GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on August 09, 2015, 11:13:45 PM
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I have seen a lot of guys suggesting that it would be a good idea to try and push bullets, in particularly ones with a good Ballistics Coefficient, fast enough to maintain supersonic flight all the way to the target.... I wanted to investigate what happened to the wind drift when you did that, and found a very interesting trend, pretty much regardless of the bullet shape.... The tool I used was the JBM Trajectory and Drift Calculator.... http://www.jbmballistics.com/cgi-bin/jbmtraj_drift-5.1.cgi (http://www.jbmballistics.com/cgi-bin/jbmtraj_drift-5.1.cgi)
It has a huge library of bullets, and many of them have several BCs over a wide range of velocities.... Using this tool, and three completely different shaped bullets, I plotted a graph of wind drift in a 10 mph crosswind, at a range of 400 yards, for muzzle velocities from 500 to 3000 fps, as follows....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Wind%20Drift%20vs%20Velocity_zpsaxlbda63.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Wind%20Drift%20vs%20Velocity_zpsaxlbda63.jpg.html)
Notice that all three bullets follow pretty much identical trends.... They have minimum wind drift at 850-900 fps, it increases to a maximum at 1500-1800 fps, and then falls again.... Even at 3000 fps, the 110 gr. Round Nose and 150 gr. Flat Nose bullets still have more wind drift than they do below 1000 fps.... Even the 168 gr. MatchKing Spitzer Boattail had to exceed 2300 fps to have less wind drift at 400 yards than it has at 800-900 fps....
Trajectory is easily corrected for by using MilDot scopes, an adjustable mount, or dialing up the POI with the turrets.... The wind, however, is very difficult to correct for, and is ever changing.... It seems to me there is absolutely no point in driving a bullet supersonic, with the hopes of keeping it there for 200-400 yards.... when all that does it put you smack in the worst zone for wind drift.... If you can't get up to serious PB velocities, in the 3000 fps range, you may just as well stay subsonic and deal with the holdover.... It will actually be easier to fight the wind that way....
Note, I'm not suggesting you shoot jacketed bullets in a PCP, they were just used because detailed Ballistics information was available for them, and it was the basic variety of shapes I wanted to represent....
Bob
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8) As usual great info post Bob
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Nailed it! from the looks of that graph, we need to be at 3000+fps at 400yrds. ;D That will keep the drift down for sure. I guess to put it in a nut shell, and I'm sure you've already done this Bob. If you develop a gas/power plant combination that can push a chosen bullet to sonic speed, find a bigger bullet that will shoot stabile from your platform.
Bill
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I always thought the drift was more from the effect of the bullet/pellet spinning and was only affected if the wind was pushing in the direction of the spin?
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Actually, Bill, for most of the bullets used by airgunners (round nose, or large Meplats) you would have to be well over 3000 fps to have less drift than the same bullet at 900 fps.... :o
Norman, there are three types of drift, Wind Drift, Magnus Effect and Spin Drift.... The Wind Drift is due to the bullet moving through a "moving sea of air", and is the same deflection (downwind) to the right or left for a given wind strength, range, bullet and velocity.... That is what we are talking about here.... The Magnus Effect is when the bullet rises or falls due to lift created by the spin of the bullet in a crosswind.... A wind from one side causes it to rise, from the other to fall (depending on twist direction), and rougher bullets can have increased effect, like the dimples on a golf ball.... The Spin Drift is present even in no wind, and is a gyroscopic effect caused by the nose of the bullet pointing in a different direction than it's line of travel (called the Yaw of Repose).... Longer bullets with higher spin rates have more Spin Drift, and the deflection is opposite depending on twist direction....
Bob
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Ok thanks for the explanation, makes perfect sense.
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Well I'll take a little bit more wind drift over a flatter trajectory. Cos at some point the muzzle is going to be in your scope.
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Gravity is a constant and can be dealt with.
Range it and the projectile will be where it is supposed to be.
Wind is never constant and switches need to be dealt with.
You takes your best guess and trust it was right.
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Looking at the graph that goes to show why speeds of 900-930fps are a great over all speed for drift as well as accuracy. The right speed is the key....
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Good point about the muzzle appearing in the scope eventually, Marco.... I just did some quick measurements on a few rifles, and a typical arrangement might have a scope 16" long with the objective ring 2.2" in diameter (for a 44mm).... If you use a 50mm scope, it will be closer to 2.5".... Since that has to miss the barrel (ie you will have to jack up the middle of the scope on an angled mount), you can take the scope height at the objective at a minimum of 1.2" above the top of the barrel.... With a 32" barrel, the muzzle would usually be less than 24 inches ahead of the objective.... with a longer scope, less than that.... so let's take the slope to place the top of the barrel in the middle of the line of sight as 1.2" in 24", or a 1 in 20 slope.... That works out to 2.9 degrees, but the scope has a field of view of about 1 degree at high power, so we need to allow for that.... so the edge of the field of view would see the top of the muzzle (on high power) at about 2.4 degrees (144 MOA) of elevation between the barrel and the scope.... Looking at it another way, for the bottom of the cone of the line of sight to be obscured by the top of the muzzle, the gun would have to have a near zero (if the barrel was 0.8" in diameter) of (1.2 + 0.4) = 1.6" x 20 = 32" from the muzzle.... So let's call it 1 yard, which you can put into the JBM Trajectory Calculator, and it says that works out to 143 MOA.... good enough agreement for me....
Using the JBM Trajectory Calculator linked above, I set the sighting in range to 1 yard, and I got the following ranges for the second zero....
110 gr. RN @
900 fps - 520 yds.
1500 fps - 780 yds.
150 gr. FN @
900 fps - 560 yds
1500 fps - 900 yds
168 gr. HPBT @
900 fps - 600 yds.
1500 fps - 1040 yds.
So you are absolutely correct, at extreme ranges the muzzle will obscure the line of sight.... If you are using a suitable bullet and scope, that will occur at somewhere around 550-600 yards at 900 fps, but not until about 900-1000 yards at 1500 fps.... So, if you are shooting at over 500 yards, you may need to use over 900 fps so that the muzzle will not obscure your view of the target.... Incidently, using the 168 gr. Matchking HPBT, at 1000 fps, pushes the usable range to 700 yards.... Of course a taller scope mount would extend that further....
Bob
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Thanks Bob for doing the math. And the problem gets worse when using muzzle brakes, it doesn't hafto show a lot in the scope to blur the image.
Marko
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I take flatter trajectory and less need for elevation adjustment because there certainly comes a limit for scope inclination. If I was shooting 900 fps I would need tools to set elevation adjustment from 100 meters to 500 meters and beyond. Now my current setup allow me to adjust up to 800 - 900 meters using reticle in addition. Something like 700 meters only with turret.
(So far we have shoot "only" 509 meters, or 556 yards. And yes, we hit our 12 oz. cans...)
Less wind drift would be nice to have but you can't get everything in one package. IMO, it doesn't count if you miss your target by 18 or 32 inches, its a miss anyway. In long range you always need some wind correction and it is not a problem to have enough of it but elevation is different thing.
There is nothing wrong in math or theories but not everything can be calculated. Sometimes it is just very enlightening to go out and shoot.
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I take flatter trajectory and less need for elevation adjustment because there certainly comes a limit for scope inclination. If I was shooting 900 fps I would need tools to set elevation adjustment from 100 meters to 500 meters and beyond. Now my current setup allow me to adjust up to 800 - 900 meters using reticle in addition. Something like 700 meters only with turret.
(So far we have shoot "only" 509 meters, or 556 yards. And yes, we hit our 12 oz. cans...)
Less wind drift would be nice to have but you can't get everything in one package. IMO, it doesn't count if you miss your target by 18 or 32 inches, its a miss anyway. In long range you always need some wind correction and it is not a problem to have enough of it but elevation is different thing.
There is nothing wrong in math or theories but not everything can be calculated. Sometimes it is just very enlightening to go out and shoot.
At long range shooting there are 2 very important requirements practice and then there is the what I call the "Magic Factor" which is the human mind/ calculator all the best shooters have this ability to incorporate all the science and then add their instincts to the equation to take the impossible shots and make them happen
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I guess that's what we get for trying to do things with airguns that are realistically more logically left to PBs.... I mean after all, 143 MOA of drop from the boreline at 600 yards is something like 70 feet, with typically 5 feet of windage in a 10 mph wind.... I have huge respect and admiration for those who will even try such a feat.... and if you all tell me that trajectory is more important than wind drift, I bow to your experience....
Bob