The "Danger" of this occuring dates way back to the regular use of Card & Filler wadding. These wads did not obturate & were not large enough in diameter to form a seal in the "Chamber" dia of the gun. This was why older guns have short forcing cones, so the wad column could bridge the gap prior to the wad leaving the case mouth. If this did not occur there was a possibility of the hot gases flowing around the wadding & fusing some of the shot. This of course did not always occur, most times it just erroded a little lead from some of the shot, but the possibility definitely existed. There is sufficient corrabarating evidence I believe to confirm it.
The plastic wads with an obturating base cup have virtually eliminated this concern however & no doubt that is what Benelli is counting on.
Also, though I don't have proof for this, I think that shells loaded with high quality felt fillers are less apt to have this problem than those using fiber wads. The felt is I believe much more likely to expand sufficiently under pressure to achieve a seal than the harder fiber wads.
Some yrs back I too fired a number of regular 2 3/4" shells through a 20ga having 3" chambers with no problems, but this was all after the introduction of the plastic wad & these were what I used. I would be personally hesitant to load shells with card & hard fiber wads for use in a gun having chambers significantly longer than the hulls.
