barrel length for suppressor

[Rooster 50]

I think 20" is about what the .223 is designed around.

You think that the 223 cartridge was "designed" around a 20in barrel. I am always willing to learn. Perhaps you can explain that.

edited to add..........i am not being smart with you I assume that you are referring to the military use of the 223. The problem with that is the OP said he was using it out to 400 yards
the loss of speed from 26 to 20 in could easily amount to 120 to 180fps. I don't care which bullet you are using that is a lot of speed and energy lost on a load that didn't start out that fast
to begin with.