RE: 870 Sabot slug barrel
I would not want to disagree with Bigcountry...
So, I will simply post a quote from Robin L. Sharpless, Ithaca's former director of business development concerning the ACTUAL RESEARCH they conducted when developing the Ithaca Deerslayer III, designed specifically for shooting todays modern rifled slugs.
"The additional barrel length, which may at first seem unnecessary, is also functional. Modern saboted slug loads featureSLOW-BURNINGpowders. "IN OUR TESTINGwe found that the longer barrel allowed us to keep the pressure up for a longer period of time, and so we gained velocity," says Sharpless. "It allows for a more complete burn, rather than expelling excess powder out the end of the muzzle."