First off, the Lyman GPR is a excellent rifle. Most of your better "mass produced" rifles are good quality rifles. Traditions makes a couple, Pedersolli, even Thompson Center. For the occasional shooter they usually are more then enough of a rifle.

So you asked what does a custom rifle give you. Normally an excellent quality barrel, also the higher end lock systems, excellent hardware to stock fit, and it is an original. Something only you own and no one else owns one like it. So you get that knowledge. The better barrels mean usually better accuracy. The higher end lock also mean a faster igniton, more sure ignition, and less wear on the flints. That is as long as the custom gun maker tunes the lock, which they normally do. As we all know, even the best barrel is nothing without a quality lock. Also the rifles are normally made to fit you. So a better fit, better hardware, and better balance usually mean better accuracy. At least less excuses when you miss.

The fact that your custom rifle is a one of a kind might mean a lot more when you hand it down, although I take a lot of pride in the rifles my father have passed down to me, even if they are factory weapons. They still mean a great deal to me.