008 variance would be negligible in any air gun distances. Same gun, barrel, fill, sized pellets, weighed.

Elevation, temp, humidity, winds of course, would have more of an affect than variations in pellets from same barrel & gun. Though this does raise this question for me.

A tighter choke that spits out more uniform projectiles would have less variances than say what I'm about to see in unsized pellets & cast slugs from my .251" straight, non-choked Sumatra barrel.

I think between variances in factory barrels, and variances in pellets from various dies, we all know by now if you want complete accuracy the only way to get it is to swage your barrel, have a perfect die or sizer made up, spend the time weighing, and enjoy watching them go where you send them. 

Hello? Waitress, may I please have another cup of coffee? Thank you Mam. I'm not awake so I probably shouldn't have even answered this. I was lost when dude started in with the math formula. Straight up disability on a retarded level in math. Sorry.

 

[John "spysir" Blount]

Y'all are pretty well over my head having started out with a career 707II & no chrony but many shots fired in leaning originally & havent seen chairgun since all the monet went to save dogs & cats.

I would think if using " the same" or equal rifles BC should be pretty close. But would think if the bore is tight enough ( or has a tight spot) to resize pellets BC will change slightly , right? Slightly but some change.

One fact I have seen using 2 chrony's. With one chrony I got a "BC" number that was useful but with both used one at range ( say 50 yards ) with an LW barrel that had very DEPP SHARP engraved rifling marks - causing extra drag I believe) the BC changed a good amount. Sorry I dont have note with real numbers but after enough lapping to get the rifling marks to look normal down range BC was notably improved.

If your going to be really picky each and every rifle has a slightly different BC if you can measure well enough - and it seems a lab radar unit might make it all easier?

John

 

[Cherokee140]

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.

 

[Arzrover]

Seems like the point was that it was surprising that they were essentially identical, given they were 2 different rifles and barrels. I agree. 

Recently I have been involved in testing some .25 poly barrels...all on the same rifle and pellets with a LabRadar. Looking primarily for differences in accuracy but also keeping track of the pellets bc's as another measure of the barrel's superiority. As a generality, the more choke, the worse the bc. No real surprise there but there seemed to be a couple of exceptions. I'm guessing at this point, the samples weren't large enough or the conditions were different. 

Thanks Sean for continuing to post the results of your testing. Always interesting. 

Bob 

 

[SMH77]

Yeah, I understand what you're saying, I guess we just have to assume the pellets are close enough to exactly the same so that any difference in the velocity has to be caused by the gun alone and it's effects on the pellet when doing these calculations? If these minor variations in each of the guns affects the pellet that much then I totally agree for these test to be accurate, you would have to switch barrels and shoot them off the same gun, otherwise the starting velocities could never be exact enough to get an accurate measurement. The inputs I was talking about that can't be entered into the ballistic calculator are inferred from the difference in velocity. That's just doesn't seem accurate enough to me and that's what I was missing. When doing these types of calculations in the past, how much variation in velocity does it take to make a noticeable difference in the BC? Say from like .0501 to .0509 like you mentioned above? I realize I can look that up on my ballistic calculator but I don't have it here. Thanks for sharing with me.

Stoti

A couple of answers to your questions / comments above (I tried to highlight them to show what I'm responding to):

1.  I actually could adjust my muzzle velocity to match my buddy's just fine-I just left them where they were set as I didn't see much reason to match them. In that regard-if I wanted-I could easily match velocities for the comparison, which would leave the power plants essentially identical, and just the barrel samples 'unique' from one another.
2. The second highlighted question is a bit of a more involved conversation-probably worth a topic all on it's own. I have adjusted the power on my .25 cal and .30 cal setups from min to max HS tension (leaving the regulator alone) and have seen (going from memory) a range in power from like 57 - 67 fpe shooting the same .25 cal pellet (it was like 74 - 82 fpe in .30 cal, IIRC). That translates to velocities in the ballpark of like 860 - 945 fps (again, my numbers may be off, but I recall seeing nearly 950 fps; in .30 cal it was like 845 fps - 905 fps-see chart below). Again, this is all at the temperature I did the testing at (warmer time of year). The problem with answering your question about this is that, after the speeds were increased past a certain point (maybe 930 fps?) the barrel leaded up quickly and resulted in the BC's dropping significantly. Once I cleaned the barrel, the BC's came back up to 'normal' (which is usually around 0.055 or so), see link for details. To your point though, the BC did change a bit (more than just from 0.0501 to 0.0509) by changing the velocity at the muzzle. I have read about the effects from others (Yarrh has documented this on old Yellow forum posts-good reads, BTW), but honestly don't want to get into playing with the regulator settings since my gun shoots lights out the way it's tuned and I don't want to mess with that at all...
   [/LIST=1]
   
   
   
   Here's the data I posted in the 'RAW Report' thread. I could pull this out as a separate topic to give people an indication of how much variation one can expect from changing HS tension without playing with the regulator setting at all. Might be a useful post as it's likely a question people ask or wonder about regularly...
   
   Here's the plot of Hammer Spring Tension vs. Velocity (muzzle and 1 yard) and Energy (@ 1 yard):
   
   
   
   
   
   
   
   
   
   
   
   
   Sean

 

[stoti]

Sweet. That’s the kind of stuff I like to read and learn from. Thanks again and have a great day!

Stoti

 

[SMH77]

I'm no expert but your example of 0.008 variance would be negligible in any air gun distances. Same gun, barrel, fill, sized pellets, weighed.

Elevation, temp, humidity, winds of course, would have more of an affect than variations in pellets from same barrel & gun. Though this does raise this question for me.

A tighter choke that spits out more uniform projectiles would have less variances than say what I'm about to see in unsized pellets & cast slugs from my .251" straight, non-choked Sumatra barrel.

I think between variances in factory barrels, and variances in pellets from various dies, we all know by now if you want complete accuracy the only way to get it is to swage your barrel, have a perfect die or sizer made up, spend the time weighing, and enjoy watching them go where you send them. 

Hello? Waitress, may I please have another cup of coffee? Thank you Mam. I'm not awake so I probably shouldn't have even answered this. I was lost when dude started in with the math formula. Straight up disability on a retarded level in math. Sorry.

1. Totally agree! Actually, what surprised me was that, while I planned to make my buddy his own, unique range card for his ballistics, I didn't actually need to-we can share the same ballistic data. For me, this is very convenient because I'm already memorizing common MIL compensations at various distances (32 yards, 75 yards, 100 yards, 120 yards - gun is sighted in at 50 and is also on at the near zero crossing point of 16 yards). My buddy isn't quite as 'technical' as I am-so for my to just spout of my MIL compensation numbers to him based on how far he's taking a shot is just super convenient and easy for me! What a nice 'treat' to have. I'm kind of bringing him into the fold, so we have a lot of conversations on the phone-so having his ballistics be identical is just wonderful-makes it so much easier for me to know how to advise him during conversations...
2. Totally, totally agree! My testing has shown a range of variation from anywhere from 0.0500-ish up to 0.0565 for the BCs of the same pellet through my polygon barrel due to environmental changes. I'm starting to read more about 'Density Altitude' and seeing if I can compile my historical data of BC vs. this parameter to make the plot easier to predict my BC for a given day (using a density altitude app to automatically spit out the DA for my day/location). True, the BC's are relatively close, but I do want to know where I'm at on a give day. If I can understand that, along with how my muzzle velocity will vary based on temperature, I will be that much closer to dialing in for the 'one shot, one kill' (whether steel or otherwise).
   [/LIST=1]
   
   In the end, that's really what I'm after. For years I got confused by having scopes with mixed units and always having to run to an app to predict where my shots would land. A LOT of my confusion came from having mixed units in the scope (MIL reticle, MOA turrets --> stupid idea for sure!). That led me on to thinking: how the heck do snipers do this stuff in the field and get it right with only one shot? Surely there had to be a better way... So that's what's really driven me to test, document and understand this better. And I like sharing what I learn for others to use also-to help them, and have them help correct errors in my thinking as I ponder observations....
   
   
   
   Sean
   
   

 

[SMH77]

Y'all are pretty well over my head having started out with a career 707II & no chrony but many shots fired in leaning originally & havent seen chairgun since all the monet went to save dogs & cats.

I would think if using " the same" or equal rifles BC should be pretty close. But would think if the bore is tight enough ( or has a tight spot) to resize pellets BC will change slightly , right? Slightly but some change.

One fact I have seen using 2 chrony's. With one chrony I got a "BC" number that was useful but with both used one at range ( say 50 yards ) with an LW barrel that had very DEPP SHARP engraved rifling marks - causing extra drag I believe) the BC changed a good amount. Sorry I dont have note with real numbers but after enough lapping to get the rifling marks to look normal down range BC was notably improved.

If your going to be really picky each and every rifle has a slightly different BC if you can measure well enough - and it seems a lab radar unit might make it all easier?

John

Sounds similar to what I showed in my data (also two chronies, that have been calibrated to each other) in this thread.

Yes, I should (eventually) probably replace my two chronographs with a LabRadar-it would simplify things. Fortunately I can 'auto collect' my data now from both chronographs via my iphone (corded connection) and ipad (bluetooth-even out to 50+ yards too!) so it's not terribly hard any longer to collect the data. At first it was a pain though, for sure...



Sean