i had CARLSONS CHOKES cut and install a turkey tube on my browning A500G FOR #4 SHOT.it is .670 dia. which they say normally gives better patterns for the larger shot.it is my understanding that .660 is for #6;.665 is for #5 shot; and .670 is for#4 shot.however,as stated above,every barrel shoots differently with various loads by different manufacturers.i have never tryed the #5 or#6 shot so i cannot make any comments about them other than they are smaller,have more pellets in a load AND have less knockdown capability past 40 yds than the #4s.if you shoot the bird at 30 yds, it really doesn;t matter which choke or size shot you use in either my 870 or a500g, but at 45 yds. it is a different story.you just have to know the effective range for YOUR gun and load you shoot.