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Model 21 Bore Dia.?

Dave M.

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June 4, 2026 - 9:17 am

Can anyone tell me the nominal bore diameter of a 12 gauge model 21?

It’s a first year production if that matters. Thanks

James W

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June 4, 2026 - 2:28 pm

I believe the standard bore diameter is .729 for the first year of production. But it can vary between .729 and .735 depending on the grade. .731 and .735 is pretty common. Chamber length for the first year was 2 3/4 inches.

Dave M.

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June 4, 2026 - 4:24 pm

Thank you James.

My 21 has 26″ barrels and is marked cyl. and mod. The right muzzle measures .729 and the left .715

That makes since with a .729 bore. I don’t have the tool any more for measuring the bore dia.

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