My 42's choke restriction, measured with a full lenght bore mic: Skeet = .002, IC = .003, Mod = .005
The Shotgun Book by the late Jack O'Connor lists the following regarding choking for the .410 bore shotgun. Data include (in order): boring, bore diameter, muzzle diameter, constriction, and percent at 25 yds. in 30-inch circle:
Full Choke, .410, .396, .014, 70
Modified, .410, .405, .005, 60
Imp. Cyl., .410, .408, .002, 50
Cylinder, .410, .410, None, 40
I've had about a dozen 42's over the years & they mic'd similar to O'Connor's list. I know if you looking at newer charts they list 410 chokes being tighter. The difference could be the period & the ammo, or finding the earlier more open chokes had too many open spots & a tighter choke in 410 was more effective. But who knows??
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