Some have referred to the underlug as being made of pot metal, but it isn't, it's steel. Maybe not as good of stuff as we'd like, but if one does fail, H&R will fix it as they have in the past. Shooting loose is usually caused by not fitting properly in the first place. I certainly wouldn't worry about it, if I had any concern, I wouldn't have the bunch of Handis/Ultras that I have and I certainly wouldn't spend the money to have them rebored and/or rechambered to calibers that aren't available like .338-06, and .280AII!!
Tim