When we say frame are we speaking of the entire frame being scaled down or only the standing breech area. There are many guns & particularly in the lower priced bracket which will use the same basic frame but have the standing breech sized for the different gauges. On my Lefevers I have 12ga guns weighing from 6˝lbs to 8lbs & with 3 or 4 different firing pin spacings & height of the standing breech but al have the same bar dimensions & essentially the same stock dimensions. This of course doesn't mean the stocks are interchangeable but they are essentially the same size.
As I recall the same situation exists for L C Smith guns as well
When you look at all the "Frame Sizes" listed by Parker Bros I believe also you will find the major variation was only in the standing breech, not the bar not the stocking.
While I do not have experience with the Nitro Special I very highly suspect B Dudley is 100% correct on the cause of cracking.
