The standard way of doing things, because it is the cheapest and easiest to make; in fact, it is not possible to make a launcher that cannot be muzzle-loaded.
To muzzle-load a spudgun:
- Get a (straight) stick or a length of PVC pipe that fits in the barrel to use as a ramrod. It is helpful to make a mark on the ramrod indicating how far it needs to be inserted into the barrel to push the projectile to the proper depth. This will help to consistently position the spud. The mark will also make it less likely that the projectile is pushed too far and ends up in the guns chamber (for combustion guns) or so close to the valve that it interferes with the valves operation (in pneumatics).
- Cut your projectile to size, either by pushing your projectile over your muzzle knife or a separate spud cutter. Cutting the ammo to size using a knife is not recommended since the resulting ammo will fit the barrel poorly.
- Ram that projectile home. Firm, steady pressure works best. If you cannot get the projectile to go down the barrel you may need to provide some sort of vent to release the air trapped between the ammo and chamber.
There are several methods to vent the trapped air.
- A groove can be cut in the ammo to allow the trapped air to escape. This is probably not the best solutions since, in general, you want an airtight fit between the ammo and barrel.
- If you are loading a standard combustion gun, you can simply remove cleanout plug to prevent pressurization of the chamber.
- A small (~1/16") vent hole can be drilled at the base of the barrel to allow the air to escape.
- Instead of a hole, a small valve can be installed as the vent. This prevents air or hot combustion gases from shooting out of the vent during firing, thus decreasing performance and/or burning the operator.
- For pneumatic launchers, it is sometimes possible to open a bleed port on the valve, or to simply open the gun's pressurization fitting to allow the air to escape as the projectile is rammed home.
It is recommended that you sharpen your muzzle if you are planning to shoot produce (spuds, zucchinis, etc.). This is called a "muzzle knife" or spud cutter. If just the outside edge of the muzzle is sharpened, a "single bevel knife", the ammo will be cut so that it's outside diameter is the same as the barrel's inside diameter. The fit will be very nearly air tight. For combustion guns, somewhat better performance can be obtained by adding a small bevel to the inside of the muzzle as well, a "double bevel knife". With a double bevel knife the ammo is cut so that it's diameter is slightly larger then the internal diamter of the barrel. As the ammo is loaded it is compressed giving a very tight fit, very little gas leakage when firing, and increased static friction which is beneficial for combustion guns.
If you do not feel safe having a cutter on the end of your gun, you can instead keep your spud cutter as a separate instrument.
It is generally not a good idea to load a gun that is otherwise ready to fire. For combustion guns this is generally not a problem since the ammo is an integral part of the chamber. It would be very difficult to properly fuel a combustion gun without having the ammo already in the barrel.
A pneumatic gun should have the ammo loaded before the chamber is pressurized. Some pneumatic gun designs will vent some air out of the barrel when the chamber is pressurized. This may make it impossible to muzzle load before pressurization of the chamber. In this type of gun extreme care should be taken while muzzle loading.