My Beretta O/U has a carrier barrel for the Kolar tubes, makes the 12 gauge barrel the exact weight of the other gauge tubes. 12, 20, 28, and .410 all weigh and balance the same when shooting. Might not be a big deal for most shooters, but it may gain you that one or two birds out of 500 that could mean going to a shoot-off and having a chance at the money or going home empty handed. I went for years with a Ruger Red Label with tubes in the existing barrel, worked fine, but I rarely shot the 12 gauge in the 12 gauge events as the recoil works against you eventually after several hundred rounds in a couple days. My 28 gauge average was always my highest.
Haven't shot competition in quite a few years, but when I started out in the early 90's many shooters were shooting Browning Citori 4 barrel sets but using a Remington 1100 in the 12 gauge event.