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I got a hold of some Viht-N120 and found a recipe on the Vihtavouri website using 20g of that powder with a 55gr Winchester FMJ (which I have mostly in my stash). I tried it in my Rock River 20" HBAR (.223 Wylde chambered) and it will not cycle enough to load another round. Just an empty chamber. But it was very accurate out to 100yds, fair at 200yds. However, when I shoot the same bunch out of my lower cost 16" Blackthorne (Hesse, I think) It shoots flawlessly, very accurate when I shoot it at 25yds with a red dot.

Is it my loads, something related to the gun specs or something else? Both rifles will cycle all other reloads and factory ammo that I've shot out of them. Seems that it is just that powder with that rifle.

What recipes do you use with N120?

Probably what he said. If your 16" has a carbine-length gas system, there's a lot less volume the gas needs into expand to in order to create proper pressures to actuate the bolt properly. The 20" would have a rifle-length gas system, which has a lot more volume for the gas pressure to fill. There may just not be enough pressure for the long system.

I know people who use VV-135 in their ARs, and have great success there, if you wanted to try that instead at some point.