I've been a serious user of snap caps, for obvious reasons, since acquiring my 686-3 and 29-5 a few decades back. That habit carried over to the 640 Pro for the proverbial thousand-dry-fire "trigger job." After that was finished, I double-checked S&W's online FAQ to see if they were absolutely sure that their new revolvers could handle dry-firing without any stated limitations. That's what they said, so I decided to give it a shot. The day the sear broke, I wasn't using snap caps.

For comparison, some of my other revolvers:
LCR No. 1 -- 1,000+ dry fires, no snap caps, no problems
LCR No. 2 -- 1,000+ dry fires, no snap caps, no problems
Super Redhawk -- Hundreds of dry fires, no snap caps, no problems.

I would be reluctant to assume S&W is wrong about the ability of its revolvers to withstand dry-firing without snap caps, when my LCRs, whose internal parts seem less solidly built than the 640 Pro's, can handle it without concern. If the paperwork that arrives with the gun lacks details, I'll be calling Springfield to find out whether they have any theories beyond chalking it up to a random defect.