Other Why am I getting low ballistic coefficient?

[MushroomPsyche]

I want to say first and foremost, I don't typically experiment with projectiles. I find one that works for me and stick with it. With that said, I have experiences really with only these two projectiles.

A few years ago, when I was tuning my Taipan Veteran Long .22, I tuned it to shoot JSB Hades. Everywhere I read, it states the BC is at around 0.026. I initially used 0.023 BC but I noticed that my shots have been consistently low. I finally decided to test it on my own and put the chronograph 30m down range. I was surprised to find that it was only 0.017 BC for the Hades. I used 0.017 and since then I haven't had to touch. The Sig Sauer laser rangefinder with ballistics calculator built in that I've been using has always been spot on with 0.017 BC and I've shot all the way out to 66 meters with this gun. Searching the forums, I've found at least one other person who posted that they got the exact same 0.017 BC for the .22 Hades when they tested themselves.

This morning, I started experimenting with slugs. I tested two sizes of the NSA .22 17.5gr slugs. The specs for this particular slug is 0.075 according to the NSA website. With the chronograph 31 meters downrange, I only got 0.043 BC.

For both the Hades and NSA calculations, I used the JBM Ballistics calculator and I input the proper ambient temperature, humidity, atmospheric pressure, and elevation. Funny enough, even when I used the calculator in Strelok Pro that only allows input for near and far velocities, I still got the same 0.043 BC for the NSA.

Could someone please inform me of what is going on? Even Hard Air Magazine got, I believe, 0.023 BC for the Hades with the LabRadar. How can I be so far off but still be on target???

 

[MongooseV8]

Pellets have an erratic BC change during flight. Air pulse can affect a pellet right out of the muzzle, and any wind deflection or too high speed will also cause wild inconsistencies. Thats primarily why slugs are much higher performing in air rifles, they are much more stable during flight. Remember you are trying to shoot a lead parachute through the air at high speeds that wants to flip around backwards because the center of mass is the opposite of what the projectile wants. Any wind deflection or abnormal flight caused that parachute to try and flip forward, but then the skirt slows down the pellet and it "rights" itself.

So you wont see the same exact numbers as anyone else due to there being so many factors affecting the pellets ballistic travel. Your twist rate, your tune, your speed, your climate, your weather, your elevation, your chrono equipment, your etc.

 

[Franklink]

What's your elevation? I'll be surprised if you're not down near sea level. From similar discussions like this, it seems the trend is that low elevation shooters see much lower BCs than high elevation shooters. And I did indeed read that you plugged in the parameters, including elevation, but I'm of the mindset that the ballistics apps don't quite account for the elevation factor sufficiently.

There's also speeds. Another factor could be if you're shooting projectiles much slower than they were shot at the speeds in the reports to which you're comparing.

 

[MushroomPsyche]

What's your elevation? I'll be surprised if you're not down near sea level. From similar discussions like this, it seems the trend is that low elevation shooters see much lower BCs than high elevation shooters. And I did indeed read that you plugged in the parameters, including elevation, but I'm of the mindset that the ballistics apps don't quite account for the elevation factor sufficiently.

There's also speeds. Another factor could be if you're shooting projectiles much slower than they were shot at the speeds in the reports to which you're comparing.

My elevation is 1,127 feet above sea level. I am shooting the 15.89gr Hades at 945 FPS and the NSA 17.5gr at 888 FPS. In my case, there are 3 apps that all corresponds with each other, JBM, Strelok Pro, and the Applied Ballistics app that the Sig rangefinder uses. As was stated above, it might very well be all the other factors such as twist rate, tune, and projectile velocity.

 

[Arzrover]

What was the wind velocity and direction?
In your face will decrease bc(increase drag) and from behind will increase bc...
Bob

 

[Franklink]

To me, 1,127 feet is low elevation.

As a field target shooter, trajectory is everything. KNOWING impact points at all distances is crucial. So I map out actual impact points on guns I plan to compete with. I live around 5600feet and have shot matches at a location that is very close to your elevation. Comparing trajectory from my house to those low elevation match sites is quite interesting. The net difference is an overall flatter trajectory at my house than at the low elevation matches, the longer shots at low elevation need more holdover/clicks. I have measured BCs at my house and they are always higher than nearly every reported BC I find online, even after plugging in the elevation. I've not yet had a chance to measure at lower elevations, but the difference in real-world trajectory during matches suggests a lower BC at lower elevations.

As Bob said, tailwind/headwind can really mess up the numbers. I once measured the .22/25.4 Monster RD with a BC of 0.074. MUCH higher than normal for that pellet, and that is because it was with a tailwind of gusts in the 18-22mph range. Tailwinds help the pellet retain speed, therefore falsely elevating BC, and vice versa would apply too (predominant winds at my house are tail-winds so haven't personally measured downrange speeds with head-wind conditions).

Also, 945 is probably hotter than reasonable for Hades. BC might actually go up slightly if you shoot them in the 880-915 range. And even if BC doesn't improve, you'll be conserving air if BC is as low as you're finding. When picking up a barrel from him, I once shot over Bobs Labradar and watched a particular pellet have the same downrange speed ( I think the trap was at 50 yards) whether they started at 900 or 960fps. Setting up a gun to push that particular pellet to 960 would have been needlessly wasting air. In other words, 960 was no ballistic advantage with that pellet, and is likely the case with your Hades @ 945.

 

[MushroomPsyche]

Thank you for the replies. I think I might just be underestimating just how much elevation and barometric pressure affects the BC.

My shot string this yesterday morning were done very early in the morning so there was no wind. I'm not a precision shooter, but for my hunting purposes, I point the rangefinder at a rodent, dial in the scope to what was indicated by the rangefinder and it's always been a hit. This is with the Hades. Yes, I'm running the Hades a bit hot but when I setup this gun, I also wanted to shoot the NSA 17.5gr on the same tune so I had to find a balance between the two projectiles. At the distances I'm shooting at (~95% less than 40 meters), I find the Hades are hunting accurate at those velocities. Also, my gun's regulator is already at 90 bars for this tune. I'm not sure if that Huma reg will go any lower. I believe Huma rates it as 110 bars minimum but I asked them if it's ok to run it at 90 bars and I was told that it's ok but they never really indicated just how low is too low.

 

[Sqwirl57]

My elevation is 1,127 feet above sea level. I am shooting the 15.89gr Hades at 945 FPS and the NSA 17.5gr at 888 FPS. In my case, there are 3 apps that all corresponds with each other, JBM, Strelok Pro, and the Applied Ballistics app that the Sig rangefinder uses. As was stated above, it might very well be all the other factors such as twist rate, tune, and projectile velocity.

Published pellet BC is based at their peak which is usually somewhere in the 800fps range. If you are starting in the mid 900s your actual muzzle BC is pretty low in comparison. This is why shooting a pellet faster may actually increase the effect wind has your shot. Slugs on the other hand are probably at their peak closer to the 1000fps range but don't hold me to that. Here's a graph for JSB pellets BC.

 

[LDP]

This is why shooting a pellet faster may actually increase the effect wind has your shot.

Anything above about 860 fps will start to have more wind drift with speeds slower than 2000 fps or so. My memory may not be correct on the 2,000 fps.

 

[Franklink]

Notice the green line in RSterns graph above.....

.22/25.4 Monster RDs, their BC hasn't peaked out at the 790 at which the line stops. Mr. Sterne must not have shot them faster in this comparison.

From slow twist polygonal barrels, I've found the best BC from the 25.4g Monster RDs to be north of 950fps. Compared to most wasp waisted (Diabolo) shaped pellets, the .22/25.4 Monster is a rule breaker.

 

[Sqwirl57]

Notice the green line in RSterns graph above.....

.22/25.4 Monster RDs, their BC hasn't peaked out at the 790 at which the line stops. Mr. Sterne must not have shot them faster in this comparison.

From slow twist polygonal barrels, I've found the best BC from the 25.4g Monster RDs to be north of 950fps. Compared to most wasp waisted (Diabolo) shaped pellets, the .22/25.4 Monster is a rule breaker.

I was curious about that myself. Thanks for adding that bit of info