Never had a A300, Had a 20ga A-400 with all the bells on it. Shot like a dream, real soft shooting with the KO. Used to shoot my brothers A-400 12ga / 32" barrels with the KO and felt like I was shooting the 20ga.
Right now I have 3x 391's 2x12ga and 1x20ga. If you get the 12ga get it with the...
I also agree with what Gordylew said. Just because there old doesn't mean they have great value. With old gun by the time you bring them up to pare you have twice what you paid for the gun
410g will bring the biggest money, then 28g, 20g then the 16 & 12 are the most common.
Try shooting it, it's a hoot, I got a 12ga BSS and a 28gaSKB, love shooting them.