I have found Winchester guns to be slightly more open bored, with less constriction, than others of the same choke. Heard a Winchester shooting rep say Winchester bored them that way so men hit more birds, not fewer birds harder. With good ammunition they thought it was more important to put shot some into every bird, than a lot of shot into a few birds. For the life of me, I can not remember that Reps name, it was the fellow who followed Herb Parson who died in 1959. That had to be 1962-1963 time frame. Also note in that time frame it was before heavy use of plastic wads, which I think keep patterns too tight in some guns. If your pattern is optimized for 30-45 yards shots by plastic wads and tight choke, and you can not hit birds at those ranges consistently, you have a gun which hits hard, but not often.
Anyways the pattern board does not lie and it sounds like your gun has that sweet spot in choke to bore we all wish for. Just a fair warning, never measure or pattern chokes of a .410 too much. It can mess your your mind and confidence. DAHIK, BTDT.
Interesting. Might be on to something here. A friend who owns two unmolested 16 gauge Model 21’s, both choked Mod/Full, one gun is from 1936, the other 1950 both of those guns Mod chokes are bored at .010. The full chokes measure at .025.
The gentleman who owns these two 21’s has done extensive pattern testing. He’s a very knowledgeable guy. He tells me his .010 Winchester cut chokes are absolutely throwing dense modified patterns!
