Over at Conceal This, the topic of snap caps has come up. Snap caps are excellent training tools, and when used can prevent damage to your firearm that may arise from dry-firing. For me, nothing beats the combination of snap caps and a LaserLyte LT-Pro. The snap caps protect the firing pin, and the LaserLyte shines a brief dot when the hammer drops, allowing you to diagnose any trigger issues that may cause problems for you on the range.
The point made at Conceal This about brass cased versus aluminum cased is a valid one. I have noticed that after heavy use, my snap caps do get dinged up as seen in the photos at Conceal This. However, this has never given me any issues with feeding or ejection. Since you have to manually cycle the action of your firearm, you will be able to address any sort of issue that may possibly crop up with ease. This comment is the first I’ve heard about aluminum snap cap problems. With the Pachmayr snap caps, they are brass rimmed, which is excellent. But I have seen in several different forums where people have had issues with the plastic tips on the Pachmayrs. I’ve never personally used them, but I’ve seen write-ups where the plastic tips can become brittle and then fragment inside the chamber. From what I hear, it is extremely difficult to remove all the plastic shards when they break.
For me, and ideal snap cap would be brass cased, and tipped with a copper (or copper jacketed) bullet. That would offer the closest approximation to live ammunition… but I’ve yet to see any offerings of the sort.