Well, you have just put your finger on one of the drawbacks of underhammers. However, a cap the has been pinched slightly will hold on fairly well. They usually had a half cock that kept the hammer NEAR the cap, and was only cocked when ready to shoot. BTW, during the American Civil War, there was limited use of an underhammer BOLT action rifle- breech loader ball and cap style with a bolt, underhammer, and ellipsoidal rifling- made by Greener. Wierder than socks on a rooster.