Little Al
It's already been said, but to reinforce it, here goes...
The longer forcing cones are a benefit to modern plastic wads
ONLY. They can be detrimental to fiber wads. The plastic wads control the column and the base is flanged to seal against the bore. Fiber wads expand some but do not expand to fill the bore just in front of the mouth of the case. If you have a "modern" shotgun with a long forcing cone, then the card/wad won't seal and it allows the gasses to push by. This causes the fiber wad to blow through the pattern creating a hole in the center. Modern barrels also usually have a "looser bore" since the wad controls the column, the choke lead is usually gentler as well.
With Fiber wads the shot column strings out in a long forcing cone and has to be "cleaned" up as the wad hits the throat of the forcing cone. It never quite gets there and most people find they don't get the pretty patterns they get with plastic wads and a long forcing cone.
A short forcing cone and the tight bore ensure that the card wad seals well and you don't get blow-by. The old forcing cones and chokes work very well with card/fiber wads. Modern guns and especially those with 3 inch chambers really don't do well with fiber wads. If people really test them the way a serious clay shooter would it immediately becomes apparent. The 3 inch chambers exacerbates the problem by just giving you more free bore allowing blow-by until the wad/card get enough constriction in the forcing cone to seal the column (by this time the gas has already churned your column in front of the wad).
Modern shotgunners want long forcing cones, "loose bores", gentle choke throats and often "back boring" between the choke and forcing cone. Modern wads take advantage of these features giving you "shorter" less strung out shot patterns and very even shot distributuion. The standard bore size for a 12 gauge is .729 thousandths and with a full choke tube size of .690 you will have .039 thousandths of choke. When the bore size is enlarged by back boring even more, say to .742 thousandths then you would have .052 thousandths of choke with the same .690 choke tube. Anything above .04" of constriction with a 12 gauge is considered to be an Extra Full choke. So factory tubes that have been back bored have larger choke diameters to keep the choke constrictions the same.
If you want to shoot Fiber wads then a older shotgun is the most appropriate. If you have a modern shotgun and especially one with a 3" chamber then fiber wads are not ideal no matter how much we wish they were. There is one, and only one advantage to fiber wads with modern chambers, forcing cones, bores and chokes. They don't put plastic in the bore. Plastic is not the monster that some make it out to be with BP. It cleans very easily and if done correctly takes no more time than fiber wads. Simply spritz with moose milk, let it sit while you clean other weapons and push out a snake skin sleeve with a wadded up 1/2 of a paper towel. Then run a patch down the bore with Ballistol on it and you are finished.
If you have a modern shotgun with the longer chambers, etc. and want to shoot tight dense patterns with Brass hulls then you will have issues. I typically shoot a Colt '78 reproduction with 3" chambers and basically no choke. I get weak patterns with Brass cases and fiber wads. Since there is so much empty case it is a simple matter to add a plastic wad in the case and even just a gas seal under the fiber wad if you just can't kick the habit. Here is the seal:
http://www.ballisticproducts.com/Powder-Cup-Gas-Seal-12ga-250_bag/productinfo/3221223/The seal behind a fiber wad doesn't work as well because you still have a very long forcing cone and no constriction. A plastic wad on top of an 11 ga nitro over powder card can be used to assure a full seal between the thinner case walls and the powder. A lot of wads will spread their flange bases and seal against the sell walls with a bit of pressure, but they tend to push back up since there is no friction between the shell walls and the wad.
Well now I'm boring everyone...
Regards,
Mako