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  #1  
Old 11-17-2013, 03:50 PM
tfrank
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Smile Semi-auto sear, bolt, hammer...



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Semi-auto sear, bolt, hammer, and disconnector arrangement. Just an FYI, but this is specifically in regards to a question that Bruce G. asked me in a PM. So I figured I ought start a thread on the answer to his question so this becomes general knowledge and I will link this to the "Technical Support Thread"!!!
This is the picture that Bruce G. sent me of the "sear tab" on the back of his bolt. It shows a good bit of wear. Not good as it could allow an "out-of-battery" release of the hammer, aka the rifle fires when the bolt isn't fully closed.

The colage below is from a project of mine and in the upper left shows an unaltered 151 bolt. Bruce G.'s bolt is a 300 series bolt even though it is from a model 453T!!! At least I am assuming that Bruce G.'s bolt is the same as a 300 series bolt.This is Bruce G.'s question...
Quote:
The area I am concerned about is shown in the second picture, the collage of bolt pics. The top left image of the rear of the bolt shows the protrusion I am concerned about. Your image shows the corner of the tab to be square and clean. On my bolt, there is obvious wear and rounding. Am I correct in stating that this is the tab that the sear engages?

Ok I will try to explain the function of that rounded part on your bolt. And yes I would reccommend that you either replace the bolt of have someone with a MIG welder build that area up so you can reshape to an unworn profile. Some other things to be aware of is the colage is of a 151 bolt which is different from the bolt that I used below in my diagram, which is a 300 series bolt that has had some holes drilled in it for tungsten carbide dowels as it WAS going to be a 17 Mach2 semi-auto bolt, but that is another story!!!
If the bolt is "out of battery" that is the bolt in not fully closed, the purpose of that worn part of your bolt is to BLOCK the sear from moving up towards and in behind the bolt on your model. This in turn prevents the rear part of the sear from being able to move downward towards the trigger, thereby preventing the hammer from being released. Which is a very good thing because if the bolt is out of battery then the rearmost portion of the cartridge case in not being supported by the chamber walls and a ruptured case is a very likely result if the sear is allowed to move with an "out-of-battery" bolt. Study the diagram in the upper half of the picture below to understand the relationship of the parts and how they work together to prevent an "out-of-battery" firing. They are arranged in the same orientation as if they were installed in the receiver.

"B" is where the rear of the sear will engage the hammer notch and hold the hammer in the "ready-to-fire" position. "A" is the front of the sear, which should be blocked from moving if the bolt is slightly to the rear because it is out of battery. The upper half of the picture shows a better orientation of the parts than the lower half of the picture. "B" is also the area that would need to be very closely inspected if you are having more than 1 shot per trigger pull. The hammer notch and the rear contact area of the sear are critical for proper SEMI-AUTO function. Any rounding of these contact surfaces can lead to multiple shot for one trigger pull.This is relative to how the parts are arranged in the diagramed picture above. The progression of steps... A. You pull the trigger, this forces the disconnector to move to the right. B. The disconnector will cause the sear to pivot on its pin, which will cause the front of the sear to rise up behind the bolt (assuming the bolt is in battery) and the back of the sear to drop, thereby releasing the hammer. Gun goes "BANG" just like it is supposed to!!! If the bolt is out of battery the sear's movement is blocked and the hammer can't drop. No bulged or ruptured case... a very good thing!!!
Bruce G. I hope this answers your question. If you need someone to weld the "sear tab" on your bolt you can send it to me and I can do that for you or you can buy a new bolt, whichever you prefer. If you want me to repair the bolt you have to pay postage BOTH ways!!!
God Bless, Frank.

Last edited by tfrank; 12-17-2017 at 02:37 PM.
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  #2  
Old 11-17-2013, 06:48 PM
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Frank,

I sit before my monitor, chastened by your extensive knowledge and your generosity in offering to fix my bolt.

I am hesitant to impose on your generosity. Let me do some checking locally, if I cannot find anyone willing to do the welding, I will accept your generous offer.

Your explanation is very enlightening, you have indeed answered my question. For some reason I was having difficulty envisioning how it was all supposed to work. Now I understand.

Thank you very much for taking the time to assess my issue and respond so generously.

I also noticed some rounding at the edge of the disconnector notch on the sear. I see similar wear in your picture so I won't worry about it. I did dress it slightly to remove some very small burrs. It becomes obvious then that as the disconnector overrides the sear notch, the front of the sear is then able to drop below the bolt, allowing it to move backward, eject the spent casing, move forward, strip the next round from the magazine and load it into the chamber whereupon the cycle is set to repeat itself.

It would seem that this design could be somewhat sensitive to very high velocity ammunition, possible resulting in some interference between the disconnector and the sear and between the sear and the bolt. If the bolt attempts to forcibly move to the rear before the disconnector has overridden the sear notch, I can see that the sear tab on the bolt and the corner of the sear disconnector notch could easily be shaved or worn by the premature forced movement.

While it is disappointing to find these issues in my first Mossberg semi-auto, I have learned a lot and will know what to check before purchasing my next one. I took a look through some of the current semi-auto offerings from various manufacturers just to see if a modern design would appeal. Unfortunately all I saw was cast or molded receivers of one variety or another. While I am sure the designers have employed the latest analytic tools and manufacturing processes to make sure these designs will function effectively and will be cost effective to produce, they just don't appeal at all. There is something about the round blued steel of the older designs that just satisfies my inner being. I will continue to prefer them until they absolutely cannot be repaired and I am forced to adopt the less expensive designs.

Thank you again for your help, I will post updates as I get the 453T back together and running properly.

Best regards,

Bruce G.

Last edited by Bruce G.; 11-17-2013 at 06:58 PM.
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  #3  
Old 11-17-2013, 10:08 PM
tfrank
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Smile You've already seen...

You've already seen this picture in the pm I sent you, but for everyone else I am adding it to the thread. And again these are 151 parts but they will FUNCTION exactly the same as your similarly named 453T parts. In the second picture the disconnector/sear spring is hidden so that is another reason that I figured that this picture would be good to include as it should help to clear up some questions before they were asked!!!

The picture below shows a 151M(a) trigger assembly. I am using the 151 because the safety lever on the side of the disconnector bracket on my 350KA (same as your 453T) hides important detail that needs to be seen. The lower half of the second picture is simply a closer view of the top half giving a bit better detail. Also on the 151 the disconnector pin is a pretty tight interference fit in the disconnector, but on my 350KA the disconnector pin is a slip fit and it is retained by an E-clip, and that E-clip also hid detail I wanted to be seen. In the bottom half of the picture you can see the slot in the disconnector bracket that allows the disconnector to move forward, when the trigger is pulled, which forces the sear to pivot on the sear pin. If the bolt is in battery the front of the sear will be allowed to move upward in behind the bolt, which means that the rear part of the sears can move downward, releasing the hammer. The hammer moves forward firing the round in the chamber. Then the entire hammer/bolt assembly moves backwards, and in its backwards movement hits the "ears" on the disconnector that protrude up into the receiver. The "ears" are directly below "sear pin". This puts the right end of the disconnector on top of the sear notch. Releasing the trigger for the next shot allows the trigger spring to return the disconnector to its "ready to fire" position as is seen in both halves of the second picture. Again if the bolt is "out-of-battery" the sear tab as pictured in the first picture of post #1 should block the sear from moving... as it is supposed to.

You mentioned some
Quote:
some rounding at the edge of the disconnector notch on the sear
I believe that rounding was an intentionial manufacturing step. It looks to me like that rounding of that corner allows the disconnector to more reliably drop down into its notch in the sear but that is just a guess on my part.
And the repair to the "sear tab" on your bolt is about a 1 second spot weld with my MIG welder. The real cost will be the postage to and fro!!! The offer still stands. Unless you know someone with a MIG welder the postage will probably be the cheaper option, plus I am familiar with this repair!!!
God Bless, Frank.

Last edited by tfrank; 01-08-2015 at 12:19 AM. Reason: spelling
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  #4  
Old 11-18-2013, 04:06 PM
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Frank,

You make a compelling case.

I am on the road this week, returning home on Friday. I will get the bolt packed up and shipped to you this coming weekend.

Thanks again for the great info you shared and for your comments regarding my bolt in particular.

As I look at my picture again, I see what might be a fine crack near the front of the tab (left part of the tab in the photo), I don't know if it is my imagination or if it really is there. If it is a crack, it looks as though it may wrap around the side of the bolt. I will have to check that closely when I get home

Bruce

Last edited by Bruce G.; 11-18-2013 at 04:22 PM.
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Old 12-14-2013, 11:10 AM
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I had a new thought on the wear on the bolt's sear tab. What causes it?

My first thoughts were hot ammo but more thought about that suggests that does not make sense. The front of the sear should already be well below the bolt by the time the firing pin hits the rim and fires the cartridge. I suppose tolerances would affect this but I would think that as the disconnector slides past the stop on the bottom of the sear, the nose of the sear would drop. Perhaps it depends on the condition of the disconnector spring.

I also wondered if the retracting spring and the hammer spring affect the timing of the bolt moving back after the firing of the cartridge.

Are there any specs on free length of these springs and what is acceptable or unacceptable?

My goal is to make sure that I don't destroy the tab after Frank rebuilds it by welding it up. I would like to make certain that everything is functioning as it should when I put it all back together.

Thanks in advance for any thoughts and comments.

Bruce G.
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