Doverham is right in the most practical way. The value of the gun is not improved by altering it this way. If you think the barrels are unsafe run away from the gun as fast as you can. Face it we have no proof laws here so there is no legal consequence to altering the forcing cones. And as it was pointed out that even in GB it is more of a technical issue than one easily or obviously encountered. If you admit that it was done then by rights you should have the gun reproofed is my take. Don't ask, don't tell.

But more to the point is that it is a alteration done that is almost totally unneeded. The few percent improvement in pattern is not worth removing metal at the highest area of chamber pressure. Just as porting is not needed in a fine double, long forcing cones are just not needed. Leave these guns as original as you can or think long and hard before you have them worked on.