I have the misfortune of a Lyman GPR also. Mine came with what can only be described as a lock with sand for a lubricant, a gap between frizzen and pan that Lyman should be ashamed of, and a barrel with the serial number smack in the center of the recall group. Well, the lock was easy, an L&R took care of that, and while reinletting for it, I shifted the flash hole liner to a more advantageous position.
The barrel is another story. The original barrel didn't settle in for a little over 200 rounds, then only marginally accurate, a 50 yard gun at best. The recall seemed like a blessing in disquise, untill I shot it. The replacement barrel blew holes clean through any patch I used, totally unhuntable. Rifling is so sharp you might cut yourself looking at it. Tried xxxx steel wool, no help. polishing compound, no help. Scotchbrite on a 50 cal jag, (GPR is a 54), no help.
Finally someone on another forum suggested valve grinding compound to fire lap the barrel. He gave instructions as to how he did it, and it at last would group! Three days running I got 1/2"-3/4" three shot 50 yard groups cold and clean. Took it on our Pa flintlock hunt. Missed a good doe at 66 yds, for no known reason. Back to the range.
The sight-in WAS 1" high @ 50 YDS, NOW 5"high @ 50 YDS. But it still grouped. The rifle still feels rough loading it, but I'm a bit apprehensive about any more firelapping, maybe more "patched ball lapping" will finish breaking it in.......MAYBE.
To anyone thinking about buying a new Lyman I would suggest finding an alternative brand, one that comes able to shoot groups out of the box.....It's a shame T/C dropped out of the side lock market.
