How can I ascertain the degree of choke on a shot gun?

Shoot on paper. % of shot in a 30" circle determines what choke it is. Note: it may shoot a different % with different shells/shot sizes. You will also see haw well it patterns. It may not shoot what it shows on the bbl. Patterning is based on 40 yards.

IC 55%
M 60%
F 70%
 
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A bore gauge will give the constriction; a pattern plate will give you an idea on patterns

If it is an older European gun, there will be proof marks, including the barrel diameter and choke constriction, usually expressed in MMs.
 
Also be aware that the percentage of shot in the circle, determining choke, applies to the ammo the gun was designed to shoot.

A full choke gun made in the 1920s has a tigher (smaller diameter) choke than one made today. This is because starting back in the 50s, shotshells have been improved (shot cups, wads, etc.) and the shells themselves give tighter patterns than the old fiber and card wad style shells.

My Grandfather's "test" for a 12ga full choke was that it would balance a dime on the muzzle. This was absolutely TRUE for guns made in his day.

TODAY, a dime will drop right down a full choke 12ga barrel. The larger diameter choke of the modern gun still delivers that 70% pattern to make it full choke, because of the efficiency of modern shotshells.

By today's standards, and using modern style shells, the old full choke guns are now "extra full" or "super full" chokes, by pattern density.
 
What make? Lot of that kind of info is available on-line even for older odd stuff.
"...so that correct loads can be id'ed..." That's done by shooting the thing for pattern. The choke doesn't determine the load.
 
A 12 gauge bore can have a nominal diameter ranging from .725 to .750, so patterning the gun with the intended load is the only way to go, regardless of what the name of the choke says.
 
Well, all this is partly to see if slugs can be fired through either or both.

One is a Beretta "Companion" 412 in 16g, the other a Verney Carron Franchi Automatique in 12g. They date back to the 50's and 60's.

I'll have to pull the barrel off the VC to see any additional marks.

My initial investigations about two years ago had proved tricky to just get the proof-mark before firing regular 12/70s: lots of random online proof-mark websites.

The Beretta should be easier to spot although I remember markings were sparce. I'll try that before doing any shooting on paper but, just in case, how big is the circle and at which distance?
 
Last info on slugs was you can shoot the rifled lead slug thru any choke, even full choke. They tend to be more accurate thru a cylinder bore. You'll have to try them and see which is best in a particular gun.
 
Good to know but...

You'll have to try them and see which is best in a particular gun.
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My only concern was about their being safe to use with an unknown choke.
If that is settled, I'm happy to play around with different ones.
 
Many companies used to use different barrel lengths to designate different chokes, with longer barrels being tighter.
 
Forster type "rifled slugs" can be safely fired through ANY choke.

the slanted "ribs" on the slug, the ones that make it look like it was rifled, are NOT there to make the slug spin, they are there to allow the slug to compress, and pass through any choke (including FULL) without problems. Slugs don't spin (unless fired through a rifled barrel), they maintain stability because the nose is heavier than the back end, like a dart.

Conventional wisdom says slugs are most accurate fired through cylinder bores (no choke at all) but each gun is an individual, and might be the exception. Only test firing will tell the actual situation.

Sabot type slugs (Brenneke) of course, have no issues with chokes, as the slug is subcaliber anyway, and the sabot material (usually plastic) easily compresses and passes through tight chokes without issue.

Measuring the bore constriction and getting a number, only tells you what the maker intended the choke to be. Only shooting (at a pattern board) will tell you what the gun and THAT ammunition actually delivers.
 
they maintain stability because the nose is heavier than the back end, like a dart.
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Apply pressure to the rear of a dart as a shotgun slug does to a slug and tell me how that works out :) Not saying that isn't the idea, but I certainly would not say it really works like a dart. One reason I avoid hunting slugs out of shotguns.
 
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