The Beretta inertia single trigger is good with light loads (in my experience).

Here (UK) 21g loads are widely available and popular in 12 bore. I use them almost exclusively on clays and have used them in a 7 1/2 lb Beretta SO6 for 10 years or more. NEVER had a problem. I would expect in a lighter gun the recoil to be a little 'less damped' by the gun's weight (i.e. a bit sharper), so there should be even less risk of the inertia mechanism not working.