Airgun slugs can come in many different shapes, weights, lengths and designs. Most of the time, these slugs are manufactured to a generic size and not to the specific size of each gun. A common misconception is that these rounds will immediately shoot great out of your gun, this is not the case. These slugs are intended to be sized by the shooter for their particular gun and a sizing die and press must be used to do so.

Question: Why do you want to size your slugs?

Answer: Every airgun is different and so too are the barrels. Even barrels from the same production line and batch can have slight variations from one to the next. As stated previously, most ammo manufacturers produce generic ammo such as .25 or .45; your gun, however, could be sized .250, .251, .252, .253, .254 or .450, .451, .452, .453, .454. This means that you would need to have a sizing die and press that matches your guns specific need. By doing this, you will see better accuracy out of your particular gun.
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The key to shooting slugs accurately from an airgun barrel is to match the slug diameter to the groove diameter of the barrel, not the bore diameter. In the next section, we will talk about another feature that may or may not allow your gun to shoot slugs accurately.

*The pictures on the left show one type of press and several different sizing dies*

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Choked or Un-Choked, That Is The Question...

What does it mean to have a choked barrel?: A choke is a tapered constriction of a firearm or air gun at its muzzle end. A choke is designed to alter or shape the projectile as it exits the barrel, potentially increasing accuracy and stability.

There are some aiguns that are choked, which really are designed to be used with pellets, there are some that are un-choked that are designed to use slugs, and there are some airguns that can use both slugs and pellets. Knowing your gun and what ammo it can use, is imperative to the proper function of the gun itself.

As you can see from the image on the left (Fig. a1), a choked barrel tapers at the end and compresses the round as it leaves the barrel. 

Fig. a1

So why is it not advised to use a slug in a choked barrel?: A normal pellet has a hollow skirt that allows for deformation (squishing) as it passes through the choke, contouring itself to the rifling in the barrel. The rifling adds some measure of spin to the projectile and the choke creates uniformity and stabilization. A slug, on the other hand, has much more solid surface area which resists deformation. You may be able to force out a slug at your higher pressure ranges, but the chances of jamming increase drastically as you drop in pressure. You can imagine if you get one jammed in there without knowing, then continuing to fire thinking you might just be off target, how fast those rounds build up in the barrel as they slam into each other. It then could take a melting pot to extract the lead (which is costly to have someone do) or end up needing a new barrel completely.

Exceptions: There are some exceptions to the slug rule, certain guns and certain ammo are specially designed to handle both pellet and slug. A gun / barrel example would be the FX standard X-Twist barrel that comes with the gun is suited for both pellet and slug. As for ammo, FX Hybrids have shown to be very successful in choked barrels and this is due to their design:

Fig. a2

As you can see by the image (Fig. a2), the FX Hybrid round is a hollow round with a flair skirt. The outer design is that of a hollow point slug, but yet can crush to the groove diameter like a pellet.

Again though, it comes down to knowing your gun. For some gun models, this round has a tendency to tip forward as the probe pushes the round into the chamber. This can cause the round to travel awkwardly through the lateral length of the barrel, increasing the chance of instability in flight.

Another example can be found in the image to the right (Fig. a3). These are banded slugs. These slugs are a little more forgiving than the ones without. Although it is still not recommended to send these through a choked barrel, it does reduce the risk of jamming marginally. The bands on these slugs act as a crush area rather than having a completely flat and solid surface. These rounds tend to be on the longer side, so knowing what your magazine can handle, is very important. 

Fig. a3

Slug Summary

So when it comes to shooting slugs, it really ends up being the shooters choice, what gun they have, what their intended target and distance is. Slugs are an excellent ammo for hunting and quite often, for accuracy as well. Slugs can be used in 2 different ways. Under 100 yards they are used more for impact force and over 100 yards they are used for longer range accuracy, whereas pellets are usually used under 100 yards. The most important thing is knowing your gun, and we cannot stress that enough. Know what your gun can handle and what it likes to "eat" when it comes to the rounds you plan on putting through it. By doing all of this, you can avoid costly mistakes, that are not the responsibility of the manufacturer or the reseller. Most of the time, if you use the wrong ammo for your particular gun and it needs to be repaired, the manufacturer won't cover it under warranty since the gun was not designed for that particular ammo.