I recently purchased a Traditions inline. I've noticed that the base of the nipple is visible with the bolt closed.

It seems obvious to me rain is going to be an issue when hunting with this rifle.

Q- Will a steel or nylon spacer dangerously elevate pressure? Obviously, sidelocks can vent nipple backpressure by virtue of being spring-loaded, not locked down by a bolt. I'm concerned confining the nipple may force too much combustion through the nipple, elevating pressure. In addition it may raise pressure in the nipple and damage it.

I'm trying to waterproof the action as much as possible and I am presuming o-rings won't handle the firing. I will probably try them first anyway, though.

Any thoughts?

If ALL the fire and gas from your cap enters the chamber, it WILL NOT cause excessive pressures. When you use a 209 primer instead of a No. 11 or musket cap, there is MUCH MORE flash and gas entering the chamber!!