This is one which may be of interest to some or not. Just seeing how it goes. There are dozens of things which will affect group size, probably the biggest effect being you. However, after your pellet has left the gun there are a whole new set of things which will affect the group size over which you have little or no control. The biggest is probably wind; your pellets are highly vulnerable to the slightest wind. Another variable which people worry about is velocity changes though at most of the ranges we are shooting the effects are fairly minimal. If you are attempting to fire a 155mm shell 60km then changes in muzzle velocity have a major effect but not really at 40 yards. One thing which does have a large say in your group size which most people may not be aware of is the aerodynamic overturning moment normally written down as Cma. Cma is involved in both aerodynamic and gyroscopic stability and a lot of other ballistic behaviour characteristics. Amongst those are how pellets react to yaw and yaw rates as they leave the gun barrel. It is an important little devil. Cma is just the shortened way of saying C M alpha where alpha is the angle of yaw. The coefficient Cma dictates the size of the aerodynamic overturning moment about the projectile centre of gravity as alpha varies. The aerodynamic overturning moment is just the product of the aerodynamic normal force on the pellet multiplied by the distance between the centre of gravity and the centre of pressure. All projectiles have a value of Cma; the problem with diabolo pellets is that it tends to be very small. To show you the effect of Cma on your group size compare two pellets. The two pellets are identical in every way except Cma on one is the computed normal value for a pellet and on the other it is three times as big. Please don’t ask how you can find a pellet with a big Cma, I don’t know of any. Each of the two pellets has been fired in the same windless conditions and each of them miraculously leaves the gun with the same Muzzle velocity and the same yawing rate, caused by a very small pellet centre of gravity offset. The initial yaw rate does not have to be caused by the offset centre of gravity it is just an easy one to model. The diagram below shows the group size in inches you could expect to obtain with the two pellets. Before you get excited remember this pellet does not exist. The point of the diagram is to show the importance of Cma to group size. It also smooths out the flight path of the pellet as the maximum yaw angle will be much smaller even though the initial yaw rate was the same. This relationship has been known for years but knowing it and getting a large value of Cma are two different things. One way of getting a much larger Cma is to go from a diabolo to a bullet type shape. Unfortunately in air rifles the bullet shaped pellet weight tends to get very high and the barrel twist rate is too low for gyroscopic stability to be easily obtained. One of the favourite ways to reduce the weight is to have a hole in the back or the front of the bullet but, if the hole is in the back it reduces Cma making it little better than a diabolo, or if the hole is in the front you are immediately creating a high drag losing one of the advantages for a bullet shape and you may need a higher twist rate for gyroscopic stability. Another way to obtain a higher value of Cma is to use a very long Diabolo shape. Unfortunately, you would need to use something other than lead in the flare, the pellet drag would probably increase and you would end up with a very long pellet which would not fit in any magazines. In the end using a modified bullet type shape, designed specifically for air guns, using a mixture of materials seems the only way to go to get a significant increase in Cma along with a stable pellet. Such a round will be much more expensive to make and buy so it will be up manufacturers to decide if shooters are likely to buy one should it be made and sold. But at least you now have another excuse for when you miss that vital shot.