Jump to content

M4 bolt keeps locking in rear position (FTF)


srg433

Recommended Posts

Hi,

I recently got an M4 that now has about 200 rounds on it, cleaned it a few times already. The problem is that with some ammo, I see FTF 5%-10% of the time. Lighter slugs / buckshots (1150 FPS), some (1450 FPS) slugs. Had no issues with 1500+ FPS slugs.

When it happens, the round gets to the carrier but the bolt gets locked in the rear. Pushing bolt release button or racking the charging handle chambers the round with no issues and it fires fine after that.

Any ideas are appreciated.

Thank you.

Link to comment
Share on other sites

Just throwing this out there but, it might be that you have a weak magazine spring that is not throwing the shell far enough back onto the carrier that is jamming up the gun.

You will notice that when the bolt is locked back and you press the bolt release it causes the carrier to flip up into the loading position.

Possible with your heavier loads, the recoil is helping to nudge the rounds further back onto the carrier and that's why you don't experience the failure with them.

With the lighter loads, does it happen on the last couple of rounds from the mag when the spring is less compressed?

It's worth looking into.

  • Like 1
  • Upvote 1
Link to comment
Share on other sites

3 hours ago, Evolution said:

Just throwing this out there but, it might be that you have a weak magazine spring that is not throwing the shell far enough back onto the carrier that is jamming up the gun.

You will notice that when the bolt is locked back and you press the bolt release it causes the carrier to flip up into the loading position.

Possible with your heavier loads, the recoil is helping to nudge the rounds further back onto the carrier and that's why you don't experience the failure with them.

With the lighter loads, does it happen on the last couple of rounds from the mag when the spring is less compressed?

It's worth looking into.

Thank you for suggestions. I didn't notice a definitive pattern, it's definitely not every last round but it's also not every first out of the mag...

Anyway, I just ordered a couple of OEM m4 springs, easy to try it out.

Link to comment
Share on other sites

1. Check magazine fore-end cap is tight; magazine cap detent engaged. Inspect cartridge drop lever looking for dings or bent.

2. Fully load magazine with "dummy" rounds and manually cycle, as fast as you can, the action until magazine is empty.   Did the bolt lock-back inappropriately?

3. When the untimely bolt lock-back occurs, instead of hitting pushing bolt release of cycling charging handle, strike/bounce the butt of the M4 on the ground. SAFETY - Keep muzzle oriented in safe direction. Does the bolt close for the next shot?

Agree with 'Evolution's' initial assessment of a weak magazine spring.  

 

  • Like 1
Link to comment
Share on other sites

I had a similar issue with my new M4 last year, about 2-3 times in every 20-25 shells a shell would get stuck on the carrier ramp.  The bolt carrier was not locking back it was simply hung up in the rear position without enough spring tension to move it forward.  Basically at the time it seemed like the bolt carrier group didn’t have enough momentum to push it forward and load it.  In each instance I was able to either tap the bottom of the shell carrier or the nudge the bolt handle forward and it would load the shell. Initially I thought maybe it needed shells with a little more velocity but this seemed to occur regardless of the type of shells I was using, and always towards the end of the magazine usually around shell 5, 6, or 7.  I tried some Federal 3 Dram 1-1/8 ounce #8 field loads, Federal Power Shock 00 Buck, Winchester 00 Buck Military Surplus (Brown Box), Wolf Power Shot 00 Buck, and a few Federal Law Enforcement slugs, nothing low velocity and nothing low recoil and all 2.75" shells.  When I say a few total shell count was around 200.   

I went ahead and took the entire shotgun apart, barrel, magazine tube, gas pistons, plugs, bolt, trigger group/housing, bolt carrier, and thoroughly cleaned and oiled it, sans the gas pistons which the manual states not to oil.  Everything looked in good working order but still the issue persisted. Even with dummy shells and testing on the bench it would occur once or twice out of every 3-4 magazines.

The aftermarket parts I swapped are listed here:
Benelli OEM 7 rd mag tube installed, for 922r purposes I put in the FFT trigger kit (trigger, hammer, disconnector with springs), one of the FFT Mag Followers, an A&S Trigger Housing (since I had to take the whole trigger pack apart anyway to put in the FFT Kit I figured I might as well get rid of the polymer housing plus gain one 922R part), a FFT Wolff Magazine Spring, and FFT Handguard.  I also swapped the recoil tube for a 3 position tube but that's Benelli OEM.  Basically I've added parts to stay in compliance plus the mag spring but nothing superfluous.  No side saddles no aftermarket Bolt Release Buttons, etc…

Here was the fix (at least for mine M4 anyway) if anyone else is having this issue, Per Stranger Danger's guidance I went ahead and polished the inside of the magazine tube, used some #1 Steel Wool wrapped around a brass 12 gauge bore brush and attached that to my Drill.  Went back and forth for about 20 minutes then switched to a #0000 polishing steel wool and did another 20-30 minutes.  That mag tube is smooth as glass.  Even went ahead and used some light polishing compound on a cotton wheel on the Dremel on the ridges of the Magazine Follower which is also smooth as butter at this point. 

However that didn't completely resolve the issue but what did was removing the FFT Wolff Premium Spring.  For some reason my M4 was just not happy with the FFT Wolff Premium Spring.  I switched over to a Carrier Comp premium spring (oddly also a Wolff Spring but made to different specs) and I am rocking shells up the feed ramp at this point.  Even the original factory spring is working albeit a little less aggressively.  Interestingly the OEM Spring, and the FFT Spring are close in length, with the FFT being just slightly longer maybe 2-3 inches.  The Carrier Comp Spring is over a foot longer then both the factory and the FFT Spring.  I talked to Todd over at FFT and to his credit he refunded me the price on the spring I bought from him since it just doesn’t seem to work in my gun.  Excellent Customer Service and that was certainly appreciated!

Granted I had some aftermarket parts and it sounds like yours is still stock but really in the end it came down to using shells over 1300 FPS (at least initially) and using a magazine spring with greater tension.  If you don't want to swap the spring the gun should smooth out a bit with use and that may help.  But if you are already so close to the margins that certain shells aren't loading then I'd head over to Carrier Comp and put in one of their springs.  However nothing will overcome using light power loads that just don't produce enough gas to run the gun.  

 

  • Like 1
  • Upvote 1
Link to comment
Share on other sites

Thanks. I'm getting new OEM springs delivered today and do some suggested experiments.

My POV is that a high quality shotgun should "just work" majority of the time, with no 3rd party mods. So, polishing the tube and using after market springs is a bit of an anti-pattern for me, I appreciate all the info shared though.

Link to comment
Share on other sites

Interim update on it... 

1) I couldn't reproduce this feeding issue with A-Zoom snap caps. They cycle with no issues.

2) I inspected BCG and upper receiver grooves for signs of irregular wear and couldn't find anything concerning. There are some spots on edge where black finish came off but nothing that would visible show any change in shape or anything. All-in-all, it doesn't appear that BCG has much to do with feeding issues.

3) I got two replacement OEM springs. While they looked similar but weren't quite the same. Replacements springs are 1 inch longer, have mate finish and appear a bit stronger. So, I replaced the sprint that came with the gun.

4) I went through about 100 rounds of four different ammo types and got one FTF, which is a marked improvement over 4-5 I saw before.

 

Considering the about, it does look like replaced spring made it better but didn't fully resolve the issue. Upon further analysis, I can to conclusion that the ammo load doesn't have much to do with these FTFs but manufacturer is what's most common between rounds that have issues and the ones that don't. Any Winchester load appear to work fine with 0 FTFs, Federal, Fiocci and Stars and Stripes have issues.  Changing the spring made Fiocci rounds cycle reliably, Starts and Stripes had that 1 FTF.

It looks like some manufacturers have wider tolerances for round's metal base width, that's why some round have more friction going from the magazine to the far edge of the carrier and end up not fully engaging carrier latch. The carrier latch itself appear to have rather stiff spring.

I attached a picture of the wear on the receiver from cycling the shell and the wear on snap caps. Basically, it appears that rounds going out of the magazine face rather still resistance from the carrier latch that pushes them hard against the receiver wall (wear pattern is shown on the picture). You can also see aluminum scrap from grinding the edges of the snap caps. While I can't reproduce FTF with A-Zoom's, I ordered Fiocci dummies as these are cheap and have more feeding issues as per the Internet anecdotes.

At this point, I'm going to check if I can polish the carrier latch and the receiver wall a bit. I also have a spare carrier latch spring I may consider replacing to see if the new one is softer.

Frankly, I'm not a mechanical engineer or a gun smith or even a power user of shotguns, so if the analysis above appear to be critically flawed, please let me know.

IMG_3135.thumb.jpeg.012e81e57519d588784e044c10713a28.jpegIMG_3133.thumb.jpeg.54dff88625a64aeb132be4ea44e38465.jpeg

  • Like 1
Link to comment
Share on other sites

Thanks for taking the time for an update. From your original description of the problem, I did not think that it was a bolt or bolt carrier problem because as far as functioning, they were doing what they're supposed to do...the piston impulse was enough to shove it all back enough to lock the bolt open when empty and empty shells are ejecting.

I'm still kinda swaying towards the spring, somehow with your particular weapon, not being powerful enough to throw the shells back to actuate the lifter and trip the bolt to go forward.

I'm not an expert on these guns either. I was hoping someone like StrangerDanger or unobtanium, would chime in on this thread. Both of those guys could be considered experts IMO.

I've never handled shotgun snap caps, so I don't know but, are they as heavy as a live round of buckshot? I would think not, so any spring would be able to toss them back easier than a live round, no?

If not, then I would also think that using snap caps for this type of determination would not be accurate? Anyway, I wish that you would have gotten one of CarrierComp's springs to test as they are around 12" longer than OEM springs.

I am in no way saying that there could be something else like the rough spots you found contributing to the shells not smoothly getting thrown back onto the rear of the carrier.

Keep us up on it! 

Edited by Evolution
Link to comment
Share on other sites

4 hours ago, Evolution said:

Thanks for taking the time for an update. From your original description of the problem, I did not think that it was a bolt or bolt carrier problem because as far as functioning, they were doing what they're supposed to do...the piston impulse was enough to shove it all back enough to lock the bolt open when empty and empty shells are ejecting.

I'm still kinda swaying towards the spring, somehow with your particular weapon, not being powerful enough to throw the shells back to actuate the lifter and trip the bolt to go forward.

I'm not an expert on these guns either. I was hoping someone like StrangerDanger or unobtanium, would chime in on this thread. Both of those guys could be considered experts IMO.

I've never handled shotgun snap caps, so I don't know but, are they as heavy as a live round of buckshot? I would think not, so any spring would be able to toss them back easier than a live round, no?

If not, then I would also think that using snap caps for this type of determination would not be accurate? Anyway, I wish that you would have gotten one of CarrierComp's springs to test as they are around 12" longer than OEM springs.

I am in no way saying that there could be something else like the rough spots you found contributing to the shells not smoothly getting thrown back onto the rear of the carrier.

Keep us up on it! 

The A-Zoom snap cap are actually heavier than every variety of ammo I have. I measured and weighed what I currently have on-hand. See the screenshot. Granted, I currently don't have a place to shoot buckshot, I only tried the slugs from that list and some other slugs I had earlier.

You can actually see how the snap caps metal base diameter is now smaller than live rounds as the M4 cycling kept grinding it.

54 minutes ago, Brandon101884 said:

Carrier comp spring solved my issues. Get that one. This is the one I got, I have the full 7+1 tube. I’m not 100% of this will work for you since yours isn’t the full tube. Anyone know?

 

https://shop.carriercomp.com/product.sc?productId=1

Yeah, I don't mind buying and trying it but It's 5 round tube that already won't fit 5 of the longer 2 3/4'' rounds in it as it takes the same 7 round OEM spring.687661815_Screenshot2022-10-30at9_43_31PM.thumb.png.bac877d8c0c49d9ce40bab8a31621e4a.png

Link to comment
Share on other sites

  • 4 weeks later...

I finally got more time to get to the bottom of it and happy to report that the problem is now resolved.

TL;DR I have 5 round mag tube and I replaced the spring with Carrier comp one AND shortened it to 20'' (most important part). Additionally, I polished the carrier latch with my Dremel, this step is likely optional as the spring's role is more important. The longer version is below.

More details:

After cycling many more rounds through it and replacing the spring with new OEM one, the number of FTFs reduced but was still at about 3-5% for some specific ammo brands. 

Since reliability of a weapon is the most important thing, I wasn't satisfied with that. I wasn't able to replicate the issue with A-Zoom dummies or other "no-name" dummies and live fire gets expensive really quick. As per the Internet anecdotes, Fiocci dummy rounds have more FTF issues than the others, so I order them. Once the dummies arrived, I could clearly replicate the issue with FTFs happening 30% of time. It was great as I finally got a way of reproducing the issue reliably and with no extra expense for live rounds. As for any engineer's satisfaction, the ability to reproduce reliably is like 50% of fixing the issue. (Side note, Fiocci dummies are NOT suitable for dry-fire as their cartridge head is just flat, no primer, and it bends if you dry fire).

After studying the anatomy of the feeding system, I saw for myself that if the mag tube is properly installed all the way in, there were limited number of variables in play:

1) Friction inside the mag.

2) Friction from the receivers wall.

3) Friction from the rim engaging the carrier latch. It basically the carrier latch surface itself and the spring that pushes it outwards. 

4) The power of the spring pushing the rounds out.

So, before going into troubleshooting 1-3, I decided to order a couple of Carrier comp springs and try them out. I asked CC rep on the phone if they tried it with 5 round mag tube but was largely referred back to this focus, so I figured I'll need to experiment by myself... Once the springs arrived, I noticed that the springs are stiffer and LONGER that the OEM ones. The OEM ones when NEW are about 27'' long, after few hundred cycles, they shrink to about 25''. The carrier comp ones were about 31''. So, I installed CC spring and found that while it's longer, you can still fit 2 3/4 slugs into it, so it wasn't that big of a deal. 

After about two hundred of cycles of Fioccy dummies and rotating them to account for the rim wear, it was clear that the behavior has changed. Basically, I wasn't getting any mid mag TFTs anymore, the ones I was seeing with OEM springs, but was consistently getting 70% of FTFs on the very last round from the tube. It didn't make sense to me, so I re-installed the OEM one and it was back to "random" FTFs but no FTFs on last rounds. GOOD, I could clearly reproduce different failure modes with different springs. What didn't make sense to me was WHY the STRONGER AND LONGER spring was giving me such a high rate of FTFs on that last round. 

Anyway, I decided to see if reducing the friction will make things better. I replaced the carrier latch spring with a new OEM ones. It actually made things worse as that new spring needed some break it time likely, so I re-installed the old one. Then, I decided to give Dremel with some rough polishing compound a go and polish the carrier latch. After making the entire round rim facing surface slick and shiny, I immediately noticed an improvement. No, CC spring was giving me about 30% FTFs on that last round (down from 70%) and I could clearly see that on average, the rounds would get pushed further away from the follower and into the carrier. OK, it was great but still not enough. I did understand that slow and manual cycling wasn't the same as firing live due to no inertia that could help to push the round further into the carrier 

However, the fact that CC spring had these reducible FTFs on the last round didn't sit well... I came up with the idea that the bent shape of the spring in the mag tube may have a bigger impact on consistency and applied back pressure, which I was committed to test.

Knowing that I could easily measure the impact by looking how far from the follower the last rounds get pushed, I decided to start chopping it in. The springs in 5 mag round M4s are exactly the same as in 7 round mag tubes. I made rather simplistic, and potentially flawed, assumption that I should be safe to go up to about 18'' inch length by using the "post break-in" OEM spring length of 25''/7 (size of the mag it was designed was) * 5 (size of the mag tube I have).  Because it was easier to reproduce the issue on CC spring, I decided to use it and apply and start cutting 2-3 inch of length at a time. 

The end result was unexpected, the more I trimmed the spring, the farther away from the follower that last round was going. Once I got to 20'', the round was making it it to the retaining notch 50% at the far back of the carrier 50% of the time and was always at a solid distance from the follower. WOW.  After manually cycling the dummies about 100 times, I saw 0 FTFs. Then, I installed NEW Carrier comp spring, untrimmed, and it was back to about 30% FTFs I had after polishing and before trimming. So, the experiment was clear to me, I switch to 20'' Carrier comp and went to try it out with the same mix of previously problematic live ammunition and saw 0 FTFs after 100+ rounds, half of which were cycled by only loading one round into the mag. 

I suspect, the OEM spring will exhibit similar behavior but had no time to experiment with it too. 

All in all, I learned a lot about the M4, the way in functions and can probably disassemble it with my eyes closed and replace the carrier latch / spring in minutes lol. Thank you for all the hints given in this thread and hope the information above helps somebody else.

 

  • Like 1
Link to comment
Share on other sites

Good job! Glad we were able to at least point you in the right direction. Still...I have to say that it is a bit weird that you had to modify your CC spring to effect perfect reliability. That really should not be necessary.

I'd love to see you install a standard 7 round mag tube, then install an unmodified CC spring and see what results you get....anyway, as you say, it was an experience which you learned from.😀

Link to comment
Share on other sites

Yeah, it was surprising to me to learn that reducing the length of the spring in a 5 round mag tube actually led to increased energy on the follower. If I were to guess, it likely because of the unnatural shape of the spring leading to excess energy applied to the sides of the mag tube rather than straight down on the follower.

For 7 round mag tube, I live in MA, which makes it complicated... Two extra rounds for me aren't worth being in some gray area of the law where people keep arguing what a "feeding device" is...

 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Unfortunately, your content contains terms that we do not allow. Please edit your content to remove the highlighted words below.
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...