Feeling tight when opening and closing the barrels may be tightness between the forend iron and the front of the receiver. Test for this by removing the forend and opening and closing the barrels. If the tightness is gone, it's not the width of the barrel monobloc. Removing a small amount from the rear of the under lug will eliminate this stiffness. This is very common on Browning Citoris and lots of other O/U's. Rich Cole is a top authority and gunsmith, relative to Berettas.
The precision of the modern machining makes barrel interchanging much more do-able than it was on older guns.
Hope it works out well for you.
