Bolt over Base Malfunction

[Fenrir]

First off I like to say while I been familar with shooting AR-15s since I was 8 I never seriously shot with one or owned one until August of 2011. So if some of my questions sound very beginner this is why lol. I also had no formal training as well.

I have a S&W M&P-15 MOE. According to S&W's website its chamber in NATO 5.56/.223. I was using American Eagle 5.56 with my Magpul P-Mag. I have both 2 regular versions, a 20 rounder, and 2 window versions. I had NO problems what so ever in it performing both on the range, and at my past 3 gun matches. At the beginning of Jan. I started using PMC .223 Remington. Tested about 200 rounds at the range, and had no issues so I decided to buy a case. Shot another 150-170 zeroing it, and practicing before my match Feb. 11. It was VERY cold this day (15-20 degrees), and I had two malfuctions with a Bolt Over Base on the first stage. At the time I did not realize what type of malfunction it was so I just tapped, racked, and kept going. I shot a a little over 30 rounds so I used two magazines, and the malfunctions happen right about the end of the stage. I went onto the next stage after oiling the bolt, and feed ramp. I had one malfuction of the same type on the next stage which I shot about 20-24 rounds and it happen again right near the end. Both stages I rotated the around 3 magazines so I didn't feel it was a magazine issue. Next stage my gun began having this malfunction every 3rd or 5th round almost right away into the stage. I was VERY annoyed at this point but I set the magazine aside, and oiled the gun again. My rifle worked fine for the first 20 rounds on the next stage, and began doing it again. I reloaded with a different magazine, and it continued the problem of having a bolt over base ever 3rd or 5th round. I recently changed to hopps gun oil so I thought that might of been the culpert. So I decided when I got home to give my weapon a THROUGH cleaning, and go back to my original lube which was break free. After cleaning everything throughly, and focusing especially on the feed ramp since one friend suggested it could be a dirty feed ramp/chamber. I decided to chamber a round to see if it would do it while cycling more rounds with just the charging handle. Every so many rounds with me racking the slide it would produced the same malfunction. I was worried I wasn't pulling back enough on the charging handle but when I pulled completely back, and held it, and then released it would still do the same malfunction every couple of rounds. I rotated through my mags, and I still had the same recurring problem.

Before I did the cleaning I will say I looked up the malfunction in Kyle Lamb's Carbine book which is how I found out what it was. He says the most common problems could be the lube, or magazine. Only thing I can think of is it possible by using 5.56 P-Mags that they aren't designed to properly move .223 remington? I just can't see 5 magazines all brought at different times all suddenly causing Bolt Over Base Malfunctions. I don't feel its the lube as I never had a problem using break free before. It apparently can be the ammo if its some type of crap ammo but I never really heard of this type of problem with PMC. Another reason could be something to do with the gas at least according to the book. I don't quite understand what they meant but that wouldn't explained why it continued the malfunction every couple of rounds when I was cycling it with just the charging handle. Honestly I am quite worried as I only just now reached about thousand rounds through this rifle and this is the first time I had a problem with it.

Only thing I can think of left to test is to go back to American Ealge 5.56, and see if that somehow is causing the malfunction because the P-Mags say 5.56 or the PMC ammo is causing the malfunction? My only concerned is if I can recreate this in a range envoirment before my next match as the PMC did not show these malfunctions at the range when I was practicing before the match.

Here is a picture I found online that shows what is happening with my AR.

Thanks ahead of time for any advice, or suggestions people will hopefully offer.

[dmc_md]

I know you said it happens when you use the charging handle, but are you 100% certain you were pulling the handle back all the way when It happened? This looks like a short stroke issue.

Darren

[Amerflyer48]

May get more visibility and answers in the Multi-Gun Rifles-Technical Section.

1,000 rounds,.. not sure if that could be a buffer spring going soft and affecting timing or not.

John

[raz-0]

I'm going to place my bet on ammo+cold.

1) Is your ammo PMC bronze? That stuff is VERY anemic. Fortunately for you I have a copy of my chrono session with it on my phone instead of just my log book at home.

federal xm193 55gr, out of my carbine was averaging about 3060fps.

wolf 55gr was 2852 average.

55gr PMC bronze was 2640. (this is about the same average as wolf 62gr but with a lighter bullet)

On top of that, every .223 round I have had the chance to try out in cold temps has been temp sensitive getting slower in the cold. I can say from experience that at 18 degrees F with a wind chill down at about -1, xm193 would barely cycle my rifle if I didn't keep the mag warm and just left it on the bench in the wind waiting it's turn. Wolf 55gr DID cease functioning the gun when I did that. Failure was a bolt over base failure due to short stroking. That was on a carbine length gas system with fixed stock with a rifle buffer.

[MarkCO]

But Raz-O, he said it happened during hand cycling as well. If that is in fact true, then there has to be a mechanical issue with the bolt not stripping the top round. The magazine is not seated, there is a problem with the bolt geometry, or there is a dimensional problem in the geometry of one of the receivers.

[Fenrir]

Here is a video of me doing a demo of the problem for you guys. Also if an admin doesn't mind could we have this topic moved to the Multigun Rifle Section? But as you can see except for maybe once I was pulling the charging handle as far back as I could, and the rounds were still jamming. Also note I tried to take a picture of this but the bullet tips are having scratches made from being stopped. It extends from the tip down to just at the neck of the bullet. According to what a read this is a common result of a bolt over base malfunction. I will say I forgot to mention it but earlier in the video when I was trying it I did have one failure to go fully into battery. I also had this happen twice over the match. So I dunno if the two are connected or not I didn't really think to mention it in my original post since the problem is mainly bolt over base.

http://s70.photobucket.com/albums/i91/SpecOpsDelta/3%20Gun/?action=view&current=002.mp4

Edited February 17, 2012 by Fenrir

[Religious Shooter]

That stuff happened to me.

One time the bottom of the bolt was rounded off. SOLUTION: Got a new bolt.

The other time the extractor/bolt was dierty. The extractor wasn't camming correctly because there was crap between the extractor and the bolt body. It looked like primer cup debri. SOLUTION: Punch out extractor pin and clean recess.

[superdude]

your second jam, with the round stopping on the feed ramp - not bolt over base - made me wonder if the gun has the correct feed ramp or that the ramps (receiver, barrel) are properly aligned. i'm sure it does but it would be worth checking. it's odd for the round to stop where it did, so it's something to look for. and the scratched bullets are suspicious too and would be consistent with ramp issues.

i'm not sure what to advise on the bolt over malfunction. i feel your pain!

[dmc_md]

Your video proves it's not a gas issue.

Either you're not getting a complete back stroke or your bolt is slipping off of the round. I doubt it's your magazines. Make sure there's nothing blocking the magazines from going all the way up.

Have you changed anything that would change the length of the back stroke? Changed buffer spring or buffer? Changed rear stock/buffer tube? Perhaps something got behind the buffer and is stopping it from going all the way back.

To check whether or not it's your bolt, the easiest way will probably be to find a friend with an AR and try his bolt and bolt carrier in your gun. If they work, then there's your answer.

Also, you might take your bolt completely apart and make sure it's clean and put back together properly.

Good luck,

Darren

[3gungeezer]

Tighten your magazine catch another turn that should help raise the magazine in the well. It might fix your problem.

[mpeltier]

Tighten your magazine catch another turn that should help raise the magazine in the well. It might fix your problem.

That has nothing to do with how high your magazine sits in the magwell.

To the OP...in your video you demonstrated the issue using the charge handle. How does it behave if you lock the bolt carrier to the rear, lock the charge handle in place and use the bolt catch/release to strip a round. Also, after this malfunction occurs, what does the cartridge under the one that gets stuck look like? Does it by chance show two longitudenal scrapes up from the base running towards the shoulder? If so you have a little deburring and polishing to do to the bottom lugs of the bolt. Sometimes they get sharper as you go and need a bit of attention. Feel the bottom lugs of the bolt with your finger and see if they feel sharp and pointy. If so break the edge lightly with a small file and polish. Life will be good again.

[Fenrir]

Update:

Ok so I decided to take this rifle to my dad who is retired army, and a friend of mine who is currently in the Army National Guard. I demonstrated the problem to them, and apparently I was incorrectly pulling the charging handle. While I was pulling it COMPLETELY back I was slightly riding the charging handle which was causing the malfunction. So I starting practicing the new technique they showed me where I pull the charging handle all the way back, and release it while continuing my pull back. We also sprayed some solvent on the feed ramps and let it sit for a couple of hours. We then wiped it off, and was able to get some more crap off. I am sorry to those that offered suggestions based on my video but if you watch it again you see me doing it again I do appearing to be riding the charging handle a bit.

This of course doesn't explain why the gun was still doing this while firing. So I am going to try raz-o's suggestion of testing it with different ammo. Since the PMC was firing at a lower FPS the cold would explain why it malfunction at the match but not at the practice range sessions before the match. I also noticed something very odd. I compared the round against Winchester .223 vs PMC .223, and noticed that the Winchester has a VERY slightly shorter tip than the PMC, and the necks were also slightly different. So I am wondering if the tip of the PMC is SLIGHTLY catching which might be causing or helping this malfunction to happen. Does anybody have any input on this idea?

So I am going to re-clean it, and lub it again before my next range session. I am also going to test it with different ammo as well as PMC ammo to see I can cause the malfunction again or if it happens with the PMC but not the Winchester. Either way if I shoot in cold weather again I will not be using PMC ammo, and after I shoot through my supply I plan on going to a different brand thats a higher FPS.

Thanks though for everyone's suggestion, and I am sorry about not realizing I was working the charging handle wrong. Like I said I am new carbine shooter so I am still learning the finer techniques. I actually am trying to take a Basice Carbine class from Larry Vickers before the year is out to hopefully help me learn more.

[Jadeslade]

That gun is not working. You are pulling the charging handle just fine, like any grunt would. That rifle isn't made anymore. Usually a reason for that. You should probably let a local armorer go over the gun and get it running for you. PMC is fine ammo. The ROK uses it, we use it in the ROK, and it's good ammo. If its an original purchase, I would send it to S&W. Bolt, upper receiver, springs could be problem. I hate it it when that happens. It gets better. DVC

[StealthyBlagga]

The advice to tighten the mag catch one turn is worth following - it's a 2 minute job and costs you nothing. Shoot me a PM if you are not clear how to do this.

Try different ammo before getting too stressed. There could be something about the PMC ammo causing it to drag inside the PMAG (I've seen this with other ammo - Wolf specifically). Also consider trying a regular GI magazine just to get another datapoint.

Check the upper receiver and the barrel extension area. Depending on whether the gun has "rifle" feedramps or "M4" feedramps, the bullet tip may be hanging up there and damaging the aluminum of the upper receiver. Also, check you have the right combination of upper receiver and barrel extension, per the photo below:

If the above don't resolve the issue, then call S&W. The whole point of paying a premium price for a factory rifle is that you are paying for factory support in situations like this.

[Fenrir]

Well I haven't had a chance to go to the range yet but looking at my rifle I have the M4 feed ramps based off your pictures. Also they appear to be good and not showing the problem shown in some of the pictures you posted. So we shall see if different ammo makes a difference, or if the cold weather was just affecting my current ammo. I plan on going to the range Friday or Saturday to find this out which will give me about 2 weeks before my next local 3 gun match. If it still doesn't work with PMC or other ammo I plan on taking it by the gunshop I brought it from, and let them take a crack at the problem. Worse case I shall contact S&W, and see what they can do for me since the rifle is less than a year old.

[Fenrir]

After Range Testing Update:

Well I took it to the range today. I decided to do a more through job of checking my magazines as well. So I started out with the PMC doing some zeroing with a 20 round PMAG and fired about 40-60 rounds. Everything worked fine, but then I switched over to my 30 round PMAG (both the non-window, and window versions) and all of them started having the malfunction every 2-4 rounds (or more) out of every 30 rounds (about 120 rounds fired since I wanted to go through all 4 of my magazines). Tried some Winchester .223 and it was doing the same thing (30 rounds of Winchester fired). So I brought a Brownell's AR-15 30 round GI magazine, and it continued the problem with Winchester ammo (fired 30 rounds with the Brownell's magazine). So finally I went into the range shop and purchased some Federal .55gr 5.56 which is a hotter load. I fired a total of 60 rounds and didn't have any problems except for one round at the very end with my second magazine. This is kinda of making me think its an ammo issue if it weren't for that fact that I had it happen with ONE round of Federal out of 60. I took it to my local gunshop where I brought it from, and the owner looked at it. He said it could be the buffer spring allowing the bolt to cycle faster than the rounds can feed from my magazines. Apparently with the 5.56 it pushed the bolt back further than the .223 so it 'stays' back longer allowing my next round to feed which might explain why the Federal worked better except for one round out of 60 (PMC and Winchester .223 were having at least 2-4 or more malfunctions out of 60 rounds). He suggested I change the buffer spring to a heavier buffer spring. I am planning to hit the range again next wednesday, and he offered to install one of his extra personal buffer springs to test it and borrow it for my day at the range. If that seems to fix it I can bring it back into the shop to order one for me before the next match. Now I asked why would the 20 rounder work but not the 30 rounders if it was the buffer spring, and he answer that 20 rounders feed faster than 30 rounders.

Does anyone have any ideas if this will work? I am willing to try since its not costing me any money unless it fixes the issue which means I am ordering a new buffer spring. Is it also possible its just the ammo, and the one Federal round was a fluke? As always thanks for all the advice offered ahead of time.

Edit: He also said if the buffer spring fix doesn't work we can try a new bolt carrier. I did see a blog article by VuurwapenBlog where he mentions a similar issue of why bolt over base malfunctions happen. He also mention it in a paragraph he wrote for one of his youtube videos:

Edited February 26, 2012 by Fenrir

[3gungeezer]

Find someone that KNOWS the AR platform to help you fix this. The guy you talked to who said that the gun is cycling too fast doesn't know what he is talking about. If it happens when you hand cycle then it is not "the bolt moving too fast". Also a heavier buffer spring will make the problem worse unless it is a feed ramp issue and then it might work temporarily but still isn't a fix for the problem. Your AR has a mechanical failure that needs to be repaired by a person who knows how the platform operates and not just change parts hoping that one of the changes will fix it. Ask around and be preparded to ship it someplace.

[dmc_md]

Never heard of a bolt moving too fast issue. I agree with the advice of taking the gun to an AR enthusiast for a once over.

Darren

[mpeltier]

Is it truly a base over bolt malfunction or a failure to feed. When this happens is the bolt or bolt carrier pushing the top round?

Did you look at the things in my first post?

[Fenrir]

Yes I did. I did check the botton of the lugs, and they don't feel sharp to me at all. As for locking the bolt to the rear, and releasing it to chamber a round it will do that. I have yet to have it jam doing that. It only does it when cycling after firing some rounds. The only scrapes that appear are on the bullet tip not on the casing at all. I assumed this is a bolt over base since the round is being caught underneath the bolt. I always assumed failure to feed is simply the round didn't feed up from the magazine at all. I looked up bolt over base, and failure to feed and some of the pictures look like the same thing. This is the best picture I found yet of what is happening with my rifle:

See how the bottom part of the round is sorta of caught underneath the bolt? Also how the tip is resting up against the feed ramp? I also get two scraps on the bullet tip itself. Here are some pictures of took with my camera phone of a round I saved after it caught:

Not the best picture in the world but you can see the outline of the scatch heading towards the bullet tip.

Edited February 26, 2012 by Fenrir

[3gungeezer]

Have you pulled the buffer and spring out of the buffer tube to make sure they are intact? Can you post pictures of both?

[Religious Shooter]

Those markings on the bullet happened to me too. Again, problem was a dirty extractor recess. SOLUTION: Punch out the extractor pin and clean the extractor recess.

[mpeltier]

Yeah that is definately bolt over base. I would check the complete gas system, as this is a short stroking issue. It can be ammo related, but I would be surprised if that were the case here. I also doubt its your spring or buffer. Is the gun completely as it came from s&w or have you changed anything?

[3gungeezer]

...

Edited February 26, 2012 by 3gungeezer

[Charles Bond]

Really looks like a feed ramp problem as I had exactly the same problem several years ago from an off the rack AR I purchased at a pawn shop. Checking the gas system and extractor is also an excellent idea especially if your seeing a different result from different types of ammo.

While I agree with that PMC is good ammo, do not believe even good ammo makers always make good ammo. After firing about 10K rounds of XM193 this past year I ran into some where the necks were not properly crimped.

My advice is get your gun to someone who knows what they are doing with a generous sample of your PMC ammo.