[STEINBVG]

As I said in my previous post, my rifle shoots well, but here and there shot doubles, unpredictable.
Disassembled no problem, reviewed everything, reviewed this good thread

151M-c full auto bursts / FTFs

Just returned from the range, and it fires as designed. No more 151M-c machine gun! I would have been really tempted NOT to have fixed it. :AR15firin :bthumb: I fixed an old Stoeger .22 Luger a buddy got for free once. It needed a mainspring, mainspring guide, and a magazine. One day at...

www.rimfirecentral.com

And came up with a couple of possible culprits. In the rating order:
1) obviously broken sear/disconnect spring
2) a bit worn edge of the sear
3) a slightly nicked sear edge in one area of the striker.
The disconnect legs on the bottom don,t seem to be worn out .
I have cleaned everything , lubed. And after assembly shot a few . This time no doubles, but I still ordered new spring, new pivot pins. If the problem resurfaces afterward- I will order sear and perhaps hammer.
By the way, I have never seen such a wide foot print of a firing pin on a shell, but it,s very deep and crisp as the the hammer spring seems strong. I have not had any weak strikes / misfire.
Additional comment, speculation.
This gun with completely perfect fire control group might be able to go full auto with weak ?subsonic ammo. It almost can behave like open bolt gun , while bolt/ hammer combo travels not far back and doesn,t get caught by the sear, still feeds and slams a round by protruding firing pin.
(i slowly moved the bolt/striker back, the handle just crossed the second safety hole) , fed a cadridge and the sear didn’t catch….I was tempted to let go and see if it would fire…

 

[tfrank]

The broken sear/disconnector spring could easily be the culprit, so it's good to see you are replacing it. Not 100% sure about the back end of the sear where it contacts the hammer.
I know this post was linked indirectly but here is a direct link. Study it and the text explanation. (2) 151M-c full auto bursts / FTFs | Rimfire Central Firearm Forum There could be damage to the sear contact that is not obvious even with a close visual inspection. damage that could have been caused/exacerbated by the "full auto" malfunctioning of the rifle. The pictures you posted are good pics but don't have the close-up detail I want, so that is why i am not sure of the sears contact surface.
If you are using your cell phone to take the pictures try maxing out the zoom feature and then backing up somewhere around 16 to 20 inches. The first picture gives a relative size of the piece, the 2nd picture gives close-up detail. My cell phone zoom maxes out at 6X and that was how I took these pictures.

I would like to see a picture of the back end of the sear with it laying flat on something, as close as you can get and in focus. I know, easier said than done, that is why I usually take a minimum of three pictures then pick the best one that shows the detail I want. keep us posted on this!!!
God Bless, Frank.

 

[STEINBVG]

The broken sear/disconnector spring could easily be the culprit, so it's good to see you are replacing it. Not 100% sure about the back end of the sear where it contacts the hammer.
I know this post was linked indirectly but here is a direct link. Study it and the text explanation. (2) 151M-c full auto bursts / FTFs | Rimfire Central Firearm Forum There could be damage to the sear contact that is not obvious even with a close visual inspection. damage that could have been caused/exacerbated by the "full auto" malfunctioning of the rifle. The pictures you posted are good pics but don't have the close-up detail I want, so that is why i am not sure of the sears contact surface.
If you are using your cell phone to take the pictures try maxing out the zoom feature and then backing up somewhere around 16 to 20 inches. The first picture gives a relative size of the piece, the 2nd picture gives close-up detail. My cell phone zoom maxes out at 6X and that was how I took these pictures.

I would like to see a picture of the back end of the sear with it laying flat on something, as close as you can get and in focus. I know, easier said than done, that is why I usually take a minimum of three pictures then pick the best one that shows the detail I want. keep us posted on this!!!
God Bless, Frank.

Thanks for the suggestions and your bravery. It seems using words’ full auto ‘ blows people away. I have ordered the replacement parts, In the mean time my gun disassembled and assembled back, clean, lubed is not shooting doubles so far. I made sure that the sear/disconnect spring is held by the screw very tightly - I suspect the screw head covers up the broken area by the hole and the spring is able to function appropriately.
Once I get the new spring and pins, I will post a better quality picture of the sear working edge.

I wonder still if my speculation about underpowered ammo holds water and can set this type of a gun off into a full auto rant failing to blow the bolt back far enough to catch the sear, while still feeding a round.

 

[tfrank]

Thanks for the suggestions and your bravery. It seems using words’ full auto ‘ blows people away. I have ordered the replacement parts, In the mean time my gun disassembled and assembled back, clean, lubed is not shooting doubles so far. I made sure that the sear/disconnect spring is held by the screw very tightly - I suspect the screw head covers up the broken area by the hole and the spring is able to function appropriately.
Once I get the new spring and pins, I will post a better quality picture of the sear working edge.

I wonder still if my speculation about underpowered ammo holds water and can set this type of a gun off into a full auto rant failing to blow the bolt back far enough to catch the sear, while still feeding a round.

My experience says no. If anything it will cause a jam in that the hammer and bolt were not sent back far enough to properly eject and feed the next round. My 151M(a) behaves very poorly when fed sub-sonic ammo. But when I originally got it and did the full disassembly and clean I also made sure to select the longest 151 springs that I had as I have no desire to shoot sub-sonic ammo, or even standard velocity ammo. The "retracting spring bushings" shown in the picture below also played a part in wanting the strongest springs in my 151M(a) as HV and particularly Hyper-Velocity ammo can play heck with multiple internal parts!!!

God Bless, Frank.

 

[tfrank]

Some more pictures for you to study and compare your parts to!!! Top three parts... 3 series semi-auto main spring, retracting spring and the retracting spring plunger. Bottom three parts... the retracting spring plunger, the retracting spring and the main spring for a 1 series semi-auto. The 3 series parts were S-L-LR capable rifles, so they had to have weaker springs to allow a short to be able to cycle the action properly. The 1 series parts were to LR only rifles so I guess Mossberg used longer/stronger springs in those models.

A broken retracting spring plunger, most likely due to prolonged HV or Hyper velocity use. I think the broken retracting plunger was to a 3 series semi-auto that was a S-L-LR model which has the shorter/weaker spring set but can't swear to it.

God Bless, Frank.

 

[STEINBVG]

Some more pictures for you to study and compare your parts to!!! Top three parts... 3 series semi-auto main spring, retracting spring and the retracting spring plunger. Bottom three parts... the retracting spring plunger, the retracting spring and the main spring for a 1 series semi-auto. The 3 series parts were S-L-LR capable rifles, so they had to have weaker springs to allow a short to be able to cycle the action properly. The 1 series parts were to LR only rifles so I guess Mossberg used longer/stronger springs in those models.

A broken retracting spring plunger, most likely due to prolonged HV or Hyper velocity use. I think the broken retracting plunger was to a 3 series semi-auto that was a S-L-LR model which has the shorter/weaker spring set but can't swear to it.

God Bless, Frank.

WOW. Awesome information. I didn't see serious wear of my bushing, but will look closely. However the hole in the guide road hole seems a bit stretched. I need to keep an eye on it. I think perhaps with the thinning out of the bushing, there may be ^ pressure with every bang on the pin that holds the guide/plunger attached to the bolt.
I wonder, if I place an appropriate size but thin nylon washer right under the spring against the bushing: could/would it 1) reduce the travel preventing pin from eating at the hole 2) soften the blow of the bolt traveling back. What do you think?

 

[Solddaboat]

Tfrank... This is the assembly the broken plunger came out of...

 

[STEINBVG]

Couple discoveries and fixes:
Looked at my plunger and bushing, slightly worn. Placed a thin washer on the bushing with marching diameters- let the hammer bang that. Shot a few. Good.
It,s not clear why the plunger hole is wearing out? it’s only gets springs pressure of the pin when the bolt assembly is free out of the receiver. When in, the rear plug compresses the bushing somewhat inward and relieves the pressure on the plunger , pin and the hole I believe. I can,t see how shooting should affect that hole- if it did it would broke quickly.

Also fixed the elevation dial on the s130. It was not clicking as I would think it should. The bracket fork arms were too apart. I brought them together as close as the rotating dial allowed , and the ball now has better contact with the dial indents and the ‘Y’ spring. The clicks are very snappy.

 

[tfrank]

View attachment 376638
Couple discoveries and fixes:
Looked at my plunger and bushing, slightly worn. Placed a thin washer on the bushing with marching diameters- let the hammer bang that. Shot a few. Good.
It,s not clear why the plunger hole is wearing out? it’s only gets springs pressure of the pin when the bolt assembly is free out of the receiver. When in, the rear plug compresses the bushing somewhat inward and relieves the pressure on the plunger , pin and the hole I believe. YES, correct. I can,t see how shooting should affect that hole- if it did it would broke quickly.

Also fixed the elevation dial on the s130. It was not clicking as I would think it should. The bracket fork arms were too apart. I brought them together as close as the rotating dial allowed , and the ball now has better contact with the dial indents and the ‘Y’ spring. The clicks are very snappy.

The retracting spring plunger's inertia is the answer. You pull the trigger, and the retracting spring plunger (along with other stuff) is accelerated from motionless to really fast in a split second. It is suddenly stopped by spring action and/or hitting the retracting spring bushing. The retracting spring accelerates it until the front of the bolt contacts the barrels face and it is stopped again. So, it is standing still, it accelerates abruptly, it stops abruptly, it is accelerated abruptly again, and it is stopped abruptly again. So, if there is any clearance between the retracting spring plunger pin and the hole in the retracting spring plunger, (which there is) the retracting spring plunger can slam back and forth on its pin and over thousands of firing cycles that battering elongates the hole in the retracting spring plunger. That is one of the reasons that I don't particularly care for Hyper velocity ammo, beats the snot out of some of the internal parts on the semi-autos. The retracting spring plunger and bushing are two parts that will suffer the most damage from HOT ammo as the movement of the internal parts is magnified. Physics says if you double the speed, you quadruple the energy. Hence you beat the snot out of certain parts!!! The retracting spring plunger really doesn't have any spring pressure being exerted against it. The retracting spring bushing receives pressure from both the mainspring and the retracting spring, while on the other end the bolt is receiving pressure from the retracting spring and the hammer is receiving pressure from the mainspring. The retracting spring plunger is there just to keep the retracting spring straight so that it functions as designed, and not much else.
That wear on the side of the receiver where the cocking knob slides in, and the wear in the pin hole of your retracting spring plunger tell me... your rifle has seen a lot of shooting. Your retracting spring bushing wear indicates to me, very little HV or Hyper velocity ammo has been fired in your rifle.
That washer you put on top of the retracting spring bushing in effect strengthened the mainspring... microscopically. So long as the hammer can still move back far enough for the sear grab hold of it, well then, no problem. And you said you have test fired it so... no problem!!!
God Bless, Frank.

 

[Rosscoe]

inertia.

 

[STEINBVG]

The retracting spring plunger's inertia is the answer. You pull the trigger, and the retracting spring plunger (along with other stuff) is accelerated from motionless to really fast in a split second. It is suddenly stopped by spring action and/or hitting the retracting spring bushing. The retracting spring accelerates it until the front of the bolt contacts the barrels face and it is stopped again. So, it is standing still, it accelerates abruptly, it stops abruptly, it is accelerated abruptly again, and it is stopped abruptly again. So, if there is any clearance between the retracting spring plunger pin and the hole in the retracting spring plunger, (which there is) the retracting spring plunger can slam back and forth on its pin and over thousands of firing cycles that battering elongates the hole in the retracting spring plunger. That is one of the reasons that I don't particularly care for Hyper velocity ammo, beats the snot out of some of the internal parts on the semi-autos. The retracting spring plunger and bushing are two parts that will suffer the most damage from HOT ammo as the movement of the internal parts is magnified. Physics says if you double the speed, you quadruple the energy. Hence you beat the snot out of certain parts!!! The retracting spring plunger really doesn't have any spring pressure being exerted against it. The retracting spring bushing receives pressure from both the mainspring and the retracting spring, while on the other end the bolt is receiving pressure from the retracting spring and the hammer is receiving pressure from the mainspring. The retracting spring plunger is there just to keep the retracting spring straight so that it functions as designed, and not much else.
That wear on the side of the receiver where the cocking knob slides in, and the wear in the pin hole of your retracting spring plunger tell me... your rifle has seen a lot of shooting. Your retracting spring bushing wear indicates to me, very little HV or Hyper velocity ammo has been fired in your rifle.
That washer you put on top of the retracting spring bushing in effect strengthened the mainspring... microscopically. So long as the hammer can still move back far enough for the sear grab hold of it, well then, no problem. And you said you have test fired it so... no problem!!!
God Bless, Frank.

Thanks for the time and patience. A lot of food for thought.
Today I decided to dispense with the pin that attaches the plunger to the bolt- after a deliberation I decided it serves no shooting value and is not going to be missed. Its there just for convenience of the assembly , the self contained package of the bolt/hammer/spring/plunger/guide.
As a matter of fact, the assembly without it is not much more complicated: once the bolt is dropped in and the handle installed, then the guide with the springs and the hammer are pressed into the receiver by the plug( the main small spring easily finds the hole in the bolt) and the plug gets screwed all the way in with slight resistance compressing the springs. Then the lock is screwed in, to keep the plug in place. Done

Shot a few few rounds….no difference, good, no doubles either . The reinforcement washer is perfect, cheaper to change than the bushing it protects if necessary. I put away the pin with the other small parts. Now the plunger hole will never brake.
Come to think, a lot of guns have their rear plugs going in agains the resistance of the free springs. But there are some with a contained ones.

 

[tfrank]

Thanks for the time and patience. A lot of food for thought.
Today I decided to dispense with the pin that attaches the plunger to the bolt- after a deliberation I decided it serves no shooting value and is not going to be missed. Its there just for convenience of the assembly , the self contained package of the bolt/hammer/spring/plunger/guide.
As a matter of fact, the assembly without it is not much more complicated: once the bolt is dropped in and the handle installed, then the guide with the springs and the hammer are pressed into the receiver by the plug( the main small spring easily finds the hole in the bolt) and the plug gets screwed all the way in with slight resistance compressing the springs. Then the lock is screwed in, to keep the plug in place. Done

Shot a few few rounds….no difference, good, no doubles either . The reinforcement washer is perfect, cheaper to change than the bushing it protects if necessary. I put away the pin with the other small parts. Now the plunger hole will never brake.
Come to think, a lot of guns have their rear plugs going in agains the resistance of the free springs. But there are some with a contained ones.

For some reason I'm thinking that is not a good idea. Manufacturers don't machine parts and install pins for no reason. Particularly a manufacturer as frugal as Mossberg. Time will tell though.
God Bless, Frank.

 

[STEINBVG]

For some reason I'm thinking that is not a good idea. Manufacturers don't machine parts and install pins for no reason. Particularly a manufacturer as frugal as Mossberg. Time will tell though.
God Bless, Frank.

Thanks. I also had reservations for exactly the same sentiment.
But. Will give it a shot for a while, no pun intended. What do I have to loose? Keep an eye on the parts. I can always install the pin back, correct?
What I'm curious to do is to install a soft metal loose fitting pin[? brass] or even a plastic one- and shoot, and see what happens to it, how much and how quickly it gets banged up, instead of the hole in the plunger. By the way I'm shooting 1260f/s ammo, believe it's SV?

 

[LarDaBear]

Thanks. I also had reservations for exactly the same sentiment.
But. Will give it a shot for a while, no pun intended. What do I have to loose? Keep an eye on the parts. I can always install the pin back, correct?
What I'm curious to do is to install a soft metal loose fitting pin[? brass] or even a plastic one- and shoot, and see what happens to it, how much and how quickly it gets banged up, instead of the hole in the plunger. By the way I'm shooting 1260f/s ammo, believe it's SV?

1260 is high velocity, SV is around 1070 feet per second

 

[STEINBVG]

Ok, got my sear/disconnect spring from Numrich, it appears different. Installed and discovered 2 things: weak spring on the sear end apparently was causing doubles. The new spring is too stiff, flat at the center, bent closer to the end at the disconnector pressure point. It would not allow the trigger to move the disconnector forward and reset.
I flattened it with pliers just a bit at the end and raised closer to the hole making it look similar to the original at least on the disconnector side and voila! It resets good. The sear side was good as the snaps in place on the hammer , by the sound of it very aggressively. Even if the edges are worn, the spring will hold them tight. Shot perhaps 30 rounds, no doubles any longer.

Will get me SV rounds as advised for this old working horse.
Also got a hood from eBay for ~ 20bucks. Now it looks much better. Next stop is the s 107 sight and last thing the swivels perhaps.
Thanks for your inputs guys.

 

[LarDaBear]

Get the swivels from Havlin Sales! You can also get a lot of other replacement Mossberg rifle and shotgun parts from them.Their prices almost beat anyone else's and they love Mossbergs as much and maybe more than the rest of us Mossberg nuts. They have a lot of knowledge about Mossbergs. Heck,I started collecting Mossbergs in 1992 and they were at it long before then.You can even join the NMCA,Nat'l Mossberg Collector's Assn through them which they publish and het some really nice full color glossy paper newsletters on a regular basis with a lot of informative articles.
And you can get a copy of their book Mossberg More Gun For The Money direct from them. Great folks, tremendous knowledge and very good prices.

 

[tfrank]

The 1260 fps ammo is pretty much all I shoot in my 151M(a) and I haven't had any issues, so I wouldn't worry about the HV ammo. Now if you were shooting it in a 50 or 51 CONEBACK of any sort, then I would be worried as the Coneback receiver cap had a tendency for the locking ears on the receiver to break when fed a steady diet of HV ammo.
God Bless, Frank.