I'm not sure I would trust RTV to provide much cushioning action for the firing pin. Every RTV I have ever used, except some anaerobic types, is "rubbery" when cured but still easily punctured with something like a firing pin. Once it is punctured, it doesn't "heal" shut and the pin would just continue to enter that hole.

Likewise, fired casings with a dented primer in place are a popular substitute for snap caps but the dent doesn't get any deeper with dry-firing, so using them is just kidding yourself. Spring a few bucks for a set of snap caps.

Ed