Ditto what
trotterlg says.
That was my experience with an older model Topper with 30/30, and 20 ga barrels. The barrels wouldn't fit (without considerable modification) any of my modern frames, and it would not accept any of my modern barrels because the barrel lugs (modern) were too thick for the frame.
On the other hand (reversing the process) I now have another Topper 158 frame (according to the FAQs it is a 1969) that perfectly fits and shoots with a modern 357 mag barrel!
The first (30/30) frame wouldn't accept any modern barrels, because the barrel lug was, as
trotterlg says, kind of narrow, yet the Topper frame I have now accepts several modern barrels... but I am not going to shoot it with anything but the 357 barrel!
Bottom line, it can be a "luck-of-the-draw" thing, as we all know whether any given barrel will fit a given frame...
You can see pics of it in the old classifieds
http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=86501And here's one of the receiver and barrel.
Cheers
Kerry