IMO carrying a percussion gun uncapped is the only real safe way to carry the gun.
That said, hunting with a uncapped rifle is asking for a world of difficulties.
As any hunter knows, taking your eyes off of the game animal is a sure way to lose it. Glance away and then look back and POOF! It's gone.
Capping a percussion rifle without looking at it is difficult at best so it is unlikely that it will be done by most hunters.
That brings up the question, is it better to cap the nipple and carry the gun at half cock or carry it with the hammer resting on the capped nipple?
I think everyone will agree, carrying it with the hammer resting on the capped nipple is asking for trouble. If the gun is dropped or anything hits the hammer hard, the gun can fire.
Carrying it capped but at half cock isn't a great deal safer.
If the gun is dropped and something hits the hammer the blow can break the sear or the tumbler half cock notch rather easily. If it does, the falling hammer will most likely cause the gun to fire.
Another thing most muzzleloaders don't think about is if the gun has set triggers, there is another way it can fire from the half cocked position.
Picture this: The nipple has a percussion cap on it. The hammer is at half cock.
Your walking along and a heavy branch catches on the hammer as you walk by it.
The heavy branch raises the hammer about half way between half cock and full cock.
The "fly" in the tumbler, being rather stupid moves to block the half cock notch.
As the heavy branch pulls free from the hammer the hammer starts to fall.
The fly (as I said) dumbly blocks off the half cock notch like it is supposed to do so the hammer keeps on falling. BANG!
It can be that quick and the hunter won't have the foggiest idea of why his gun fired.
OK. So what does that leave us?
How about placing the percussion cap on the nipple and then placing a small, heavy piece of leather over the cap? Then lowering the hammer so it holds the leather in place.
If the gun is dropped and it lands on the hammer, the heavy leather will cushion the blow and although it could fire, most likely it won't.
I've also heard of some who used a short, hollow metal tube that slips easily over the capped nipple but it is long enough to keep the hammer from actually contacting the cap.
Again, the lowered hammer keeps the tube in place.
If the metal tube has a small lug soldered to the side of it, a short piece of leather or wire can be attached to it and to the trigger guard so it won't be lost.
When the game animal is seen, without taking your eyes off of it, bring the gun to full cock and removing the safety pad/tube with a brush of the finger or hand and your ready for action.