I suggest reading the warning on most every box of U.S. shotshells saying to shoot them only in modern guns that were originally chambered for 2 3/4 inch. Ithaca NID's are not modern and you don't really know about the length of the chambers. Better to follow the warning of the shell manufacturers than amateur opinions on the www. The RST brand shells are made in 2 1/2 inches and they are low pressure for guns like NID's.
CB,
Your concern is noted. Such caution was the rule prior to actual test data becoming available not so long ago. There is now real test data on shooting 2 3/4" shells in a short (2 1/2") chamber. This issue was put to a test by Sherman Bell in an article series he writes. The test data was collected using laboratory pressure equipment such as ammo manufacturers use. The test yielded pressure differences from 0 increase to about 15% increase due to a true 2 1/2" steep forcing cone chamber. I also did a small scale test that didn't show significant pressure increases, using a straingage system.
But each person still should make their own choice on the issue with all the available information.
