7.0 gm pellet vs. 9.3 gm pellet, how many clicks upward?

[Guest]

I assume the effect of heavier pellet would drop the target hit site slightly lower on the 10 ring. I assume 9.3 gm would be a 32% increase in weight; therefore, there would be a significant drop in velocity. Drop in velocity would mean longer time it takes for the pellet to hit target. This longer travel time to target would translate into more vertical drop to the target. (1/2 AT square from my newtonian physics).

The heavier pellet would hit about 2.4 cm below the lighter pellet according to my calculation.

But that does not seem to be the case in reality. What gives?

9.3 gm pellet and 7.0 gm pellet have the same velocity?

How many vertical clicks are required for the heavier pellet in your experience? I am using IZH 46M, 460 ft/sec or 140 meter/sec official velocity.

[jipe]

I have measured the following velocities with my LP10 (velocity screw of the LP10 factory tuned):
- 160m/s with Vogel rifle match 0.53g 4.49mm
- 165m/s with H&N pistol match 0.50g 4.50mm
- 171m/s with RWS R10 pistol 0.45g 4.49mm

Do not know the number of clicks since the last years I only use R10 pistol 0.45g 4.49mm for real shooting. Anyway, especially for elevation, other factors like your vision, lighting conditions... have also an influence.

Even if it is often said that the LP10 works the best with heavy pellets, with R10 0.45g, the velocity is pretty stable and the grouping is the best that can be expected from the LP10, well beyond what the shooter can do !

For lower velocity pistols like ISZ46, light pellets should be better.

[2650 Plus]

I once experimented with the same 45 using 185 gr wadcutter and hand loaded 230 gr bullets chronografed at the same velocity { or as near the same as I could load them] The 230 gr shot much higher at 50 yards than did the 185. The reason this made sense was the greater angle of movement in recoil caused by the heavier bullet. Could something simular be occuring in your tests ? I must say it appears unlikely. Do yu have any explanation for the results you are getting. Good Shooting Bill Horton

[Richard H]

I assume the effect of heavier pellet would drop the target hit site slightly lower on the 10 ring. I assume 9.3 gm would be a 32% increase in weight; therefore, there would be a significant drop in velocity. Drop in velocity would mean longer time it takes for the pellet to hit target. This longer travel time to target would translate into more vertical drop to the target. (1/2 AT square from my newtonian physics).

The heavier pellet would hit about 2.4 cm below the lighter pellet according to my calculation.

But that does not seem to be the case in reality. What gives?

9.3 gm pellet and 7.0 gm pellet have the same velocity?

How many vertical clicks are required for the heavier pellet in your experience? I am using IZH 46M, 460 ft/sec or 140 meter/sec official velocity.

Your assumption is a little off at 10m neither of the pellets are in the downward part of their ballistic trajectory hence why you don't see the 2.4 cm. At 10 m they are either still slightly in the upward phase or in the flat phase of the trajectory. The amount of adjustment will usually only be a few clicks.

Another error is your assumption that a heavier pellet will drop more. A heavier body requires more force to slow it down as both pellets are exposed to the same force (friction) the heavy pellet although traveling slower also has more energy. As the pellets present basically the same cross section difference in drag is minimal.

[ColinC]

Richard is fairly well spot on but there is also another way of looking at why there is little difference in the point of impact.

I have crunched some numbers and am willing to stand corrected by someone who understands these things better than I.

Basically the drop from 170 metres/sec to 160 m/sec is only a 5.8% drop in velocity.
Imagine the barrel being perfectly horizontal at the moment the pellet comes out(therefore the pellet would not be on an upwards trajectory, flattening out, then falling as described by Richard).
The pellet travelling at 170 m.sec reaches the target 10m away in 0.0588 sec. and the 160 m/sec pellet reaches it in 0.0625 secs. That's a whole difference of 0.0037 secs which is not much in the scheme of things.
What it then comes down to is how far a pellet will fall in 0.0037 secs because objects fall at the same speed whether they weigh a few grams or 100kg. (Gravity, Leaning Tower of Pisa experiment, Gallileo??) Remember forward velocity translates to how far the pellet will travel horizontally while gravity determines that pellets of different weights will hit the ground at exactly the same time, albeit metres apart.

I would suggest that in such a short space of time, the difference is hardly going to be more than a click.

[Richard H]

Objects only fall at the same speed in a vacuum to be accurate.

But again like you pointed out the differences here are much smaller than the OP's assumptions.

[tenex]

I would calculate the change in drop like this:

1st, calculate the velocity difference. My assumption would be that the pellet energy would be the same (air pressure vs. pellet position in barrel should be about constant, energy supplied = force*distance).

so, e = 0.5*m1*v1^2, 0.5*m1*v1^2 = 0.5*m2*v2^2
v2 = v1*sqrt(m1/m2)

m1 = 7.0, v1 = 460 '/sec
m2 = 9.3, v2 = 399 '/sec

2nd, calculate the drop for each, take the difference for the change in POI

drop = 0.5*a*t^2
a = 32.2 '/sec^2

t1 = 32.8' / 460'/sec = .0713 sec.
t2 = 32.8' / 399'/sec = .0822 sec.

drop = 0.5*a*t2^2 - 0.5*a*t1^2 = 0.0269 feet = 0.323 inches = 8.2 mm

I don't know how many mm/click you get, My gun is suposed to be 1.8 mm/click.

That would be about 4.5 clicks for my gun.

Steve.

[boris]

I shot LP5 in the airpistol silhouette matches with open sights. Normally I shot 7gn RWS Diabolo basic for training and 7gn R10 in the match, but occasionally I use 8.4gn JSB or 8.2 R10. On 10M-12.5m no difference in POI. 15m-1 click higher and on 18M 2 extra clicks up for heavier pellets.
My second pistol is IZH46M scoped. For 18m extra 1 click (1/4MOA) for heavier pellet.

[edster99]

Hi Tenex

I agree with all your calcs, but possibly would question one of your assumptions -

I would calculate the change in drop like this:

1st, calculate the velocity difference. My assumption would be that the pellet energy would be the same (air pressure vs. pellet position in barrel should be about constant, energy supplied = force*distance).

I think you need to take into account pellet friction in the barrel, which increases with the square of the velocity and the power required to overcome it increases with the cube. I have to state that this is the case when racing pushbikes- the aerodynamic drag being the key thing that slows down the rider. On that basis the differential of the velocity might be something like the cube root of 1.13 (the proportional velocity chang between 460 and 399) which would be 1.04. If you plug those numbers in, then the drop is even less than Colin's 0.003 seconds worth of vertical movement.

Now I know its not that simple - but overcoming friction is pretty key whilst in the barrel, which has an impact on the muzzle velocity. Theres the lead against the barrel, the air to be pushed out of the way, etc, etc. So something thrown into the mix for discussion.

Flameaway!

Ed

[tenex]

...
I think you need to take into account pellet friction in the barrel, which increases with the square of the velocity and the power required to overcome it increases with the cube.
...

Hi Ed,
That's a great observation. My assumption is that the residual force accelerating the pellet is constant for each case. If you use the data from Jipe's post above, you can see how good (or bad) my approximation is. Using the following 2 data points:

- 160m/s with Vogel rifle match 0.53g 4.49mm
- 171m/s with RWS R10 pistol 0.45g 4.49mm

the calculated velocity of the second point is:
v2 = 160*sqrt(0.53/0.45) = 173.63 m/s

Not too far off. They are different pellets, and the could very well have different levels of drag in the barrel. The aerodynamic drag is probably close to V^2, but I really don't know the relationship of the drag against the barrel surface to V.

I did try to calculate the muzzle rise for the 2 pellets, and without going into mind numbing detail came to the conclusion that although the heavier pellet has more drop, it also has a little more muzzle rise, negating some of the difference. I think I figured a delta of 2.5 mm or so, reducing the 8 mm drop down to 5.5 or so. If you add the fact that the heavier pellet probably has a better ballistic coefficient, and slows down less, it's not inconceivable that the drop at 10 meters is negligible.

It's a very interesting problem. I've shot a .357 where my slower 158 grain loads shoot higher than full power 125 grain loads, the muzzle rise being the significant factor.

All good stuff,
Steve.

[JamesH]

5 clicks up, or down.
Try both and see how you go.

[Spencer]

...It's a very interesting problem. I've shot a .357 where my slower 158 grain loads shoot higher than full power 125 grain loads, the muzzle rise being the significant factor.All good stuff,
Steve.

relating this back to 10m air Pistol: in a CO2 pistol as the gas reservoir drops off pressure (and the pellet velocity drops) the first few shot go higher - subsequent drop in pressure will give even lower velocities and the shots then go progressively lower.

We have put this down to the (minor for the first few) drop in velocity giving a longer barrel time and the shot leaving the barrel at a later and higher recoil angle, without the (minor) drop in velocity significantly affecting the flight time/gravity effect.

similar outcomes with 25m testing in a Ransom rest - lower velocities for .22LR sometimes print higher at 25m.

Spencer

[jipe]

- 160m/s with Vogel rifle match 0.53g 4.49mm
- 171m/s with RWS R10 pistol 0.45g 4.49mm

the calculated velocity of the second point is:
v2 = 160*sqrt(0.53/0.45) = 173.63 m/s

Not too far off. They are different pellets, and the could very well have different levels of drag in the barrel. The aerodynamic drag is probably close to V^2, but I really don't know the relationship of the drag against the barrel surface to V.

Indeed, your computation is quite close knowing that the shape of the pellets is definitely different between Vogel and RWS (especially the rear part) and also the fact that the values I have put are an average of only a couple a shots with each pellets.

If you are interrested by a second test of the computation method, I did the same measurements with the same pellets on the LP@ light of my wife:
- 134m/s with Vogel rifle match 0.53g 4.49mm
- 136m/s with H&N pistol match 0.50g, 4.50mm
- 144m/s with RWS R10 pistol 0.45g 4.49mm

The computation gives: 145.42, again, not so bad ! Again a little too high, but less (about 1%, 1.5% with the LP10).

This pistol is factory set at a lower velocity to allow a decent number of shots with the short cylinder and short barrel. Trying to get the velocity of the LP10 with its shorter barrel would use more air/shot of a smaller capacity cylinder => too few shots/cylinder fill.

[superstring]

...It's a very interesting problem. I've shot a .357 where my slower 158 grain loads shoot higher than full power 125 grain loads, the muzzle rise being the significant factor.All good stuff,
Steve.

relating this back to 10m air Pistol: in a CO2 pistol as the gas reservoir drops off pressure (and the pellet velocity drops) the first few shot go higher - subsequent drop in pressure will give even lower velocities and the shots then go progressively lower.

Spencer I have to correct you on this because, until there is no more liquid CO2 in the cylinder, the gas pressure is constant (at ~800psi), assuming constant temperature.

As far as the POI changing after the first few shots, this may help to explain that:

From: http://www.airgunsonly.com/tech/CO2vsAIR.htm

We have heard of shooters experiencing a noticeable change in point of impact for the first fifteen or twenty shots after changing cylinders or with the first shots on a warm rifle. [not so noticeable for pistol] It seems to be more pronounced on certain rifles. During the firing process CO2 tends to cool the pellet, chamber, bolt and other parts of the action. It is possible that there could be some velocity changes associated with this temperature change due to the length of time that it takes to "stabilize" the temperature of the mass. It would be more pronounced on a rifle due to the greater volume of gas used for each shot. Perhaps this is why CO2 was never accepted by the rifle shooters but was popular with the pistol shooter.

[Spencer]

...Spencer I have to correct you on this because, until there is no more liquid CO2 in the cylinder, the gas pressure is constant (at ~800psi), assuming constant temperature...

The gas pressure drops off when the liquid CO2 is exhausted: the subsequent shots are 'powered' by the remaining gaseous CO2, which are at progressively lesser pressure for each shot.
This 'drop off' was fairly dramatic with the older sparklet powered CO2 pistols such as the Hammerli Single, but with the reservoir CO2 pistols there are quite a number of shots after the last of the liquid CO2 is gone.

Spencer

[tenex]

- 160m/s with Vogel rifle match 0.53g 4.49mm
- 171m/s with RWS R10 pistol 0.45g 4.49mm

the calculated velocity of the second point is:
v2 = 160*sqrt(0.53/0.45) = 173.63 m/s

Not too far off. They are different pellets, and the could very well have different levels of drag in the barrel. The aerodynamic drag is probably close to V^2, but I really don't know the relationship of the drag against the barrel surface to V.

Indeed, your computation is quite close knowing that the shape of the pellets is definitely different between Vogel and RWS (especially the rear part) and also the fact that the values I have put are an average of only a couple a shots with each pellets.

If you are interrested by a second test of the computation method, I did the same measurements with the same pellets on the LP@ light of my wife:
- 134m/s with Vogel rifle match 0.53g 4.49mm
- 136m/s with H&N pistol match 0.50g, 4.50mm
- 144m/s with RWS R10 pistol 0.45g 4.49mm

The computation gives: 145.42, again, not so bad ! Again a little too high, but less (about 1%, 1.5% with the LP10).

......

If you want to take this to the next level, consider this: You're not only accelerating the pellet, but a large quantity of air as well, and you need to include it's weight in the effective projectile weight.

The air cylinder on my K2 is about 60cc (my guess), and I lose about 1 BAR per shot, so just to get close let's assume we accelerate 60 CC's of air (at standard temp & pressure, STP) along with the pellet. It's not quite right, since not all the air is accelerated to the pellet velocity, but it's another simplification that should get us a little closer to reality. Now the equation is as follows:

v2 = 160*sqrt((0.53 + M)/(0.45 + M) = 173.63 m/s, M = mass of accelerated air
weight of 60 CC's of air @ STP = 1.14 grains = 0.0737 grams.
v2 = 160*sqrt((0.53 + 0.0737)/(0.45 + 0.0737)

v2 = 171.8 meters/sec, quite a bit closer to the measured 171 meters/sec.

If you rearrange the equation to solve for M (which I did, but won't bore you with the details), you'll find that the accelerated mass is less for the slower case in your wife's gun than for the high speed case, which make perfect sense, since you turned down the gun to save air in the first place.

Again, this is all really cool stuff...
Steve.

[jipe]

v2 = 171.8 meters/sec, quite a bit closer to the measured 171 meters/sec.

If you rearrange the equation to solve for M (which I did, but won't bore you with the details), you'll find that the accelerated mass is less for the slower case in your wife's gun than for the high speed case, which make perfect sense,

Congratulation for the nice computation, it becomes indeed very close. It also show that the effect of the pellet shape is probably very small.

since you turned down the gun to save air in the first place.

The pistol was factory tuned like that, I didn't modify the speed myself.

[tenex]

The pistol was factory tuned like that, I didn't modify the speed myself.

That's very interesting. Do you know what the difference in capacity is from the same gun? I did the reverse calculation to find out what the equivalent additional mass (of air) was for the two guns and came up with 1.7 grains of air for the faster gun, and 1.1 grains or so for the slower, which makes sense (less speed, less air).

I'm going to borrow a chronograph and try to get a handle on the speed of the pellet down range. I've got this great idea for a 6 meter target where the black spot is displaced vertically from the scoring rings so I can practice on a short range without changing my 10 meter sight settings.

I have too much time on my hands...
Steve.

[Spencer]

I've got this great idea for a 6 meter target where the black spot is displaced vertically from the scoring rings so I can practice on a short range without changing my 10 meter sight settings.

Why not make up an overlay with the scoring rings, centre it on the group and score from the overlay.
spencer

[Richard H]

Well you're at it don't forget to include humidity in your calculation, plus altitude.