If you are looking for a .410, I would suggest the Mossberg 500.  I bought one a couple of years ago for shooting pigeons in the shed.  
Its a sweet little gun for 300 bucks.  I tried to find an H and R single shot, but they are discontinued.  
When my buddy wants to challenge me to a round of clays he takes my Benelli and leaves me with my .410.  Its still not enough handicap!  He will clean my clock with a rifle, but I whip him with a shotgun.  
I would not change barrels on that Mini 14.  You can shoot .223 so cheap now its ridiculous.  I dont bother reloading for .223 at all.  I cant make them as cheaply.  
I have some AR parts that I need to do something with.  My buddy bought me a Spikes Tactical lower and a fancy new hand guard.  The hand guard is cool as it does not use a nut that has to be aligned with the gas tube.  It has a nut that is small so the tube goes above it, and then the hand guard is located with allen screws at an angle.  
I need an upper, all the parts for the lower and upper, plus a light 24 inch barrel.  I was going to chamber it in 22x6.8, my gunsmith really likes that one.  But .223 is so cheap to shoot.