Metric choke numbers on a Spanish gun?
 

I have an Armas Erbi 12ga sxs in the safe. I was trying to figure out what the chokes are. I don't have a bore gage to check. The barrels are stamped "Choke" with numbers below that. One barrel has 17,3 and then 18,2 below that. The other says 17,9 with 18,2 below.

What do those numbers mean?

17.3 mm = .681"
18.2 mm = .7165"
choke .036" Full
17.9 mm = .7047"
18.2 mm = .7165"
Choke ..012" IC or Light Mod
Bob Jurewicz

I believe those are choke diameter measurements in millimeters. For instance, 18.2 millimeters is the english equivalent of .716 inch, hence would be a constriction of .13 inches, assuming a standard 12 bore diameter of .729 inches

Thanks guys! I figured they were mm measurements. I'm still confused on how to determine the choke with the numbers though. Is the 18.2 indicating the bore diameter and the other number the choke measurement? If so, the bore is .7165" rather than .729". If 18.2 is not the bore, then what is it? Which number is referencing the choke when each barrel has 2 numbers stamped on it?

Bob is correct. The larger number is the bore. The smaller number is the constriction. Yes, not your standard .729 for 12 gauge.

Excellent choke combination.

OK, cool. Thanks guys! I'm glad to learn something.

Would these slightly narrower bores lead to higher pressures, maybe a little more velocity, and tighter patterns (compared to the same constriction differences on a .729" bore)?

Not that it matters a whole lot. Just curious.

I doubt it. The modern shotgun wads have a hollow base and petals that expands to the bore size, just as the great invention of the minie ball did for muskets.