Beating the Ballistic Coefficient horse to death

[HogKiller]

I'm using Strelok used their numbers for JSB 33.95gr, .25's and...it's off a MILE. I finally got off my butt and put a target out to 70 yards, zeroed, then worked up to 100 yards, total ws 11.3moa. Started plugging in different BC's using GA function and I wind up with a BC of something between .0199 and .022, depending on the desktop version and the iPhone app but close enough. So, my question is what am I missing. Oh forgot the numbers worked exactly the same for 2 different guns. Well it seems that the numbers aren't reality, dropped down to 30 yards and the actual MOA drop was half of what the app said.

 

[Airgun-hobbyist]

When they published those numbers it is likely that they are for short range targets, under 50 yards. The pellet mfger's figuring BC indoors at 10 yards is a good bet. They say that BC is ever changing based on range and slowing speed. Have you messed with the multiple BC settings in Strelok Pro? (I have not, but there's a lot of folks on here who use it).

 

[Sqwirl57]

My understanding is that BC is usually advertised at its peak so if your velocity is different than that specific test point (lets say 900fps) it can be significantly less. A 25.39gr at 700fps may have a significantly lower BC than that same pellet at 900fps. And then of course you have outside variables like air density and whatnot

 

[Revoman]

I use whatever Strelok, and MERO, use in the apps. They are normally very close at distances up to around 100 yards.
I have recently however, done a Tall Test on my two rifles and find that the turrets don't necessarily agree with that test. You may have the same issue there as it really affects long distance shots.
It's super easy to do and will prove itself out if there are scope turret/tracking issues and if your scope is truly aligned vertically.

Mike

 

[Kgphotos]

I'm using Strelok used their numbers for JSB 33.95gr, .25's and...it's off a MILE. I finally got off my butt and put a target out to 70 yards, zeroed, then worked up to 100 yards, total ws 11.3moa. Started plugging in different BC's using GA function and I wind up with a BC of something between .0199 and .022, depending on the desktop version and the iPhone app but close enough. So, my question is what am I missing. Oh forgot the numbers worked exactly the same for 2 different guns.

If you have a chronograph you can determine the BC for your pellet on your own. It will be much more accurate than a BC from the manufacturer. The BC that you determine on your own will take into account your elevation, your particular barrel and tune and velocity. Also keep in mind that BC will change with many environmental factors. Elevation, temperature, air pressure, humidity... All these factors come into play. The BC you find today can be way different than what you get tomorrow with the same pellet in different conditions.

Here is how I do it. Using your chronograph, shoot ten pellets and determine average muzzle velocity. Then place your chronograph 25 yards down range and shoot through it ten more times to get that average velocity. I will do it again at 50 yards and 75 yards. Depends how confident you are in not hitting your chrony. With this info and a good ballistics program you can determine the BC by velocity lost at each distance. I use this method often with many different rifles and it always works. Hope this helps.
Kenny

 

[bigHUN]

Just a quick note for checking the vertical line.... do it with a plumbing weight....
I have four construction levels from HD (bought not so cheap for my basement project years ago) and when I stack them in a sandwich no two shows the same bubble not horizontal not vertical !!!

 

[Revoman]

"Just a quick note for checking the vertical line.... do it with a plumbing weight...."
EXACTLY!

Mike

 

[HogKiller]

If you have a chronograph you can determine the BC for your pellet on your own. It will be much more accurate than a BC from the manufacturer. The BC that you determine on your own will take into account your elevation, your particular barrel and tune and velocity. Also keep in mind that BC will change with many environmental factors. Elevation, temperature, air pressure, humidity... All these factors come into play. The BC you find today can be way different than what you get tomorrow with the same pellet in different conditions.

Here is how I do it. Using your chronograph, shoot ten pellets and determine average muzzle velocity. Then place your chronograph 25 yards down range and shoot through it ten more times to get that average velocity. I will do it again at 50 yards and 75 yards. Depends how confident you are in not hitting your chrony. With this info and a good ballistics program you can determine the BC by velocity lost at each distance. I use this method often with many different rifles and it always works. Hope this helps.
Kenny

My crony has already survived a near death experience, it'll be fine. When the moon is perfect I can hit a cork at 100 yards so, not worried about killing it, besides place is a bit off center, the pellet will hit the cone. I'll have to give it a try next time I'm out to my permission. Thanks

 

[EquivalentDelta]

Scope height over bore, muzzle velocity, and air density are usually the culprits when the BC doesn’t reflect reality.

That and BC changes with velocity, but not as much at subsonic speeds as with supersonic projectiles.

I don’t think I would bother with BC or a ballistic calculator. Pellets are so far off from even the G1 shape that it’s a very poor model match.

I would shoot my pellets at intervals of 10 yards and measure the poi shift. Then plug that data into excel to extrapolate a dope.

If you have a day of constant wind, and you have an anemometer, you could do this for wind as well.

 

[HogKiller]

I use whatever Strelok, and MERO, use in the apps. They are normally very close at distances up to around 100 yards.
I have recently however, done a Tall Test on my two rifles and find that the turrets don't necessarily agree with that test. You may have the same issue there as it really affects long distance shots.
It's super easy to do and will prove itself out if there are scope turret/tracking issues and if your scope is truly aligned vertically.

Mike

Please correct me if I'm wrong but he took his measurements from the end of the mat to the target, not from the end of the barrel, now will that 18" or so matter.??? Applying this to a pellet at 100 yards is going to be interesting, I'd be inclined to do 50 or 75. That formula is going to come in very handy, hello spreadsheet.

 

[HogKiller]

Scope height over bore, muzzle velocity, and air density are usually the culprits when the BC doesn’t reflect reality.

That and BC changes with velocity, but not as much at subsonic speeds as with supersonic projectiles.

I don’t think I would bother with BC or a ballistic calculator. Pellets are so far off from even the G1 shape that it’s a very poor model match.

I would shoot my pellets at intervals of 10 yards and measure the poi shift. Then plug that data into excel to extrapolate a dope.

If you have a day of constant wind, and you have an anemometer, you could do this for wind as well.

Honestly, that's my opinion also. I know my guns shoot 905fps with a SD of 2 or less. I know the majority of the pellets will impact the aim point with the usual flyers. I'm most definitely not so OCD that I'm going to measure each and every pellet. My JSAR Raptor does well with JSB 33.95 MkII's, it does ok with the AEA Center Punch 33.9., My Maverick VP prefers the Center Punch over the JSB's but it's literally a very small amount of difference for both, right in the human error or a slight puff of wind area. All I need is 1 minute of Squirrel and I've been there. Oh I forgot to mention, i'm running Arken scopes. I've done the twist the dial thing and back to zero and they repeat well on elevation, I haven't seen any real windage issue. I'm sure there are some thermal issues as the guns get warmed from the sun but that can't be controlled.

 

[Revoman]

"Please correct me if I'm wrong but he took his measurements from the end of the mat to the target, not from the end of the barrel, now will that 18" or so matter.???"
Though not exactly stated, the measurement should actually be from the turret, which appears to be very close to end of the mat. Since it's being measured to an exact distance, I assume that the 18" (or any other measurement) would matter if not measured true to the turret.

Mike