You don't want to be walking around with the hammer resting on that percussion cap, Only a little motion lifting that hammer and then letting it drop will often set off the cap. Now, if you are walking around with NO CAP on the nipple, the gun would be safe. Make a hammerstall for your percussion gun out of a .30 cal. carbine or .25 auto casing cut to fit over the nipple and cap, but not touch the cap on the inside of the casing. Tie a string to the recessed rim, and then tie the other end to your trigger guard. To shoot, all you have to do is cock the hammer to full cock, and reach forward and pull the casing off the nipple. If you put a small "O-ring " over the base of the nipple, it will provide good water seals so you can protect your powder charge in wet weather, too.
Both Roundball and I am are assuming you are asking about walking around with that hammer on a capped nipple! That is very dangerous, and you will not be doing that on any range I am on for long, nor will you be doing so around me in the field.