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The question “Why” has lead to many interesting answers. Why do some shooters seem to know when to change lots of ammo when the rest of us just continue to shoot around the X?

The question was why one of my mentors (Gary Mitchell) always knows within 5 shots from a Lot of ammo if it will shoot that day or not. Gary always goes to the line with at least 4 different Lots in his purple box. These Eley Lots range in speed from 1044 fps to 1077 fps. The “Answer”, to the “Why” started with Paul Campbell (another mentor) from Cuthbert, Georgia. Paul was using a barometric pressure gauge and writing the readings down with his scores and changing Lots of ammo with increasing or decreasing pressure. His scores proved that he was on the right track.

I started from the barometric pressure readings and added relativity humidity and temperature; I think relative humidity is more important than temperature because we almost always shoot close to the same temperature, but temperature is the over riding factor. I noticed that as the barometric pressure went up and the relativity humidity was down I needed to tell Eley customers to shoot faster ammo, this also holds true for low pressure and high relative humidity – shoot your slower ammo. Many of us shoot two Lots, one for the spring and the same Lot for the fall season, the second Lot for the summer season. (This is where temperature and humidity come into effect.) My understandings grew when at a match last December while I was talking to Allan Hall about air pressure and water vapor, wet air is lighter (or thinner) than dry air and the effects of shooting at higher altitudes in respect to bullet speed. It’s the only discussion I ever won with Allan (hands down), though I have gotten him a few times on a target or two. So here is what I have learned and maybe it will help you.

We are dealing with air density when shooting rim fire much more than the center fire boys are because of being subsonic. Air density gets into the center fire game when they click in or out powder. Air density is the mass of air divided by the volume it occupies. Air density is a combination of pressure, temperature, and how much water vapor is in the air. The Ideal Gas Formula is D=P/(T*R) or density = pressure divided by temperature multiplied by relative humidity. This is more than any of us want to do before a match so there are several ways to get a quicker answer later in the article. If air is dry, and the temperature is 0 degrees Celsius, one cubic meter will have 1.275 kilograms of air in it. As the molecules in the air are heated the molecules move faster, they expand. This is just like taking a balloon in and out of the refrigerator; it’s larger in the warm air and gets smaller in the cold air. Also as the barometric pressure increases so does the air’s density, just like filling up a car’s tire – it’s thicker. The higher you are in altitude and it’s a hot day, air’s density is lower, the example I see the most is playing baseball in Denver, they say that it’s the place to hit a home run because the “drag” of the air is lower. The same effect can be seen in Minnesota during the winter on a clear cold day, besides being cold there’s lots of drag because the air is “thick”.

We all seem to understand the temperature side of this formula, so let’s go on to the hardest thing to understand, water or more correctly water vapor. Humid air is less dense than dry air. The higher the relative humidity the less dense a given volume of air is – its thinner (lighter) or to us, less drag on a bullet. In physics we are taught that air is a combination of nitrogen, oxygen, and all of the other gasses. 78% nitrogen (atomic weight of 28), 21% oxygen (atomic weight of 32) and water vapor has an atomic weight of 18 (less than nitrogen and oxygen). (Example – of the three, water vapor is the biggest balloon.) As the water vapor increases it pushes nitrogen and oxygen apart and the weight of the air decreases so the air density decreases – to us - less drag – our bullets move faster. Dry air is thicker than wet air, an example is, dry air has a higher drag on objects moving through, and it slows down bullets like shooting into water. The thing to remember is that barometric pressure is first, then temperature, and then last is relative humidity. Remember we almost always shoot in warm summer months, so the relative humidity is very important. When you start keeping up with your Lots of ammo and the conditions when they work, look at the whole picture of what’s happening. Think of the conditions that we shoot in as a bell curve wave, you will find a wave where two or three Lots work. Be ready for the condition when you may have to add power because the air is so thick that you have to add power to get to the target. This is a condition I have run into twice once in Jacksonville and the other in Richmond, both are closer to sea level and when you leave your home range the air can go to either extreme.

There is one group that understands this principal more than the rest of us, airplane pilots. Pilots can all tell you that it takes more power and a longer runway to get a plane up or down in the hot summer. It also takes more power to stabilize a bullet in this hot summer condition. Pilots use a number that gives them their “apparent” altitude or Density Altitude. (I will shorten density altitude to DA in the rest of the article. ) The density altitude uses the standard atmosphere’s table for the average pressure, temperature and air density for various altitudes. One of the tools that pilots use for take off and landing is density altitude computation; it gives you a number that is in relation to what altitude you are at, in respect to the air that is around you. Example – If a plane is at St. Louis with a DA of 2400 feet a pilot would know that he needs a longer run way than its normal 604 feet above sea level. It takes the barometric pressure, temperature, and relative humidity and gives you a number this is your “apparent” altitude in relationship too where you are.

I am now testing and selling a meter, Kestrel 4000 Pocket Weather Tracker, watching the “apparent” altitude or DA and seeing what Lot works. One of my rifles shoots at 1053 fps when it cold (less than 500DA), then I go to 1059 fps (600 to 1400DA), 1066 fps (1600 to 2000DA), and then 1071 fps (over 2000DA). The Kestrel 4000 performs ten different measurements; the one I use is the Density Altitude calculation. I have it set to DA so that I can see any changes in conditions as I am shooting. I might need to write another article on how I use the meter and set it up at a later date, but its setting needs to display the actual conditions you are under during a match or practicing. An interesting side benefit is that it also tells wind speed. I also have a hand held rotating paper chart that you enter the measurements into and it gives you the density altitude, but that takes time and you may need to change in the middle of a target. One thing to remember, every 22 rim fire is different, so you will need to work out your own “scale”. The other is that there are some guns out there like Gary Mitchell’s, Ole Blue Suhl, it was worn out in Europe, then K.C. Young worn it out and now Gary is wearing us out with it. Ole Blue has the widest one hole range in regard to Density Altitude of any gun I have seen. We all would like to be shooting a gun that can handle conditions like Ole Blue.

So what does this air density and density altitude mean when you are shooting Eley ammo? Eley uses a system where the first three letters are for grade, year, and crimp example U is Ultimate, F is 2001, G is 2002, and W is the crimp machine example UFW. The next four numbers are for machine and Lot number 4375 is the four machine and the 375 is the Lot number –UFW4375. The fps is printed under, example 1055 fps. This fps on the label is an average of 50 shots from one of the six test guns at the Eley factory; these guns have 26 inch barrels with the chronograph three feet in front of the barrel. There are 4 different machines that load Eley Ultimate and Match EPS. I have found that these machines are very specific to which gunsmith chambers and fine-tunes your rifle. Depending on barrel twist, length, and the gunsmith who put the gun together, you will find at least three Lots of ammo that will shoot under different conditions without turning the tuner. Each machine has subtle differences, so that different machine Lots of Ultimate can chamber and give the best accuracy. One Lot of ammo will not shoot well in every gun. Bill Calfee and James Messer guns seem to shoot best with the number 3 and 4 machine, Tim McWhorter’s (including all the Suhl’s he has set up) shoot the 3 machine, some 4. Marshall Beam, Larry Shellhouse, and Lamon Logins use the 3 machine but will also shoot the 4 machines. Bill Myers uses the 2 and 3 machines. Most factory Anschutz use the 1 machine and Winchester 52’s seem to like the 3 or 4 machine. All of this can change by the way the individual shooter holds the gun. One of Mr. Jim Cannington’s guns will shoot the 3 machine in free recoil but likes the 2 machine if it’s held. Wayne Smith is the best at finding out what a gun likes to shoot and fine tuning, he was the one that found out about Mr. Jim’s gun preference. One curious thing almost all guns will shoot the 2 machine when the wind is blowing hard, better than when it not blowing.

You will have to take into effect shorter barrels and tight bore barrels giving higher velocities or longer barrels giving lower velocities. This fps on the box is a relative number after you have proved a Lot in your gun. You can come back to it or go up and down under varying conditions. With the machine number and the fps on the box you can move up or down in Lot velocity, my guess is - 10% as a high or low in fps and not turning the tuner. This is normally 6 to 12 fps depending on the barrel. You are taking advantage of more or less drag on you bullets on the way to the target.

I am quoting a great writer and gunsmith (Bill Calfee), “As always remember that these are just my opinions” or, my two cents worth. With more time and using the DA calculator I will be able to give you more and better information. I hope that this will help shooters so that they will stop wasting good ammo by shooting it in the wrong conditions. It will also help shooters know why their targets are not as good as they know they should be and what they can do to improve their scores. An hopefully it will make it easier for us to get more shooters into the Sport of Rim Fire.

Bob Collins