heavy air poi rationale?


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Franco2shoot
July 27, 2012, 06:51 AM
I have a pretty high end pellet rifle that's used for pest control. It has a pretty nice scope mounted on the top of the barrel, and a Red laser mounted on the bottom. This combination allows for extremely accurate precise shot placement. All is zero'd for 10 yards. If its closer the red Dot drops below the cross hairs and one simply knows that the POI will be halfway between dot and cross hair. Beyond 10 yards and the opposite is true. So long as I do a good job of trigger release, shots out to 30 yards are lethal. Same thing for something closer at the base of the fence just 5 yards away.

Ok, so I did that little explanation, but here's the real question. Yesterday, I put a target out at 10 yards, and it was HOT, we're talking Africa hot, Tarzan couldn't stand the heat. From inside, I slid a window open and from a bench rest took a shot. To my great surprise, the POI was off. It was high. Now usually, I can put shot after shot at 10 yards onto a dot the size of a Nr 2 pencil eraser. I repeated the process and the shots were all in the same place, about a full half inch high.

Unless someone dropped my rifle, the only explanation I can arrive at is the Hot muggy air does something to the velocity of the pellet, but I can't quite get my brain around what's happening.

Little help pleeze?

KKKKFL

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madcratebuilder
July 27, 2012, 08:23 AM
Unless someone dropped my rifle, the only explanation I can arrive at is the Hot muggy air does something to the velocity of the pellet, but I can't quite get my brain around what's happening.


The heat may be effecting the function of the rifle and not the pellet.

Franco2shoot
July 27, 2012, 08:45 AM
Don't think that is the answer... The rifle is in the house ALL the time. I'll slide a window open and shoot a rat running along the bottom of the fence. So, it's not a "walking-around" gun. My guess is that the pellet velocity is different in heavy air (maybe slower?). That's the guru question, does the velocity increase or decrease in Cold air. What about Hot air?

KKKKFL

highorder
July 27, 2012, 08:57 AM
That's the guru question, does the velocity increase or decrease in Cold air. What about Hot air?

It's way more complicated than that. ;)

Humidity affects air density too, offering more resistance to the pellets progress.

Franco2shoot
July 27, 2012, 09:36 AM
Now we're getting somewhere. It WAS both very hot and VERY humid... so I take it from highorder that the pellet would be moving slower.... and that was my gut take..

problem is, If slower my brain wants to think that the pellet should have hit LOWER, but I know I'm not thinking this properly.. if the optics are set for dead center @ 10 yards with a pellet moving 1700fps... but on this HHH day the pellet is only moving 1500fps
didn't it get to the impact point lower? Or is it that it took longer to travel the 10 yards and hence rose more.... arghhhh..

KKKKFL

WNTFW
July 27, 2012, 11:36 AM
You are only talking about 10 yards. How much does the air have to change to put you a half an inch high at 10 yds?

Humidity makes air less dense. Still it is only 10 yds.

Drop is for time of flight, not distance.

I think you have a partial understanding & are just confusing some issues. Your parallax could be the issue, coupled with a different position.

Some days I don't ask why, I just twist the knob on top of the scope.

Good news is if you are shooting a tight group you are ahead of the game.

YankeeFlyr
July 27, 2012, 11:56 AM
Though technically influencing air density, compared to temp and pressure, humidity's effect is almost zero. In your case, unmeasurable effects.

Franco2shoot
July 27, 2012, 12:11 PM
This is not the first time that I have seen this... In both cases the distance is within a few inches of 10 yards. Most times, I go into my shop and typically mid-morning will slide a window open and have a few shots. The pattern is alway smaller than a dime. On two seperate occasions and both in the afternoon, I have seen the POI shift upwards about 3/4s of an inch. Both times it has been near 100 out. The first time I just put it down to maybe the wife bumping the rifle and the scope getting mis-aligned. This time I know for certain that nobody has been in the shop. I fired 6 shots all hit high when the cross hairs were dead center on the bullseye. I went out and put a new target up, then re-adjusted. Next 6 shots all dead on. So, I am thinking that it must be the temprature/air density. What will be interesting is to see if the POI changes back once it cools off.

KKKKFL

Art Eatman
July 27, 2012, 12:16 PM
Air density decreases as the temperature rises. I don't know if high humidity outweighs the decrease in density. If it does, that could account for more "wind resistance" to small pellets.

Acera
July 27, 2012, 12:21 PM
Humidity affects air density too, offering more resistance to the pellets progress.

Completely Wrong, read the following article about how a baseball flies. Just think of it as a really big pellet. The higher the humidity, the lower the air density.

That is the explanation of your pellet being high. Less resistance traveling through the air. But there could be other factors as well.

http://www.exploratorium.edu/baseball/howfar3.html

http://www.usatoday.com/weather/wdensity.htm

TonyAngel
July 27, 2012, 12:44 PM
Hot air is thinner and offers less resistance. Cold air is thicker and offers more resistance. This is why guys shooting in higher altitudes where the air is thinner get better velocity and can ofter shoot a bit farther than we can with a like load. Whether this affects a pellet at 10 yards, I really couldn't say.

You also have to consider the power source. What sort of rifle is it? CO2, Pump, single stroke cocking, etc?

rcmodel
July 27, 2012, 12:51 PM
One other possible WAG explanation is this.

When air is compressed in the air-rifle, it is heated.
Witness the "dieseling" effect when there is too much oil in the piston & cylinder.
It gets hot enough to light off the oil.

Perhaps very hot air is already hot and less dense, so there is not as much pressure increase when the piston compresses it.

My Dodge hemi pickup truck is way weaker when it is 110 out then when it is 40 out for the same reason.

rc

BCRider
July 27, 2012, 03:47 PM
It's hard to believe that this would affect the pellet in only 10 yards worth of flight.

On the other hand the mass of air being compressed within the gun might result in a meaningful difference in the storage chamber's pressure with changes in the density due to humidity. In effect it would be like altering the amount of powder in a cartridge. The only way to tell for sure would be if the rifle had a pressure gauge on the chamber.

Something else that may be an issue is that you're shooting from inside. Is your house air conditioned? Cooling the air inside will raise the density of the air and result in a heavier charge pressure in the rifle. Again it's hard to imagine this mattering over a mere 10 yards but it may turn up to be the case. If you notice that it only occurs when it's Hot As Hades outside and that it returns to it's normal zero when the inside to outside difference isn't as great then it could be something along that line.

If you could chrono the pellets during the different conditions I'll bet it would show that on these Hotter n' Hades days that there's a very noticable difference in muzzle velocity from shooting with a "cool" rifle through the window and then if you take the rifle outside and allow it to equalize the temperature over a couple of hours and try it again using outside air to compress into the charge chamber.

But again it's only 10 yards. A bit of muzzle velocity difference should not be a big deal. The only final issue I can think of is the recoil and how it affects the rifle while the pellet is still in the bore. Now normally I would not consider recoil of a pellet to be meaningful any more than I'd consider recoil from a .22 rifle to mean anything at this small 10 yard distance. But an air rifle has a rather heavy piston and spring moving around. Which is why it's typically tough to find an air rifle scope which lives long. So if the cool inside air is creating a higher charge chamber pressure then it stand to reason that the piston and spring is going to not move as quickly as it tries to squeeze the air out through the barrel. This could alter the recoil dynamics enough to produce the change in elevation.....

.... at least that's my final offer and I'm sticking to it.... :D

jmr40
July 27, 2012, 03:51 PM
Fill a balloon with air, place it in a cold room and the balloon will shrink. Move it to a hot room and the air inside expands enough to possibly burst the ballon. It is the same amount of air. Perhaps the hot air inside of the guns cylinder was under more pressure because of the heat.

It is not uncomon here in the south for aersol cans to explode in hot cars left in the sun. Had a guy leave a scuba tank in his car that totaled his car and damaged several others when it went.


Just a guess, I don't really know about air rifles, but I do know that some gunpowders will produce higher velocities when the air is warmer. As much as 1 fps for each degree of temperature change. A load shot at 90 degrees could be 100fps faster than the same load shot at -10.

pseudonymity
July 27, 2012, 06:05 PM
Probably two factors - hot and humid air is less dense, so POI would be higher due to less resistance during flight. It is probably more of a factor for a pellet since those have such a very low BC.

If you charged the rifle with cooler air into the chamber and shot into warmer air, you will probably get a bit more muzzle velocity as well since the cooler air expands into a greater volume at the higher temperature.

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