# Choke size?



## bigboresonly (Jan 29, 2007)

How is choke size determined? Is for example, a full choke a 15% constriction, modified 10%, extra full 20%, etc., or, a certain measurement in thousandths, or, something that is totally gauge specific?


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## darkgael (Feb 10, 2006)

Patrick Sweeney has a good discussion of this in his book "Gunsmithing: Shotguns". The simplest comment that he makes is that choke is a variable thing - it is not the same in all shotguns. The best way to determine choke is to pattern each gun. 
An object lesson: I have a Galazan choke gauge which I bought before I knew better. When I put it into the muzzle of an old Ithaca SXS that I have, which is choked F/M, the gauge measures just that - F/M. When I put it in a Parker SXS, also supposedly choked F/M, it meaures IM and 1/4. The same gauge put into a Mossberg screw-in full choke gives yet another result. 
The gauge is the same yet all the "Full" chokes are different. 
To answer your question with numbers - the amount of choke is the constriction from the bore measured in thousandths. A 12ga. bore is "nominally" 0.729". The US standard for full choke constriction is 0.030".
The best way is to pattern your gun. A full choked gun is supposed to throw 70% of its shot into a 30 inch circle at 40 yards; IC should throw 50%. None of my shotguns will do precisely that; some will throw nearly full choked patterns with modified chokes, others seem to throw the same pattern with either barrel. And a lot depends on the load.

You can find an excellent discussion of this in Bob Brister's "Shotgunning: the Art and Science".
Hope that this helps.
Pete


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## bigboresonly (Jan 29, 2007)

Wow, I didn't realize it was that subjective! I always figured all that data was already cast in stone by someone like Greener, who had done so much research on patterning. I knew things like rifle loads were subjective, because each rifle barrel and each twist has it's own likes and dislikes on ammo. It looks like the the choice of loads could be endless, with the difference in what brand and specific choice in brand of shell, plus the choke thrown in there. I just figured that a smooth tube, was a smooth tube, and it always reacted a certain way with a certain amount of restriction, and that reaction had already been cataloged 100 years ago!


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## southdakbearfan (Oct 11, 2004)

Varies from gun brand to gun brand and whether the barrel is backbored or not.


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## bigboresonly (Jan 29, 2007)

According to kwacs.org.uk, common British choke restrictions are .005 IC, .010 1/4 choke, .020 1/2 choke, .030 3/4 choke, and .040 for full choke. I kind of figured they'd have a formula, since they are THE gun people. In that formula, they have a so many inch spread per constriction, like 41" spread at 40 yards for full, 59" for cylinder, etc., and I guess if your particular gun was patterning that way, you know just about what type restriction you had. Of course, how that relates to American choke, I have no idea. I'll have to check around and see if there is a comparison to say 1/2 UK choke, being similar to the US modified for example.


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## darkgael (Feb 10, 2006)

Was away for a few days. Just sat down and checked the NRA "Firearms Fact Book" (a handy reference for your gun book library).
The constrictions listed there are: FC/0.036, IM/0.022, M/0.011, IC/0.007.
I'm sure that, if I keep looking, I'll find a different set of dimensions.
Pete


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