BARREL THREAD SIZES:
1/2x28 - Threads are everywhere. There's tons of guns & barrels with these threads, too many to list. Most USA Firearms 9mm & under have these.
5/8x24 - Most common on 30 calibers including 300 Black out, 30-06, and 308.
9/16x28-Most common on 40 cal and 45 cal pistols and ARs.
1/2x36 - Most common on USA-made 9mm ARs. Not very common on anything else.
M13.5x1LH is a thread found on many non-USA threaded 9mm handguns. Euro guns tend to be even less standardized, so, it may fit many others.
M14x1LH is the most common thread for AKs.
M14x1RH is found on some SKSs and quite a few 7.62 chamberings from Yugoslavia.
Guns use PARELLEL threads. Water pipes use COMPRESSION threads. Parallel threads screw together without the threads mashing into each other. Compression threads screw together easy at first, then get snug, then get very tight as you continue to turn them. This is because the threads mash into each other to create a water-tight joint. Never put compression fittings on your rifle or pistol because they will damage the threads. Oh, they work ok to begin with, but after a few install/removal cycles, you will begin to notice the wobble in your fitting. At this point you will need some creative gun smithing and maybe a new threaded barrel. We do not sell any compression fittings. We only sell PARELLEL fittings. You can install our fittings 10,000 times without thread damage. Our production templates also include the correct thread shape used in the firearms industry. Tip: Always clean a new fitting with a bristle brush (stiff nylon or lightly with soft wire brush) then apply a few drops of vegetable oil to the barrel threads before installing. Most folks use thread lock (from any auto parts store) or teflon tape on their endcaps and fittings threads, not on your barrel threads.
If knowledge is half the battle, then here's a 100 round clip of mental ammo.