"You do this to an AR"

"It makes the recoil feels much lighter, like it is only as bad as a .38 Special."

Huh?

What an odd thing to say. 

Is the 5.56/.223 recoil really bothering you?

It's hard for me to judge, just thinking about it.  That I could perceive a big enough difference between the two calibers in felt-recoil in a shoulder fired arm.

There is more propellant in the 5.56.  And normally a longer barrel and heavier platform to alleviate some recoil, compared to the usual platform a .38 comes out.  But a big part of felt recoil equation is the Newton part.  No, not the fig filling.  The mass.   5.56 is the 55 grain projectile and the .38 is summat like 158 grain.  So a 3X difference.

All this is apple and oranges, tho.  I don't why I have devoted the above amount of brain sweat to this madness.

I wonder, if you did make a .38 Special AR, if you could notice the difference between it and a regular AR in a blind taste test sort of range session?  I'm guessing no.

Don't make a .38 AR.