Having never owned a Nitro Special I cannot really say for sure, but I would almost bet the 20 gauge uses essentially the same stock as the 12. The NS was the economy line for Ithaca, I highly suspect the frame was the same for all gauges other than the standing breech which would have been sized for the gauge. If the stock stood up to 12ga why would a 2 3/4" 20 "Cracki It".
I also had a late model Flues 230ga which had the chambers lengthened to 2 3/4", 1924 as I recall. I never shot any "Express/SuperX" loads through it but it digested a bunch of 2DE-1oz loads in both factory & reloads without a hitch. It had been shortened to 26" (likely from 28") when I acquired it. Had also been previously reblued, probably hot salts, & finally the ribs came loose so I retired it. This I do not believe had anything to do with the loads fired through it though. T^he NS is no doubt a "Tougher" fgun than na Flues, though not quite as Sophisticated.
