Pellet sizing

What is pellet sizing and what is the purpose?

Pellet sizing refers to the process to change the size of a pellet. More specifically, the objective is usually to reduce the head size of a pellet to a specification.

The purpose of pellet sizing is to improve the consistency from one pellet to another, and to optimize the head size for accuracy.

What is the theory behind pellet sizing?

To understand the theory behind pellet sizing, it is important to understand how head size affects airgun shooting.

Head size being too small

If a pellet has a head size that is too small, then a pellet can travel along a barrel and eventually exit it not parallel to the axis of the barrel. The first effect is that as the pellet exits the barrel, some parts of the skirt clears the crown first. As residual compressed air finds the first opening to enter the atmosphere, the turbulence further disturb the alignment of the exiting pellet relative to the axis of the barrel. The second effect is that the axis of rotation of the pellet is no longer aligned with the center axis in terms of shape. The causes the pellet to cock screw in flight and decreases the ballistic coefficient.

This is a long way to say that when the head size of a pellet is too small for a barrel, accuracy suffers!

But then what about a head size that is too big?

The skirt of a pellet is, by design, much thinner in terms of material. This allows the skirt to expand as the compressed air wave front contacts a pellet. The cavity inside the skirt provides a small volume for compressed air to expand from.

Even though a pellet is made of relatively soft material, the head of a pellet is solid. Even if a pellet is made of pure lead, the shape of the head does not deform nearly as easily as the skirt. As a result, when a pellet has a head size that is too big, more energy is expanded to for the rifling to slightly shape the head. The loss of energy is more significant for choked barrels.

Even after the head of a pellet is shaped, a head size that is too big will have a larger contact area with the lands and grooves of the barrel. This also increases the kinetic friction between a pellet and the barrel that it is traveling through. The bigger the head size, the larger the contact area, and more energy is lost due to friction.

But does a head size that is too big affect accuracy?

The answer is yes, and there are many factors.

When a pellet is loaded into a barrel, a larger head size increases the amount force that is needed. If the vector (direction) of the force applied to push a pellet into a barrel does not line up with the axis of the barrel, then a pellet can be shaped misaligned relative to the barrel axis as the pellet is being loaded. The entrance of most barrels are tapered to ease the loading of pellets, this means there is more "play" for a pellet to be loaded misaligned.

Once a pellet is shaped misaligned, it is likely to remain misaligned as the pellet is propelled by compressed air through the barrel. This leads to the muzzle/crown and cock screwing issues already mentioned.

Furthermore, as much as manufacturers do their best to control quality, the head size of pellets right out of a tin varies. This translates to a variation of the contact area between a pellet and the barrel, which then translates to a variation of friction and hence acceleration in the barrel.

For a piston airgun, this variation causes some, but not much variation of muzzle velocity. The amount of energy released is, for the most part, constant.

However, for a PCP/CO2 airgun, this variation gets a little tricky. With a relatively weak exhaust valve spring and a relatively high compressed air pressure (regulated or not), the opening of the exhaust valve is a complex function of time that also depends on how the projectile accelerates. This is because it is not the pressure that helps close the exhaust valve, it is the movement of air that helps close the exhaust valve. In the most extreme case, if the projectile does not move (this can happen when someone loads a .22 pellet into a .20 gun!), the dwell time is maximized. Conversely, when there is no projectile, air flow is free, and the dwell time is minimized.

As a result, a pellet with a smaller head size decreases the dwell time, and therefore changing the amount of energy released from the airgun itself. This, in return, can cause variations of muzzle velocity.

What is the right head size, then?

Each barrel has its own characteristics due to manufacturing tolerances. This is a given, so there is no such thing as the perfect barrel.

This means determining the right head size is a process that is applied to each barrel.

The bottom line is that the head of a pellet should still contact all the lands in a rifled barrel to align the orientation of the pellet. At the same time, the lands should not "cut into" the head of a pellet, either. A pellet can be pushed through a barrel and then have the head examined.

The author finds that accuracy is optimal when the lands of a barrel leave behind impressions (flattening of contact area) but not indentations (with depth).

A choked barrel is a little more tricky because the pellet is shaped by the choke. It may be best to push one pellet only up to the choke, and push another one past the choke, to examine the effect of the choke. The author's own experience is that it is okay not to have impressions from the lands prior to the choke, but it is important to make sure all lands leave impressions after the choke.

Finding the right pellets

You can do the experiment as outlined in the previous section to all the pellets and find none meets the criteria! The author had exactly a barrel like that.

This is partially why some barrels are not pellet fuzzy while some others are. The majority of .177 pellets have a head size of 4.50mm to 4,.52mm. If a barrel can barely put impressions of the lands on these pellets, the barrel is likely to shoot these pellets well.

However, the author has a barrel that is optimal (based on the previous section) for a head size of .176 inches, or 4.47mm. No one makes pellets with this head size, let alone choices to head shape for longer range accuracy!

No wonder this rifle never shot right no matter what pellets it was fed!

Pellet sizers

A pellet sizer is not a pellet sorter. A pellet sorter sorts pellets based on their head sizes. This means that from a tin of pellets, a sorter categorizes pellets into groups based on the actual head size of each pellet. Sorting pellets this can be quite time consuming!

A pellet sizer, on the other hand, actually very slightly reshapes a pellet to a certain head size. There are several types of pellet sizers.

One type is a push through. A precision made instrument has a tapered hole to push a pellet through. This type can be found fairly commonly on eBay. The author has not used this type, so not much can be written about it. By design, however, a push through sizer makes the head the same size as the skirt.

The second type, sold by T. R. Robb from the UK, is an adjustable rig. This is the type that the author uses. precision taper cavity is machined, and a plunger is used to push a pellet into the tapered cavity. Instead of pushing the pellet through, there is an adjustable stop. This allows a pellet to be sized based on how far down the cavity it goes. Furthermore, the taper also allows a finished pellet to have a skirt size that is a little more than the head size. For example, a H&N Field Target Trophy Green pellet (in .177) has a distance of 0.2" from the base of the skirt to the widest portion of the head. A sized pellet has a head size of 0.176" and a skirt size of 0.179" (unsized skirt is 0.184"). The theory is that this allows the skirt to engage the rifling better and seals better.

The third type, only seen in videos but cannot be located for ordering, is a modification of the first type. There are two dies, one for the skirt, and one for the head. The die for the head has a stop, and then the two dies are inserted into a tube where the ODs (outer diameters) of the dies match the ID (inner diameter) of the tube. A pellet is then pushed into the rig, the head portion into the lower die for the head while the skirt remains in the upper die.

The first two types of pellet sizers cost about US$30 to US$40. Keep in mind, however, that the first type is specific to a head size, while the second type is adjustable to work with a variety of head sizes. The third type cost more than US$1000 (presumably for a set of dies).

Conclusion

While the accuracy of an airgun depends on many factors, most shooters may not expect the humble pellets, specifically their head sizes, may play an important part of accuracy. Understanding the importance of pellet head sizes can potentially "rescue" an airgun or a barrel that has been deemed flawed or inaccurate.

The price of a pellet sizer is not prohibitive, it is about the same as 2 to 3 tins of pellets. It is the time that it takes to resize pellets that may be a show stopper.