First the Lee slugs are for rifled barrels. They are loaded inside a shot wad that acts as a sabot holding the slug. So take the diameter of the slug and add the thickness of the wad petals to it 2 times to come up with the diameter of what you will be pushing thru each choke. I doubt the Lee slug will compress as readily as a rifled slug when squeezing thru the choke. I have often heard that SxS shotguns are poor choices for shooting slugs.

 

Good to know. How about the Lyman 475gr Foster slug? It’s a bit larger at ~.700 and is made to be shot with smooth bore, no shot cup, through tight chokes.

 

It's not a good idea to shoot slugs though a modern gun with tight chokes. You have been advised not to with your older gun. If you want to shoot slugs buy an appropriate gun.
The Lyman slug also is loaded inside a wad just like the Lee. That's were Lee got the idea for there slugs.

 

The seller advised me not to use COMMERCIAL slugs. There is a large difference. Commercial slugs are, at least where I live, loaded much hotter and heavier than smaller shot, and are nearly always made for use in rifled shotguns. Of course I can’t use a .730 diameter commercial slug in a .700 diameter choked barrel. That’s why I’m using a ~.695 diameter cast slug instead. Also shot cups are NOT necessary for Lyman Foster slugs.

 

How about a round ball? They come in various diameters.

 

If you shoot slugs in this gun there is a significant risk of bulging the barrel(s), separating the ribs from the barrels at or near the muzzles or both.

If your BSA was proofed between 1887 and 1954 it should have the word "choke" as part of the proof marks - prior to 1887 the term stamped on the barrel flats was "not for ball". Either term means that the gun is not suitable for shooting solid balls or slugs. Starting in 1954 neither term was used because it was now assumed that after roughly 100 years all shooters in the UK knew not to shoot slugs in choked barrels.

As a previously posted "Do whatever you want to it's your gun" but if you do shoot slugs and destroy the barrel(s), which effectively destroys the gun, it's on you not those of us who advised you not to.

SxS's are often notoriously inaccurate with slugs so in addition to risking damage to the gun you could also have an inaccurate gun. Single barrel guns such as semi-autos and pumps tend to be much more accurate with slugs and, with adjustable sights, can hit somewhere near the point of aim.

 

The shotgun is not suitable for solid slugs. I know this. This shotgun was not designed to be used with slugs, but the Foster slug WAS designed to be used in a smoothbore tightly choked shotgun as this one is. What I have been looking for, and have not received, is an explanation as to why the Lee foster slug (or Lyman’s foster slug which I’m leaning towards) would be dangerous to the gun. Just a lot of talk about ‘slugs are bad for tight chokes, slugs are bad for double barrels’. I’m not going to shoot a foster slug until someonequalified gives me the go ahead, but I’m not going to toss the whole idea aside without proper reasoning behind it.

 

Just shoot moderate speed/pellet count buckshot through your double and don't worry about damaging it.

There are lots of concerns about high pressure impact against the chokes when shooting slugs. Especially with tight chokes - modified through full. As well as common results of poor accuracy compared to a cylinder or improved cylinder choked barrel. Solid copper sabot slug boxes are often covered with lots of pressure warnings and limitations. Why risk your old BSA?

good luck, garrisonjoe

 

The Lee 12 ga, 1 oz key drive slug is cast with pure lead. Slug has a taper, being smaller at the base.
On firing the slug will form to barrel diameter sitting in the plastic wad. A full choke will swage it smaller, if larger then the choke diameter.

https://www.thehighroad.org/index.php?media/img_2928.508/full

 

The foster can not be loaded in a wad. It was meant to have the groves swaged into it or use pure lead and have it set back and fill the bore. But you then run into a problem with it as the nose is so thin you can blow the nose right out of the slug. If you really want to make the foster work, send it to Eric @ HPmolds and have him open the mold up to 0.729 and shoot it full bore. While it is there have him make a new pin to get nose thicker and maybe the skirt a little thicker.