Caliber Changes For The AR-15 Rifle

In most cases, all that is required is a barrel, bolt change.
Switching from 5.56x45MM to 7.62x39MM requires a bolt, barrel and magazine change.
I assembled a new top end with the 7.62x39MM components, so now all I have to do is knock out the two breakdown pins, remove the 5.56x45MM top end, and install the 7.62x39MM top end in the reverse order, change magazines and I'm back in business.
This takes less than 1 minute...

The exact same process nets me a variety of available calibers for under $300 (USD) each.
Current available calibers are .222, .223, .300 Whisper, 7.62x39MM Russian and many others.

Pistol calibers are a little more involved, depending on the conversion you use.
Some conversions require a magazine well insert to hold the smaller pistol caliber ammunition, and some require changing the recoil buffer spring.

Some short barrel conversions will have cycling problems.
Pistol calibers are normally blow back instead of gas operated, so cycling problems with them is limited.

Rifle calibers with very short barrels have a tendency to try and cycle too soon and too fast.
This is due to the gas port being moved so close to the chamber, delivering too much pressure (the bullet would normally be much farther down the barrel, reducing pressures)
and delivering that gas pressure way too soon in the cycle.
Cyclic rates of fire on full auto will go through the roof, if you can keep the rifle functioning long enough to empty a magazine...

There are some aftermarket fixes for these problems, like gas regulation valves on the front sight post, or installed in the gas tubes.
One aftermarket company is actually using a full length gas tube, and just coiling it around the barrel.

I have found an unorthodox solution for this problem, and it is VERY cheap...
Remove the front gas block, and thread the gas port from the bottom. You will find the gas block has been drilled straight through from the factory, so tapping the hole is very little work.
Install a short allen head set screw in the threads.
Drill the set screw with a much smaller hole that the original gas block had, and test the rifle.
If you don't get enough gas to make the bolt cycle properly, remove the gas block and drill the hole a little larger...
Repeat until you get the cycling you want.
If you screw up the hole size, just replace the set screw and start again! Costs $0.20!
It is possible to slow the cyclic rate of fire this way until even on full auto the rifle is controllable...

Current pistol caliber conversions are available in nearly all rimless calibers.
I've seen .32 Auto, .38 Auto, .38 Super, .357 Sig, 9MM, 10MM, .40 Cal, .45 ACP, and the ridiculous .44 Auto Mag and .50 Mag.
I'm sure there are many others I haven't listed, but these are conversions I know are commercially available right now.

All of these pistol caliber conversions require magazine well adaptors for the pistol caliber magazines, and most require changing out the buffer and buffer spring.