Welcome to tfl!
Welcome brandan!
You are worried about nothing. Slugs are made to be safe when fired in ANY choke gun (including full choke). Safe.
Where you get your best ACCURACY is usually from cylinder bore (no choke) barrels. (Rifled slug barrels are a different matter, discussed separately. I'm talking about smoothbore barrels here
)
The standard Forster type slug, is often called a "rifled slug". This is because the slug has angled fins on it, that resemble the rifling in a barrel.
Many people think these spin the slug for accuracy, like the rifling spins a bullet. This is not true. Slugs are as accurate as they are, because they are nose heavy, like a dart. It is the weight distribution that keeps them on target, NOT SPIN.
What the "rifling" fins on the slug do actually do, is allow the slug to safely pass through any choke without any serious deformation of the slug or build up of pressure due to the choke restriction. (the fins will squish/flatten a bit as they pass through the choke).
Let me repeat, so its clear, you can SAFELY fire slugs through ANY choke.
You will probably get a better group with an open choked barrel than a full choke one, but both (and everything inbetween) are SAFE to shoot slugs through. Commercial "Slug guns", in the decades before rifled slug barrels became common, were simply regular cylinder bore barrels fitted with rifle style open sights.
The old timers used a coin to determine if a barrel was suitable for slugs.
Drop one down the big end and if it sticks in the small end, don't.
If it passes clean through, then do.
Wish I could remember what sized coin it was, though.
But they did a lot of things that are frowned on now-a-days.
This reply wasn't meant to be of any real help.
Well, you were right, it's no real help!
I had never heard that one before. Amazing what one still hears.
What I do know, is what my Grandfather taught me. His (and his contemporaries) test to see if a 12ga was full choke was to use a dime. You can balance a dime in the muzzle of a full choke 12ga. If the choke is more open, the dime will fall down the barrel.
I have done this, it is absolutely true,
BUT it only applies today to guns made in my Grandfather's day, essentially. Modern guns, a full choke 12ga, will let a dime fall right through.
This is because the amount of choke is was not a fixed standard bore diameter. In the old days what choke a barrel had was not determined by the inside measurement of the barrel (although that was the starting point) but by patterning results. The percentage of shot pellets inside a 30 inch circle at 40 yards was the standard. Full choke meant 70% (or more) of the pellets inside the circle.
What ever degree of barrel restriction (choke) it took to produce this result,
with the ammo of the day, was what it took. Two different guns with the same choke barrels could be several thousandths inch difference in choke diameter.
This is where my Grandfather's rule came into play. In his era, NO 12ga that would let a dime through the muzzle would produce a full choke pattern,
with the ammo of the day.
And,
the ammo of the day was a critical factor. Up until the 1950s, when plastic shot cups came into use (along with plastic hulls and star crimps) shotgun ammo was loaded with fiber wads under the shot, and a thin card wad on top, held in place with a roll crimp. Shot pellets could touch the barrel on their way out, resulting in flat spots on the pellet, which meant they wouldn't fly straight. (air resistance causes the damaged pellets to veer off from the main shot column)
With the general introduction of modern ammo with plastic shot cups, deformed pellets were drastically reduced, and therefor more pellets would be in the patterning circle, for any given load.
So, to get the 70%, with the "new" (modern) ammo, a barrel didn't have to be choked as tightly. So a modern gun, 12ga, full choke will let a dime fall through the muzzle, and still deliver the 70% concentration.
Rifled slugs come from the old days too. BECAUSE of the possible wide variation in barrel diameter due to different chokes slugs were made to be able to SAFELY squeeze through the tightest choke you might find.
brandan, don't waste your money on a choke gauge (even though they are cheap), its only an approximate measure anyway. To know what you really have, you have to pattern the gun.
Take your gun, or just the barrel (if possible) to a gunsmith, and they will have a choke gauge. (check first) IF they charge you anything to check your barrel it should be minimal, and ought to be less than the cheap price of the gauge. If they try to tell you something outrageous, go elsewhere (for EVERYTHING!)
Hope this helps...