First, let me say I read through the FAQs and did a search on the boards. I did not find a topic that specifically addresses my problem.

I just picked up a pre-ban lower this week and was putting it through it's paces at the range yesterday. I had quite a few FTF's (w/different mags) and I think the culprit is the mag catch. Here's why I think that.

Using 30 rnd USGI mags (with new spring and new followers), round will sometimes get caught under the bolt and not get fed. The rounds themselves end up getting big dents in them from the bolt's claws.

So then I tried a 10 rnd DPMS plastic waffle mag. After I fire one round with it, it won't feed another round. I noticed that after the first round, the mag slips down in the mag well just enough so it's not fully engaging the mag catch but not enough so it falls out. I know I'm inserting firmly and it's engaging the mag catch initially. This same mag works fine in my other "new" (I'm the original owner and my "new" lower only has around 500 rnds through it) lower. I looked at the mag catch inside the mag well and it look a little smooth. I checked my other "new" lower and it looks and feels much sharper.

BTW, all of the mags I used work fine in my "new" lower so I've ruled out the mags.

Do the symptoms and diagnosis sound correct?

TIA

Is mag catch release button screwed all the way onto the shaft of the mag catch? The end of the shaft should be flush with the exterior face of the button. Are you certain that the upper is not shortstroking?

The mag catch release button is screwed in all the way. The screw is flush with the button and the mag catch level itself. The bearing surfaces of the mag catch are a bit "smooth".

As far as short-stroking, I've read up on that some more and that is a possibility. I'll check the carrier key and make sure it's on tight. The upper and bolt I'm using is relatively new (< 700 rnds) and it's worked fine with my other lower. The gas rings look find as well.

I know there are varying opinions on greasing the buffer spring but I believe the buffer spring in the pre-ban lower (collapsible stock) is not greased. That could cause a bit of binding, at least when the buffer parts are brand new, correct?

I'll probably get all new mag catch/release parts since they're cheap (< $10 for all 3 parts) and I've got a KAC M-4 ambi swivel assembly coming in so I'll have to remove the buffer assembly anyway. At that point, I'll check it out to make sure nothing's causing any unnecessary friction and probably slap some Tetra grease on it.

One last thing. I'm using plain Winchester 55gr White Box Value pack ammo. I've been cleaning the rifle after every use and it's sufficiently lubed with FP-10 (except for the buffer tube as I stated earlier). Also, this problem is consistent. It's happening once in a while, around 10% of the time which is a heck of lot [:(]

BTW, thank you for the response. I really appreciate it.

Never mind, I'm a bonehead. I just downloaded the Armorer's Manual and, if the buffer spring isn't greased up, I'm going to do as I stated earlier and grease it up.

Matt_B,

Does that mean you found the source of the problem?

Greasing the buffer is no big deal as long as you don't get extreme about it. Don't fill the coils with grease, or anything close to that amount.

Sorry 'bout that Tweak. I'll try and be more vague next time [:)]

When I checked out the M-16A2/M-4 Armorer's Manual (TM9-1005-319-23), I read through the section on short stroking so now I've got a solid list of everything I need to check out. I don't know exactly what's causing the problem but I can figure it out now I've identified the actual problem (short stroking).

I will be sure not to grease up the buffer too much. A little bit of that Tetra grease goes a long way.

Thanks again for the help.

Matt_B,

There is a simple check for shortstroking, Load one round in a mag, lock and load and fire. The bolt should lock back with its face behind the that of the bolt catch.

If the bolt does not lock back then repeat the above WHILE pushing on the bottom of the bolt stop. The bolt should lock back. If it doesn't you have a short stroke. If it does lock back then you have a bolt catch and/or mag problem.

Repeat several times to be sure.

Well, the bolt was locking back on an empty mag every time. I was getting FTF (with the bolt over a round) "occasionally".

Then the piece isn't short stroking.

So it's not possible it could be short-stroking "sometimes" because of the buffer spring not being greased?

If so, that narrows it down to either a problem with mags (unlikely since the mags in question worked fine with my other lower) or a mag catch problem (the DPMS mag will get loose after firing one round).

I order all new mag catch parts since it was cheap ($10 for everything) so I'm going to replace those anyway. No harm in that.

I really appreciate the help Tweak. I can only get to the range every two weeks so that's the only time I can really test out any fixes.

Well, since I was installing a KAC M-4 ambi sling swivel, I removed the buffer and spring. No grease on them so I put of bit of Tetra grease on both and a little bit inside the buffer tube. The action's smooth as silk. I also installed all new mag catch parts.

I went to the range today and when through 360 rounds with only 1 problem. I had 1 FTF because the mag wasn't seated all the way.

When I compared the the old mag catch to the new one, the new one was a bit wider. The old one wiggles a bit in the receiver's mag catch recess.  Seems like that was the culprit. If the mag wasn't seating properly, the rounds weren't getting stripped from the mag smoothly.

Thanks again for the help Tweak.