How to interpret metric shotgun chokes?

M1Garandfather

M1Garandfather

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Hello!

I have just got a SxS 12 ga , a Gorosabel... What I am trying to understand is the choking that is stamped on the bbl's... One is 17.9 and the other 17.4..
Now I am pretty sure this is the degree of constriction in millimeters - but , how does one interpret them the usual definitions of choking we are used to , i.e Full , Imp Mod , Mod , IC , Cyl and so on?

I have searched the net for a good explation , and gotten some idea of what they are... Is this how you go about determining the choke?

Since the gun is stamped 17.9mm = .704" , and 17.4mm = .685".... And if a 12 ga bore is .729" in dia. is all one does is subtract the actual muzzle measurement from .729 (like above conversion) and compare the amount of constriction to a chart and therefore arrive at the choke designation?... If this is correct , it looks like I have a SxS with Imp Mod and Extra Full chokes... Have I got it correct here?

Thanks

Bob
 
Last edited:
Somewhere on your barrels there should be a number starting with 18 which is the bore measurement, probably 18.1 or 18.2. Subtract the 17.x and what's left is the choke in millimeters. Multiple by .4 to find out the Imperial measurements.

You may have over estimated your chokes. Until recently most Euro guns had bores smaller than the standard .729. A bore size of 18.1mm (.724) isn't unusual in an older Spanish or Italian gun.
 
Claybuster said:
Somewhere on your barrels there should be a number starting with 18 which is the bore measurement, probably 18.1 or 18.2. Subtract the 17.x and what's left is the choke in millimeters. Multiple by .4 to find out the Imperial measurements.

You may have over estimated your chokes. Until recently most Euro guns had bores smaller than the standard .729. A bore size of 18.1mm (.724) isn't unusual in an older Spanish or Italian gun.
Click to expand...

I have to point out Claybuster's conversion math is either out, or he's let a typo in....

18.1mm is 0.713"
or 18.4mm is 0.724" ( a typical tight Spanish or Italian barrel )

Certainly nominal bore sizes do vary. I have an old Beretta that actually measures 0.726" ( 18.44mm) and a like-aged Union Armera (Grulla) that measures 0.732" ( 18.6mm)

A dial gauge barrel bore micrometer is the only definitive means of determining choke constriction ... and actually patterning the gun
( 40 yards in a 30" circle) the only way to reasonably assess what choke the barrel produces with a particular cartridge/shot size.
 
Thanks for the help fella's , the fact about Euro shotguns having a slightly smaller bore size was something I didn't know... Now it makes more sense about the amount of choke constriction...

All the best , good hunting this fall!
 
There's a table showing constriction for various chokes and a second table with metric to standard conversions conveniently at the bottom of this page:

w ww.scgunworks.ca/Shotgun%20Frameset.htm
 
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