S&W M&P misfiring in DA only, random chambers!

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MatthewVanitas

MatthewVanitas

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Greetings,

I've been mucking around with my pawnshop M&P, the one I posted about a month back. It was originally a .38 S&W, and someone reamed the cylinder to .38 Spl. No military ordnance markings.

THR members said to test it with .38 Spl and see if it splits cases, and if not it should be fine with either S&W or Spl. I fired a few cylinders of .38 Spl, and had no splits or bad swells. Had a very, very slight fireformed "shoulder" on each case, but so slight I wouldn't have noticed it had I not been going over the brass minutely.

Here's the problem: in DA mode, the revo will usually fail to fire two out of six cartridges. I tried a couple cylinders, noting the failed chambers by their proximity to the cylinder stamp. Unless I'm missing something, I'm reasonably sure that the misfires occurred at _different_ chambers on the two or three strings I shot. Not totally positive, but misfires might have happened in the same order (i.e. third and fourth shots of each string).

Firing in SA, had no misfires at all.

The only other notable quirks: sometimes the cylinder is hard to close because the rims scrape against the frame while closing. Second: the hammer-mounted firing pin seems a little wiggly.

Suggestions? Thanks,

-MV
 
Matt - I am not familiar with that specific gun but - will say, all S&W's I have or have had will have a slightly lighter hammer stirke when DA - the hammer fall from SA cocked position has a smidgeon more travel and so energy.

The floppy hammer mounted pin should be fine - it finds its line as it falls and engages the mouth of the FP hole in back of recoil shield.

Now - does that gun have a leaf spring, as I imagine - inside the grip frame? Does it if so, also have an adjustment screw in front strap? It may well be the smallest quarter turn or so on that will increase spring tension a tad and cure the prob.

I have had Smith's set up that will fire cases Fed primed but not CCI's because they are harder. You may find too, a second strike can fire an apparent DA misfire, even with that being DA again.
 
As was said, check the strain screw tension. It should be all the way in. Also check to see if the strain screw has been bobbed.
 
Strain screw is all the way in, and there seems to be a folded piece of metal placed between strain-screw and spring? Is that standard, or did someone add a shim?

Should I pull the screw, measure the threads, and buy a longer one?


Oh, regarding tightening the ejector rod (eyes downcast, scuffs dirt)... umm... how do I do that exactly? I'm looking at it, and nothing jumps out at me.

-MV
 
Matt

Sounds like if screw all the way in and some shimming then this has been a problem before. Maybe spring has gone soft and weak - no more adjustment left. A new spring would help if obtainable. Longer screw not likely to help - the pressure is applied near base of spring and that will only take so much pressure.

Ejector rod - on older Smiths that will probably be RH thread - just check to see if it turns either way - and ensure therefore it is done up fully - a small loosening of that can make cyl hard to close.

I assume also, the FP is original and so full length - some folks have made replacement FP's and it could be short. Do rounds fire on second tap?
 
IRT ejector: I'm still not quite sure what I'm supposed to be turning, but suffice to say nothing turns in either direction on the rod. Am I trying to hold the rod itself still and twist the knurled part that one presses to eject, or hold the rod still and turn the extractor star?


Repeat strikes in DA do no good. After the initial string of fire, I cycled through the whole cylinder a couple more times, and no bang.

New spring? It's $12 for a new Wolff mainspring (just labeled as K-frame, compatible?) or $6 for NOS specifically M&P from Numrich. Which one? Could an inebriated ape change out a mainspring? If he can't, I sure can't.

If I order from Numrich, should I get a spare FP too? I'm only $150 or so into this sucker, don't want to make it a money-pit.


-MV
 
You might just need a new strain screw. An unaltered screw has a dog-point, or reduced diameter unthreaded portion at the end. Some people grind the dog-point off down to the threaded portion to lighten the trigger pull, find that the screw is now too short for reliable ignition, and then put a shim between the screw and the spring. Many times this shim will be a spent primer cup. It's a crummy fix.

You can remove the strain screw and look at it with a magnifying glass to see if it show signs of grinding or filing, but replacement parts are cheap. Go with Numrich and buy both a $6 mainspring and $2.50 strain screw. That most likely will solve your problem. Parts from a Model 10 will interchange just fine.

-Bob
 
To check the ejector rod:

open the cylinder which is unloaded, place two fired cases opposite eachother in the charge holes. Then holding the cylinder motionless in one hand use the other hand to turn the rod to the right or left, if it does not move do not force it its ok. If it turns its loose, depending on the age of the gun it will tighten counter clockwise or clockwise I forget which.

My experience is that if its loose you need to take it appart and locktite it or it will just unscrew again. Here is how: Remove the front sideplate screw and take off the Yoke and cylinder Hold the cylinder motionless in your hand while you slide the yoke forward, this avoids scratching the cylinder and frame. Unscrew the ejector rod all the way until it comes free (leave the fired cases in the charge holes while you do this so you dont bend the locator pins that protrude through holes in the star.Then being careful to note the parts that come out and not lose the two springs seperate the ejector /extarctor star assebmly pulling the star out its side. Next clean the threads, and use a little locktite on it, the threads only, and then reassemble.
All of my 4 N frames (made from 1967 to 2002) have had that problem and needed locktite. Once this is done the problem has not recurred.

The other possibility for the binding is that the front sideplate screw is too tight, and binding the crane. So loosen and retighten, note that could be caused by somone having removed the sidplate screws and put them back in the wrong holes, the front screw was fitted when the gun was made so it was the right length to hold the yoke but not bind it (a little shorter than the other screws).
 
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Have you tried moving the rounds that didn't fire to other spots in the cylinder?

Had a very, very slight fireformed "shoulder" on each case,

sometimes the cylinder is hard to close because the rims scrape against the frame while closing.

I am not a gunsmith in any form, but I have to wonder a bit about the "reboring" of the cylinder and the deformation you are seeing in the fired cases and the difficulty in closing the cylinder.

Maybe some bored too deep and some not enough??


Somebody that knows more than me will have to step in here with some insight.
 
Have you tried moving the rounds that didn't fire to other spots in the cylinder?
Click to expand...

Yep, they fire just fine once moved.


Maybe some bored too deep and some not enough??
Click to expand...

I'll check again next time I go shoot, but I believe that all the rounds "seat" properly, in that their rims all rest on the cylinder.


So the bored-out chambers aren't a problem so long as they're tight enough to support the brass? Definitely sticking to non-+P in this one.

-MV
 
I would send the gun back to S&W via a trustworthy gunshop near you.
Quite often the factory will fix'em free of charge.
 
I would send the gun back to S&W via a trustworthy gunshop near you.
Quite often the factory will fix'em free of charge.
Click to expand...

Does that work on a gun from 1947?

It'd be an interesting complaint letter:

"Dear S&W,

My 1947 M&P revolver is exhibiting light strikes due to a worn mainspring. I am shocked that the current management (or their grandparents, as the case may be) is not capable of providing the American consumer with a revolver that can make it to the 1-Century Recommended Service Interval without repair. This is clearly not the S&W brand that my great-great-grandfather loved so well. Please repair and return to me at your cost.

-MV"
 
Do the firing pin strikes on the misfired cartridges look shallow? Possible weakened mainspring, or someone backed out the strain screw to "adjust" the trigger pull (it is not an adjustment), or the strain screw backed out by itself. Is there excessive space between the rear of the cylinder and the recoil shield? Could there be a difference in rim thickness between the .38 Spl. and .38 S&W? If the Special rim is thinner, the primer will be that much further away from the firing pin.
 
If it is a 38S&W that has had a 38Special reamer run through it, it could be that since the 38S&W has a larger diameter case than the 38Special, the Specials are "flopping" around in their chambers and the firing pin is not making good contact with the primer. Try some 38S&W ammo in it and see how that works.
 
IRT questions:

1) Yes, the primer strikes on the non-firing cartridges are shallow. The mainspring screw is not only screwed all the way in, but is shimmed. It seems a pretty safe bet that the mainspring is substantially weakened. Might need to send $5.95 + shipping to Numrich, but shipping costs as much as the part does.

2) Rim size: actually, the rims are _too_ close to the frame, so have some problem closing the cylinder at times.

3) Flopping around: the cartridges seem pretty snug in the chambers. Only the faintest fireformed line after firing. Primers are still being struck about dead-center, don't think it's really an issue.

I think it seems pretty clear that the mainspring is the obvious culprit. Might have to such it up and put in a new one. I'll keep checking the ejector rod to make sure it's screwed all the way in.

Is a new mainspring hard to install? I have to remove the sideplates, yes?

-MV
 
Here's a question floating around in my head. Is it possible the original mainspring for .38 S&W isn't strong enough to hit the primers on modern .38 spl? Definitely change the mainspring Matthew. It shouldn't be hard, typically it requires removal of the grips, a pair of pliers and about 30-45 minutes. Most of that time is spent cursing as you attempt to install the new mainspring and get everything to line up. Remember an exploded diagram is invaluable in this process!

-Rob
 
It would probably be a good idea to change the strain screw also.
Click to expand...

On the assumption that someone might have shortened it? It doesn't look shortened from what I can see, but it's hard to tell.

It shouldn't be hard, typically it requires removal of the grips, a pair of pliers and about 30-45 minutes.
Click to expand...

Umm, are you kidding about the 45 minutes part? Any one single spring that takes 45 to put in place doesn't qualify as easy in my book. I checked out the revo, and I can see how the spring interfaces w/ the notches in the hammer. It _seems_ relatively straightforward, but like it'll take a little muscle to get it into place.

-MV
 
Matt, how long it takes depends on how many times you lose your grip on the spring and toss it down in frustration. It's not a difficult or delicate process, just requires some patience to get it right. It's kind of like installing the main spring of a 1911, it's not HARD, just takes a bit of time and patience.

I think you shouldn't have a problem with it bro! As for taking 45 minutes, let's just hope it only takes 15! :evil:

-Rob
 
To change a flat main spring on a S&W is easy
1. unscrew grip screw
2. push on grip screw with screwdriver to remove first grip panel
3. push off second grip panel from inside
4. unscrew the strain screw
5. slide mainspring to side then unhook forked end from hammer
6. slide forked end of new mainspring into hammer.
7. slide other end into the slot in the botom of the grip frame
8. tighten strain screw
9. replace grips and tighten grip screw.
done
It takes me longer to type this.
 
Which mainspring???

Okay, not quite sure which MS to order from Numrich.

In my previous thread, I was told that this revo is an M&P, that it's a S&W .38-200, and that it's a pre-Model 11.

See thread and pic: http://www.thehighroad.org/showthread.php?t=218293

Here are all the different MS that Numrich offers, I assume that I want the last one?

Model: SMITH & WESSON 581,24,657-1,25-2,25-5,581-1,25-7,25-8,25-9,27-3,581-2,27-4,27-5,28,28-3

Model: SMITH & WESSON 29-3,581-3,29-4,29-5,57,57-1,57-2,586,57-3,610,624,625,625-2,586-1,629

Model: SMITH & WESSON 629-1,629-2,657,586-2,10,66,10-10,66-3,10-9,586-3,67,12-4,67-2,13-3,13-4

Model: SMITH & WESSON 681,14,15-4,15-5,15-6,681-1,16 32MAGNUM, 16 32 LONG,681-2,17-4,17-6,18-4

Model: SMITH & WESSON 19-5,19-6,681-3,48-4,617,64,64-3,64-4,686,64-

Model: SMITH & WESSON 1917

Model: SMITH & WESSON BRITISH,K-22,K-38 OUTDOORSMAN,MILITARY & POLICE,VICTORY
Click to expand...
 
It really doesn't matter; they're all the same. In fact all K, L, & N frame mainsprings are identical. They're just listing the mainspring for each model since they don't know what the customer will be looking for, but any of them will work since they're all the same.

The strain screws for the round butt vs square butt ARE different. If you order one of them (and for $2.55 I would) then make sure you specify a square butt strain screw.

You do have another option. Brownell's carries S&W mainsprings for $5.11. It's part no. 940-050-470. They also have strain screws for $3.25, P/N 940-050-350. Shipping might be cheaper than Numrich, I dunno.

-Bob
 
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