A youth length stock with a good eye relief optic i.e. holographic, red dot, pistol scope scouted. might serve well with the shortened barrel and clear up the choice of folding stock and still be very light and compact . I tried one of the ATI colasping stocks a couple years ago and it did what I wanted it to do , Of course that stock configuration may be the most cost effective and practical for a real light/short rifle, Then say a heavy and akward home designed swing style stock.
As for the lightest caliber choice, the larger bores are going to win that one I would think. A 44 mag or 45 colt maybe the ideal one for the short and light rifle , weither the total over all weight is signifigantly lighter then a 243/223/22 hornet light contour youth style barrel chopped down to 16 1/4 may not be negligable enough to matter. I think that is when you need to ask what caliber for what type situation you foresee using / needing the super light shorty for.
You may find that your ideal short and light rifle may be filled better by the Kel-Tec sub 2000 ( rifle folds in half and can fit in a good sized range bag of lap top case) rifle line in the 9mm or 40 cal options , with magizine choices to match your side arm , Unless you are just commited to the H&R platform ?
That might be an option worth investigating.
