I would double check the interchangeability of 870 and 1100 barrels before buying one
Sound advice.
As for the 870 20ga, standard weight or LW?
Standard.
Standard to me means the 870 20 gauges built on 12 gauge frames. If the SN ends in an X or a N it is a 12 gauge receiver.
I have a 12 gauge 870 and this guns frame is much smaller. I didn't know that at one time they put 20 gauge barrels on 12 gauge frames, kind of crazy I think.