So, Mr. Override, you moved into your son's basement, I conclude. BZ!
Until today I have been at home in southern NH and can confirm there aren't many sandpit areas that aren't actually in the business of selling sand. Any open space you find is likely to be either wetland or a park. You're pretty much limited to joining a fish 'n game club, of which there are plenty. I can personally recommend Nashua Fish and Game, and Pelham Fish and Game. Both have 600 yard ranges, which is nice.
That said, I am in the process of moving to Florida, where there is less snow and more family.