Looked at a used 640 PC yesterday. Gun has night sights and a crimson trace laser grips, (that I would remove and sell). Gun looked good, BUT!! the trigger is the worst I have ever pulled. The initial pull is very hard, (guessing in excess of 12-14lbs) and just before the break, becomes very hard and rough. I detect a "click" just as the hammer drops. I believe that previous owner tried to improve the trigger and did not know what they were doing. I may go back on Monday for another look. They are asking $899.00 for the gun, as is. Since I cannot disassemble the gun on their counter, is there anything I can do to determine if it is repairable?

Everything is reparable with the proper application of cash and experienced workmanship

There were several variations of the model 640 produced in the performance center.

Even without knowing which variation this particular revolver is just based on your description, that is a fair price. And I would be a buyer if it were in my local shop

But that does presume that you are talking about a performance. Center offering and not a pro series revolve

This is my performance center model 640 quad port that I have owned for more than a decade now. It is product code 170068.

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"Acta non verba"

I believe it to be a PC edition, as it looks like the sights are replaceable. The pro edition has fixed front sight and a grove for rear sight like your picture. I am more than willing to replace springs, etc, but if someone has "stoned" the hammer, or sear beyond repair, finding replacement parts could exceed the price of a new gun.
I will have another look on Monday.

What you're describing sounds like a possible issue with the hand binding. When you take another look at the store, try cycling the gun with the cylinder open, while holding back on the thumbpiece. (cylinder latch)

If it cycles ok with the cylinder swung out, the issue may be just an over-sized ratchet problem, and the hammer/trigger may be fine.

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Ret. LE, FA Instr, S&W Armorer

This appears to be the gun I looked at, used yesterday.
https://www.smith-wesson.com/product...ries-model-640

Pro Series firearms are production guns that get a few PC features

All true Performance Center firearms have the PC logo on the firearm in place of their usual S&W logo. PC guns

You are describing a mechanical problem not a spring problem

I personally would never buy a used firearm (with or without problems) to save 10% off of MSRP

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"Acta non verba"

Anything (or at least most) are repairable. The question is how much are you willing to throw into it!? For $900 I would not advise buying and then trying to get it working to your approval when for the ssame or less you can find one that works better.

I will not try and guess what the issue or issues are without being able to inspect it, too many possibilities. If you don't have internal parts on hand and are not capable of fixing these revolver's yourself, I'd take a quick pass and move on to look for a better one. If the price were $400 my response here might be different.