.410
You are not going to beat the H&R singles for price. For a barn gun, used occasionally, that may be all you need. Slightly up in price is that Mossberg pump gun...I have had one for years. Nice and light (a Browning, it ain't but then this is a barn gun). Also, Stoeger ( remember...barn gun) makes a handy little SXS .410 coach gun for a few hundred dollars.
Pete