Not really familiar with the brand name "Great Plains", but I'll help where I can. In short, flintlocks can be converted. The flash hole linner is removed and replaced with a drum and a percussion lock can be purchased. Some gunsmithing may be required.
For the life of me, I can not understand why anyone would want to do that, but to each his own.
There may be a bit more required to convert a flintlock to a percussion gun. The lock may need to be replaced and the barrel. The lock may cost over $100.00 and the barrel from MidSouth or some other places is about $120.00. The gunsmithing route of the existing barrel may work, but it may be costly. I have bought excellent used muzzleloaders for less than $200.00 so that is the route I would take and leave the flintlock as is. Good luck on your project. Take care...
I have two .54 Great Plains Rifles, one cap and one flint. Oberndorf is right. You would have to replace the barrel and lock. You'd be much better off just picking up a percussion rifle and leave the flinter alone or use it for trade. With the price of caps going up, you may want to think it over before doing away with that flinter.
If you have a flintlock and want to trade for a percussion lock model, I am sure if you posted that desire on some different forums you'd find someone more then willing to trade. You have a great rifle. Instead of getting rid of it, I would learn to use it. You will find that once you learn a flintlock, they are as dependable as a cap lock.
It's really not rocket science. All you have to do is re-tap the vent liner hole to 5/16x24, buy a 5/16x24 nipple drum and install it. You don't have to buy a new lock, just discard the frizzen and spring, hacksaw off the pan and open up the lockplate to fit around the nipple drum. That's the way most flintlocks were converted back in the day. You can either buy a new percussion hammer of if you like to be primitive (and cheap) just make a steel block to fit into the flint jaws and shape it to strike the cap. That makes a really interesting looking rifle and is the backwoods way it was done.
If you have any skill at all with hacksaw and files and just a bit of ingenuity you can make a very neat conversion in a couple of hours and the only cost is a nipple and drum, say five bucks.
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