I we are talking about small arms, I don't think there has been anything truly revolutionary since 1900. There have been some pretty big steps and I would say the autoloading rifle is probably the biggest, but I see it as an evolution to repeating weapons before the turn of the century.
If we are talking about all weapons system, I would have to say flight.
The self contained metallic cartridge was the breakthrough that allowed the modern repeating firearm as we know it.
But, since you want to keep it to the 20th century and later then probably improvments in metallurgy may be the biggest single continuing advancement. Aluminum, scandium, titanium, stainless steel... This could include the use of plastics for frame and stocks.
I think the ability to remote fire is changing things quick. I'm sure remote controlling a UAV or a robot with weapons does not feel the same as pushing that button in a cockpit or looking down the sights at your target.
ICBM's. I believe thats what keeps us on or near the top of the pile with a lot of smaller countries that hate us and the threat of mutually assured destruction keeps all the big superpowers playing relatively nice. The threat of nuclear war, human extinction and the devestation of a nuclear attack has really shaped how, why, when and with who we will go to war.