Breech loading

Breech loading refers to loading the ammo from the rear of the barrel, that is, near the valve or chamber. It generally provides faster reload rates than muzzle loading.

Many types of breech loaders are employed. These 3, which utilize un-modified commercially available parts, are described in rough order of their speed of operation:

One common method is to use male and female threaded adapters to connect the barrel to the chamber, as seen here
To operate, one has to turn the barrel several rotations at a significant amount of torque, especially if the threaded joint is under a load due to the weight of the barrel. The ammo is then inserted, and the barrel screwed back on.

As an improvement over NPT threaded adapters, one can use a union.
Operation is similar to that of the NPT threaded adapter method described above, except that unions require fewer turns, don't require the entire barrel to be rotated, and don't require much torque except at the very end of tightening. The union in unscrewed, ammo is inserted, and the union is screwed back together.

As an improvement over even unions, one can use a camlock coupling fitting to connect the barrel and chamber. A cannon with one can be seen here
To operate, one rotates the levers outwards, pulls the 2 fittings apart, inserts ammo, puts the fittings back together, and returns the levers to their original positions.

Valves that open by sliding open exist. These are typically put on one of the co-linear parts of a tee.
The valve is simply opened, the projectile inserted, and the valve closed.

These methods which require more work to make (ranging from some work with a dremmle to installing O-rings) are described below:

One fairly common method is to simply hold the breach-end of the barrel into a socket (such as a coupler). Such a method is suitable for pneumatics, but the hot gases from a combustion can cause erosion and discoloration of the material, unless an O-ring is used to make it completely air-tight.
To operate, one undoes the latch mechanism holding the barrel, moves the barrel away from the coupler, inserts the ammo, moves the barrel back to the original position, and redoes the latch. Example, Example

In a manner that emulates the last stock-component method, one can make a plug to be inserted into the end of a tee. example
To use, one removes the plug, inserts ammo, and replaces plug.

Another fairly common method of breach loading involves covering a hole in the barrel with a coupler that has had the inner stop removed, as in bushing modification. The same leak issues that exist with holding the breach-end of the barrel into a socket exist, and the solutions are the same. Use entails sliding the coupler forwards, dropping the ammo in, and sliding the coupler back.