If the gun and choke manufacturer states Improved cylinder, you should probably follow their stated recommendation.

Remington Slugger box:

 

This is the information that i think brought up this post. Posted above is a picture of a box saying Remington slugs can be shot through any choke. From my research is that this is an old box, like 20+ years old which is not relevant to modern ammo.
Again from my research that there was a time (70's 80's) that ammo makers went from a .690 slug to the current .730 slug, Remington being the last to change over. So the "this slug can be shot though any choke" is no longer a thing. While the old .690 slugs could be shot thru any choke since it was smaller then the choke to begin with, i know of no manufacturer using this size in modem ammo.
So why would they change the size of ammo. Two words deer slayer. The deer slayer came on the scene laid down some accurate shots at distance. Why? because it was underbored to fit the current .690 slugs of the time at .708 compared to the more common .730 bore. Ammo manufacture took note and to increase accuracy of there product went to the .730 slug size so it would be the best accuracy for the common barrel at the time.
I went down this rabbit hole cause there is alot of different chokes that are said to be safe but manufacturers saying that this is not safe. I can see that some of this is going by old data points or old info. I do know that a modern Federal slug had to be forced thru a cylinder choke with improved rifling. So i would encourage anyone looking at a choke for slugs to open one up and see how tight it is thru that choke. Short story, i agree with the manufacturer that anything tighter than IC is risky and that is what i will be sticking to. Thanks.

 

That picture, I posted above, is of a newer Remington Slugger box with the larger diameter slugs. The note on the older smaller diameter slugs says this:

The old small diameter Sluggers always shot better thru a modified choke for me.

 

I think I'd pattern the buckshot through your spectrum of chokes to verify the "tighter is better" expectation before agonizing too much over the "how tight can I go with the slugs" question. Maybe the buckshot will actually pattern better with something less than full choke.

Beyond that, you may want to add Brenneke slugs to your consideration. They state that some of their slugs tolerate well being run through tighter chokes.

 

Not all chokes are equal. Years ago I was playing with home loaded slugs (and other fun stuff for pumpkin season) like Lee 1oz and 3/4oz and .69 round ball and buck&ball out of a 18" cylinder bore. Better accuracy than you would think at 50 yards. I had the same question about the chokes. So I dropped one through each choke just to see what would pass. In the 30" Rem factory barrel (fixed full choke) the .69 ball would pass. Not enough to be safe but it would pass. I also had a old side by side. The .69 got stuck about 1 inch from the muzzle on the full side. Both guns full choke. . For me the less choke the better. Might be best to keep your slugs in a different box painted red just to be safe? Pumpkin Season-- Target is .69 ball standing no rest. .

 

Cylinder Choke is even better than IC or Skeet
No choke is needed for slugs.
And if you have the great Federal Buckshot with the Flight Control Wad again No choke or cylinder choke is preferred.
M4 Benelli and Browning A5 Slug Gun
We use this slug for Grizzly Bear protection in Alaska.

Brennke Original Magnum 1 1/8 OZ in 2 3/4”
Federal Premium 00 Flight Control

 

Strangely I always thought strait cylinder was best for foster slugs but after trying a few different chokes in a few different guns Imp/Cyl does have a slight edge, more some guns then others.
Ive shot many fosters through full choke but one thing I definitely noticed that would be a problem if you shot a lot of them was that a full choke builds up lead fast.
I wouldn't go over a modified choke, I did a lot of pattern testing with buckshot and found in my guns that Mod or Imp/Cyl gave the best patterns with different buckshot.
The tighter chokes did shoot tighter but in some guns they threw high and a bit off Center or they started stringing especially with 00 buck.

But you would be fine for the occasional slug through a Full, but I'd test the buckshot out up to Mod and then use the one that patterns best.

I'm doing similar with an U/O that I picked up for Pigs, it's a "Taticool" with rails so I can mount a 1-4 spot hog scope.
I'm putting a rifled choke in the top barrel and an Imp/Mod in the bottom.
That way I've got good buckshot spread when pushing through the thick stuff and I can shoot slugs or RB for longer range out to 50 yards. But with the rifled choke I should be plenty good enough to take a 100 yard shot with the top barrel if the need arises.

 

Shot foster and breneke slugs through my Mossberg 20ga bolt action with a fixed full choke. No issues. Breneke slugs were especially accurate from this gun

 

I have one 12 ga that does not shoot slugs well through the cyl. choke (keyholing/not accurate) but does great with an IC. Have another that will print 8" groups at 100 yards with IC (don't have a cyl choke for that one) but has trouble hitting a 17" steel plate consistently at 50 yards if I switch to the mod choke (no bueno).
FWIW cyl/IC are a safe place to start but each gun may have it's own preferences. It wouldn't surprise me if two of the 'same' chokes from different mfr's act differently.

 

Which "fixed choke" (if any) Would a Winchester 18" Defender barrrel be considered as being ?

 

Most likely cylinder bore, which is little to no choke constriction at all.

 

Probably cylinder bore. No choke.

 

I'm guessing @Tunafish389 is gone now, but I'll try and clear up some misconceptions anyway.

Rifled slugs were not smaller in the past. I've never heard such a claim, I've never seen any evidence to that, and that goes for slugs as far back as the 60's. For anyone still alive, rifled foster slugs have always been .730" plus or minus a little. Maybe the original foster slug in the 30's and 40's was different. It was invented in 1931.

Along that same line, there have been some undersized slugs around, but I can't think of a factory load for such. There were, and still are some foster slugs that can be reloaded in a lead shot wad. Lyman's foster slug used to have a swage kit to make it about a .680" rifled slug for use in a lead shot cup. There were various rifled slugs sold such as BPI's segmented rifled slug for reloading which could fit in a shot cup. Today the most common is one of the Lee foster slugs which are heavily tapered, but they too fit in a shot cup. Again, I'm not aware of anything like these ever for sale from a factory loaded round.

There are also currently 2 variations of rifled slugs you can buy today. One is a classic foster slug, such as a Remington slugger or Federal Truball. The other style I can only call a "Brenneke" style because Brenneke invented it in 1898. It is also rifled, but it doesn't rely solely on a huge hollow base for stability, they also incorporate attached tail wads for stabilization. Some examples would be slugs from Brenneke (duh), but also Fiocchi Aero rifled slug, and I'm sure others. The reason I bring this up is that these types of slugs behave differently, despite doing the same job. A foster slug such as a Remington Slugger does no good to slide it through your bore to check fit. It will not be that size when you shoot it. When you fire, it will set back in the shell and expand larger. When it hits the forcing cone it is going to be over .750". It will then squeeze down to bore size. The reason they do not harm choke tubes, besides being softer lead, is the rifling allows them to squeeze down much easier than if they were solid. "Brenneke" style slugs do the same thing, but they do not bump up in the chamber as much since they are not nearly as thin walled. Brenneke classic slugs are relatively hard, but something like the Fiocchi Aero rifled slug is dead soft. Both are safe due to the thin and deep rifling on the slugs allowing lead to displace.

What choke you can use depends on the choke tube. In this instance, the best I can find is that Tristar uses the Berretta/Benelli Mobil thread tubes. There's a decent selection for these out there. Carlsons is the brand I always go with. You can shoot rifled slugs through any constriction Carlsons up to X-Full. I'm not saying it will be the best for slug accuracy, but you can safely shoot a slug through most chokes.

One last bit, that is a modern Remington Slugger box pictured above. They still say that on them, they always have, and always will be safe in any choke constriction. The only exceptions are specialized choke tubes such as turkey chokes. Those tubes say right on them no steel shot or slugs.

 

You are wrong about the diameter of slugs not being smaller in the past. Remington Sluggers for a fact had slugs that were a smaller diameter in the 80's.

 

Prove it.

 

This isn't proof, but other people know of slug diameters changing thru the years.

"Foster type rifled slugs were of a reduced diameter from the 1930s to about 1980. Winchester was the first manufacturer to increase rifled slug diameter. The 12 gauge Winchester Super X Rifled Slug was increased from .680" to .730" diameter at the base. Remington was the last to increase Foster slug ammo to nominal bore size."

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Foster Slugs in 12 gauge

Had some time to kill in Topeka today and went through a good antique store I hadn't been through in a while. There's always interesting stuff in there, but I don't buy much most of the time, though I've found a few treasures over the years. Today, in a display case, was a Lyman 12 gauge...

castboolits.gunloads.com

I am not at a place where I can cut open slugs and measure them to prove a point. Just because you have never heard about it does not mean it did not happen.

 

I am a older man and I remember the smaller did slugs of old. I also remember when I saved to by a deer slayer and first shot it Remington slugs were pretty poor but Winchester slugs had been increased in size and shot very well at 75 yards. Brennekes shot the best but would stick in my chamber after firing so sadly I could not use them.
I bought some Federal Truballs to try in a over bored gun I had and was surprised that they shot has well has they did.

 

I hate spell check and posting while tired. It was a known fact that slugs were under sized. Jack O'conner used to say they bounced down the bore for so-so performance. About the time Rifles were legalized in my state they were fixing it with .730 slugs and things got a lot better. I haven't used a slug on deer in twenty years but one of my sons has a 870 20 ga. that when shooting Federal tru bores with the rifled choke tube groups about 3.5 inches at 75 yards and I know its a deer killer. You need to keep the choke tube clean though.