Scott, They are the same size, but...
Generally, rifle primers have tougher cup to handle higher pressures and is harder ignite. Some guns, especially tuned revolvers, might not have enough force and depth of the firing pin to ignite them reliably.
The other thing is rifle primers have a hotter (if that's the right word) mixture in order to ignite what is probably a larger amount of powder in a longer case than a typical pistol round. In other words, there is some risk involved. Pressures will possibly change. So proceed with caution. Might check out the THR forum's reloading section to see what others are saying.