Help with feeding issue with URG on new lower

AR/M4 AR General Discussion
1

I just finished a recent build and took it out today to fire it and make sure everything was fine and encountered the following issue:

Approximately every 2-3 rounds I would get a failure to feed wherein the BCG would begin to travel forward but only about three quarters of an inch. The bolt lug would begin to move the next round forward and out of the mag, but the nose of the round would get stuck just as it would start up the feed ramp. The round was then essentially stuck between the bolt face and the feed ramps. This happened approximately 10-12 times out of only 40 rounds (unfortunately I didn’t have time to work with it further). Some of the time pulling the charging handle to the rear and releasing it would chamber the round, other times it would not and I would have to remove the mag, re-insert the round, re-insert the mag, and charge the weapon again. Pushing the forward assist never did anything to help feed the round. Also, there was no issue with extraction and ejection; both functions worked perfectly. This was a feeding issue only.

Here’s the specs on the gun, mags, and ammo:

DDM4V5 URG (everything is the same as it came out of the box save for the addition of a BCM CH) on an Anderson Mfg. stripped lower with PSA LPK, RE, buffer spring, and H buffer.

I used two different mags. One unknown manufacturer standard USGI mag, one D&H 30rnd teflon-coated mag. Both had Magpul followers.

I shot about half Lake City XM193, and half Speer 64r GDSP.

I’ve never had any issues with any of the above mags or ammo in other carbines, and have shot the same DD URG on a complete BCM lower (which it used to be on before I decided to use it for another build) and it functioned flawlessly.

So unless I’m missing something, I’m assuming theres an issue with the lower. I’ve detected no play between the URG and lower, they’re honestly a very tight fight, every bit as much, if not more so than my complete BCM carbine. I’m by no means an SME, but I have a decent understanding of the operation of the M4 platform. I’m leaning towards it being a buffer spring and/or buffer problem. I’m planning on taking the buffer and spring out of one of my BCM lowers and try it in this gun to see if that’s the problem, but again, didn’t have time to today.

If anyone more knowledgable than I am can chime in and point me in the right direction or tell me if I’m completely off I would greatly appreciate it.

2

It sounds like the magazine is sitting too low in relation to the upper. This is probably being caused by the lower, that is easy to find out, just put your upper on another lower. When the mag is slightly too low in relation to the upper it causes a more severe feeding angle (which is already bad in GI style mags due to the feedlip geometry). This increase in feeding angle causes biding.

This is all just a guess based on what you are seeing, but it would fit your symptoms.

3

I’m not at the house at the moment, but I’ll check the height of the mag in the lower receiver as soon as I get home. If this is the issue, how, if at all, can I fix this issue?

Thanks for the quick response!

4

If it’s the lower, you probably can’t. But just because this lower has this problem with this particular upper doesn’t mean it won’t work with another upper. Sometimes you get a magical storm of tolerance stacks that equal a non functioning gun. This is really why I usually prefer to by uppers and lowers from the same company. It’s a lot less likely to happen then.

The first step here is to nail down what is causing the problem.

5

Ok, so I just got done looking in to the problem further. From what I can tell (and without being able to actually fire the weapon at the moment), putting the DD URG on two different BCM lowers, in addition to back on the Anderson, it looks like the BCM lowers cause the mag to sit slightly higher in the mag well, but by less than 1mm. Is this enough to cause the above-mentioned problem? Clearly the Anderson is being compared to something we all know is going to be in spec, and I know the AR platform has tight tolerances, but damn, less than 1mm? I’m hoping I’m not learning a hard lesson out of trying to build a beater lower…

6

I once had a receiver extension that was noticeably shorter than a stock colt tube (~.25")… I would measure yours to be sure. Sometimes when I ride the charging handle even a tiny amount, the round gets jammed up as you describe.
Did you smack the forward assist or just push on it? I usually have to smack it to get it to work.
Maybe try swapping buffer springs or maybe try smoothing barrel extension feed ramps?

There was another thread on here mentioning BCM lowers having a tight fit when inserting loaded mags (bolt catch to carrier space) So your other lower may not be as out of spec as you think.

As you have seen, the rounds get angled a good amount, so hopefully that 1mm difference isn’t the problem.

7

1mm is = to .039" and I’d say this is a differance that most likely is your problem… .005" to .010" I’d say no way…
Maybe look into trying acouple different brands of mags. as well.?

Please whatever it is you find, report back letting us all know what was your FIX…?

Peace Jeff

8

I agree with everyone above. 1mm is a lot with regards to the position of the mag. It could have something to do with the receiver extension, but I would bet more on that lower given the info you posted. I assume it cycled fine with the BCM lower?

9

And is it possible to change the title of the thread? At this point it seems there is no evidence the DD URG is out of spec.

10

I’m not riding the charging handle; the issue doesn’t happen when loading/charging the weapon. It’s only once I’ve started firing that I start having feeding problems. I hit the forward assist several times with no results. :confused:

However, after you posted this I did go look at the lower in question and compared it to one of my BCM lowers and it looks like I may have possibly over-inserted the RE on the new lower. It appears that it may be 1 full turn farther in than the RE on the BCM lower I was comparing it to. If so, this would shorten the length of the RE in terms of compressing the buffer spring would it not? I work night shift and was on the road last night and am about to head back out soon, so I haven’t got to move the RE yet, but I’ll see if that changes anything. If that doesn’t work I may try changing out buffer springs and buffers to rule that out. Just hoping it’s not the lower itself, but we’ll see.

Oh, and ETSGroup, it functioned flawlessly on the BCM lower.

11

Sorry about that; wasn’t thinking about that when I typed the title. You’re absolutely right, the DD URG is certainly not out of spec. It’s been taken care of.

12

Ok, cool…I mean if it is the problem, and DD was made aware, then that is fine of course.

13

Did not mean to insinuate that you ride the charging handle (my problem I do it when I dry fire) but that there maybe not enough space for the carrier to gain momentum to chamber the round. (if you have a short buffer tube) perhaps check the space between bolt and cartridge when pulled all the way back between your two setups.

Since you mentioned the forward assist did nothing, perhaps see if there are bullet marks or a lip where the feed ramps blend into the receiver?

14

I’ve been out of town for the weekend and just got home. I’ve pulled the gun back out and am looking at it now. I double checked the RE tube, and the length was fine and I didn’t install it one too many turns, as I thought may have happened. On the URG, it does appear that there is a slight lip between the barrel feed ramps and feed ramp extensions in the lower. There were some very minor bits of what appeared to be copper jacketing right where the slight lip was. Do you guys think it’s possible that the combination of the mag sitting >1mm lower in the mag well and this lip that could be causing the problem together? This slight lip did NOT cause any problems when the URG was on a BCM lower.

I’m about to go shoot it some more using PMAGs this time and see what happens.

15

I do think the lip could be the hangup, i usually see bullet tip marks middle of feedramp, if its hitting low enough to catch that lip, then perhaps your lower is out of spec, but I bet smoothing that lip might fix it. Im thinking this is why m4 feedramps were introduced? but you still need it smooth where it transitions. On your BCM lower, the bullet tips should impact above the lip. Might be useful to ride the charging handle and watch the rounds chamber and see where they impact.

16

Ok, so I just got in from shooting the gun again. I lubed the BCG up with EWL and put 60 rounds of 55gr FMJ through it, out of multiple Gen 2 PMAGs and had 0 issues. The gun went bang and cycled like it’s supposed to every time. And just to compare again, I shot a few rounds out of the same GI mags and had the same results as last time. The D&H mag did somewhat better than the standard GI mag, but still had the same issues, just not as frequently. I did notice that the feeding issue only occurred on the left round (if you’re looking at the gun/mag from behind the round on the left side of the stack, moving up the left feed ramp).

Obviously I’m not 100% confident with the gun at the moment, but at least I feel comfortable that I could put a PMAG in the gun if I needed it and it would work.

That said, I have a couple more questions:

  1. I’m hearing about polishing the feed ramps. How do I go about doing this?

  2. (opinion question) If you guys were in this position (and after further shooting the gun more with PMAGs and the problem no longer occurred with PMAGs) would you keep it as is, or scrap the lower and re-do it? (And for some background here, this is not my only carbine. I’ve got another BCM gun, and am in the middle of another BCM build currently.)

Part of me isn’t 100% comfortable with the idea of having a gun that won’t use any mag I stick in it, but then again, I have plenty of PMAGs and can always (at the present time anyway) stock up on more while they’re cheap.

Thanks again for all the help!

17

I know that you have found a work around by using Pmags, but there is obvisously still something wrong with that lower. If you only plan to use it for the range then it’s not a big deal. But if you ever plan to use that gun in a defense situation, there is no way I would trust it with that lower. It’s just not worth it. Lowers aren’t that expensive. Why have to work around and worry about the gun working when a $100 part will fix, (probably less if you shop around).

I know it sucks to have a part that you paid good money for and not be able to use it, but it doesn’t suck as bad as needing that gun, pulling the trigger and hearing a click sound.

Just my $.02.

18

Anderson lowers are cheap. I have seen a few other issues with them being slightly out of spec as well. I would trash it and get another or find another lower to verify again.

19

  1. Has the upper been tried on any other lowers?

  2. Has a lock back check been performed?

20

Yes, apparently my attempt to build a beater lower was an epic fail. I’ve already been looking into Aero Precision and some other companies for a quality stripped lower to replace this one.