If they are a production gun, doubtful they will gain value. A quality hand made rifle should gain in value by the time your great grand kids are ready to sell them. If they keep up with inflation.
On the other hand, I bought dozens of milsurps over the years, that have sky rocketed in price. Look at guns on Antiques Roadshow. Grandpa's old musket isn't worth as much as the family thought, in most cases.