I have a bunch of 40 grain bullets to shoot and I am looking for what would be the lightest charge to try in my 5.56 which would still cycle the action. Looking at my loading manuals it looks like 4198, Reloader 7, and VV 130 would give decent velocities with the lightest charges. Has anyone tried these powders or others with 40's in an AR 15?

Mid length, rifle length, or carbine gas? Size of gas tube?

Stuff matters in regards to your question.

It is for a S&W M&P Sport II.

Tunable gas port might let you go quite low.

I have found .222 Rem loads (the data, not the actual rounds, of course) to function ARs, so you might get some ideas there.

Thanks. It will be a good starting point.

I absolutely have too many AR-15's, that said one of my favorite's I shoot nothing but 40gr BT's out of it. I tinkered with many different powders and in my rifle the best powder was Benchmark, start at 25.5grains and work up from there. If you cant find Benchmark VVN133 came a close second.

What utah708 calls a Tunable gas port the AR-15 snobs call an AGB (adjustable gas block). one of these is not really necessary but you can fine tune your rifle with one if you wish

Adjustable gas block is essential if trying to run a short barrel or subsonic loads (I’ve used one on a 7” 300 BLK)

Opposite end of the spectrum from a 300BLK :D, you could never get a 223 with 40's to even function in the realm of subsonic with an AGB or magic pixie dust...............doubt the OP has a 7" barrel either.......guess we will find out