K frame mainspring replacement, interesting outcome, and a question


July 2, 2013, 11:51 AM
I decided to replace the mainsprings on my 2 k-frames recently:


I bought one of the reduced power springs, and a couple of the standard, just to see what the difference was.

I dropped the reduced power spring in the M&P, and it was REALLY weak, like it was barely engaged with the strain screw turned all the way in. Nice light DA pull, but I doubt it had enough juice to reliably set off even soft primers.

I had put the full power spring in the 15-3, and it was WAY too stiff! Like, way more stiff than the stock spring, and this gun hasn't had many rounds through it, as far as I can tell.

So I swapped them. Interestingly, the full power spring in the M&P is great, much smoother pull, and seems to match the REDUCED spring in the 15-3. Both now have much improved triggers, the SA pull on the 15 is especially nice.

Was there some change to the geometry of the action between the two? Both are "short" actions, to my knowledge.

I've got one problem left with the 15, though. Right at the end of the DA trigger pull, after the cylinder has locked up, there's a bit of a gritty feel, but only when using my right hand. And gritty may be too strong a word, it feels like something's rubbing. If I make a conscious effort to push the trigger over to the left, it goes away. Any idea on what the culprit is, or do I just need to continue to dry fire and let the parts mate a little more? This one didn't have a turn ring on the cylinder when I bought it, as I said, I don't think the previous owner put much more than a box of ammo through it.

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July 2, 2013, 12:44 PM
Measure the two strain screws and see if they are the same length.

Somebody may have shortened the one in the Model 10 before you got it.


Dave T
July 2, 2013, 03:18 PM
My guess would have been the same as rcmodel. Sounds like the strain screws are not equal.


July 2, 2013, 06:26 PM
Welp, that was a good call! Screw on the M&P measures .565", and the 15 is .605".

Anyone have any ideas on the rough spot in the trigger pull?

July 2, 2013, 08:08 PM
K-Frame Model 66 strain screw.


The rough spot in the trigger pull is impossible for me to diagnose sight unseen.

But strip it clean it, and re-lube with Gun-Slick graphite grease if you can find some.

Then dry-fire the snot out of it.


July 2, 2013, 08:34 PM
My experience aligns with rcmodel.

Glad you solved your Goldilocks and the three bears spring situation. If the revolver has any age and experience on it I have found that dried up, calcified lubricants and debris are the first enemy to suspect in "perfecting" the action.

I have become an advocate of starting over by a soak in "Ed's Red" and as detailed a cleaning as possible before going forward. Then a "proper" revolver lube, then evaluate. Sometimes a handsome (externally) example was kept that way by the uninformed by years of over-lubrication. If you are not the original owner, God only knows what they used.

I actually met an old "gunnie" one time that had used vegetable oil (yes, Crisco) for years and swore by it. Get a baseline if used by eliminating the foreign matter variable.

Old Fuff
July 2, 2013, 10:41 PM
As rswartsell and rcmodel explained, dry lubricant and fouling could be the problem, especialy if it is packed around a small lever and spring (called a sear) that's mounted in the lower hammer face. If it's stuck I'd expect what you are getting.

A second possibility is that the trigger is moving side-to-side as you pull it, but I'll address that if a good cleaning doesn't work.

evan price
July 3, 2013, 06:45 AM
Also check your rebound slide...where it rubs inside the frame it can get quite gritty. Polishing it helps.
Another thing to consider is that the rebound spring and mainspring work together to give you what you feel as trigger pull.

July 3, 2013, 07:07 AM
If the revolver you refer to as an M&P is really a version of that model, it will have the old long action as opposed to the short action of the M15. That can make a difference. I have and use both but have found the old action to be capable of the smoothest pull with heavy springs.

July 3, 2013, 08:47 AM
Well, the older one is an M&P as far as I know, no model number is stamped anywhere, I believe the serial number puts it somewhere between 1948-1951. But it's definitely not the long action. I also replaced the return springs when I did the main springs, and gave the return slides a little polish. I will try "de-gunking" next time I have it apart, and see if it helps!

Elm Creek Smith
July 3, 2013, 11:27 AM
If the grinding on the Model 15 doesn't go away with a good cleaning, the trigger might need to be shimmed. IIRC, there are thin TeflonŽ shims that you can install on the trigger pin to keep the trigger from rubbing either side of the slot. (They make/made them for hammers, too.)


July 9, 2013, 12:18 AM

Was it clear that the picture is unnecessarily muddled until you clean it up? Tell us what it is like then and no doubt the action gurus here will set you straight. You start changing metal over an unidentified lump of schmutz? This will not end well.

July 9, 2013, 09:56 AM
No "lumps" of schmutz. Also, no grinding! What little polishing I did was by hand, with a very soft stone.

Cleaning the interior, I did find some fine surface rust on one side of the trigger, but other than that, it was really just some carbon, didn't see an overabundance of grit, grease, dirt, rust, etc.

We'll see if it improves with time!

July 9, 2013, 10:34 PM
Years ago I read a interview with a S&W Factory Rep who was asked what the best way to smooth the action on their revolvers. His answer was "Shoot it...a lot."

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