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The Slide Bump

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floydthecat View Drop Down
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    Posted: Apr 27 2023 at 3:03pm
I ran across a sticky issue that might be of interest to others. I was having sporadic ejection issues on a newly acquired carbine. One trip out with no issues, but the next shooting session may-or-may not present random feeding and/or ejection issues. Took some time to run it down and all the usual remedies were employed to no avail....until I found the problem.

Along the way I noticed a bump in the slide action, I discovered that if I lifted the charging handle up during the rearward stroke, I could actually lock-up the bolt in the action at about the point where hammer cocking started. I eliminated everything surrounding the cocking sequence which left one other possibility. After examination of the dismounting notch in the left barrel rail recess, I determined that the faces in the notch were basically square. Paying close attention to this dismount notch on other carbines and spare barrels, the faces of the rail on both sides of the notch are sloped (for lack of a better term). Contoured at an angle that allows the lug in the slide bucket to more easily "jump" the notch. I reformed the notch faces on both sides and the lug on the slide as well. I can still "feel" a slight bump if I lift the charging handle upward as I draw it to the rear, but not much.

I hand cycled the action on another couple of carbines and to varying degrees, can feel this slight bump, so it is obviously a built-in feature by design, the lug has to jump the dismounting notch every time. I am sure that the condition of the slide lug in the receiver right rail recess could affect things as well. If that lug is tight and in spec, the slide cannot be lifted up as easily or at all and if the slide is allowed to lift up, it rotates the bucket down on the left side aggravating the step of jumping the dismounting notch.

All the slides I checked measured close enough to each other not to suspect a slide and any slide I tried exhibited the bump I refer to in the problem action.

This could all be just useless reading or might be of interest to someone having a random issue with a problem-child action that has not considered the slide dismounting action between the barrel and slide.
If there is a problem in this area, it will slow slide action down enough to prevent proper extraction, ejection and feeding.

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Matt_X View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Matt_X Quote  Post ReplyReply Direct Link To This Post Posted: Apr 27 2023 at 9:30pm
Funny you bring this up.  Last week I came to a similar conclusion about the slide operation on my Underwood.  There was a little bump outward from when I tried forcing the removal of the slide a little forward of the opening (there's an embarressing post elsewhere about this).   The fix was to lightly, but firmly tap on the metal.  Probably shouldn't post about using a hammer to fix things but it was within my experience.  :) 

Its the only thing I could find on the carbine itself that might have been slowing the speed of the bolt closing, and possibly contributing to a misfeed.  Magazine might have contributed too.  We'll see how it does next time out.
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Jond41403 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jond41403 Quote  Post ReplyReply Direct Link To This Post Posted: Apr 27 2023 at 11:34pm
I always use a fair amount of grease in that area in the rails. Not a ton but enough to ease the jump across the notch. If anything is off a hair it should still make it across if everything is lubricated enough without slowing it down too much or causing any excessive wear on the slide lugs. AKs have a similar issue with a small bump or catch like feeling pulling back the action but for a completely different reason. The AKs issue is overriding the hammer which causes a slight bump in the action but a fair amount of grease on the hammer and in the rails solves that issue as well and makes the action smooth as butter.hth. have you made sure on the brand new to you carbine that the barrel is perfectly indexed? If it's off even a hair that can cause issues with the slide
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floydthecat View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Apr 28 2023 at 9:09am
I would not want to over-rotate the barrel to bring the left side of the barrel flat down to lower the recess in relation to the slide lug, which would help. That rolls the front sight to the left, which aggravates the orientation of the rear sight. Over-rotation causes positioning of the rear sight to the left. We see many rear sights skewed way left on receivers. I am not anal about a rear sight being positioned left a bit, but I do have my limits on how much.

This carbine has never seen a action wrench and I’d rather work it out without moving the barrel…..which I think I have.

When I first pursued the problem, slide retraction to full-cock was close to 10-pounds. I have it worked down closer to 5-pounds, which is where my other 30’s measure.
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Jond41403 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jond41403 Quote  Post ReplyReply Direct Link To This Post Posted: Apr 28 2023 at 12:46pm
I agree, I wouldn't want to mess with the barrel either as long as everything looked okay. I was going to ask you if it looked like the barrel had been replaced maybe by someone that didn't get it quite right. But by the sound of it,  that's not the case. Hopefully she'll run like a clock next time you take her out. Good luck
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painter777 View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote painter777 Quote  Post ReplyReply Direct Link To This Post Posted: Apr 28 2023 at 1:07pm
Ole FTC is a trouble shooting SOB, give him time he'll find the problem.

Index Marks: I don't trust them. I always check with a pair of magnetic digital angle finders. Angles match, I'm good. Using these I've found front sight key ways that were slightly off and found on a early WRA I own that had 3 index marks like this / l \  that was indexed correctly on the far left one. Had it been a loose barrel / receiver I dont know if the 2 bars of steel method would have worked for me.

I recall reading about Gus Fisher talking about the numbers of mis-indexed carbines he would come across at the CMP shoot at Perry, thou I can't recall the figures it was higher than you'd expect, many of these were the off set cut for the front sight key.

Just for thought......
Living Free because of those that serve.....
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floydthecat View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Apr 28 2023 at 3:55pm
Thanks for the vote of confidence Mister painter.

I consider index marks as a mass-production starting point. They are usually close enough, but I have been known to rotate a barrel a degree or so.

I never really even considered the dismounting notch to be an issue until I had extinguished about ever other possibility that I could think of and try. With the TH removed to get the hammer out of the picture, I noticed the amplified bump in slide action. The slide lug jumping the dismounting notch occurs at about the same moment as hammer cocking and you won’t notice it much due to that. If one removes the TH on his carbine/carbines and hand-cycles the action, he will likely feel this bump. This one was so pronounced that it would lock the bolt in the receiver if I held the charging handle a little awkwardly. The faces in the dismounting notch were practically perfectly square. I bet if you observe that area in your carbines you will discover those faces have been slightly tapered.

Dang……I wish I never had discovered this as a problem…..now I will be feeling the action on every carbine I come into contact with😁.

It might bump a bit just due to the necessary design in order to remove the slide, but a sloppy fit in that location can be problematic. This one runs just fine now.
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