New AR build problems, insight (Update: All problems bad mags)

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I recently sold my old AR build (my first build) on NES to fund my new build. My old rifle ran flawlessly. In 5-600 rounds it NEVER had a malfunction. that was mostly with steel cased tula. Myself and a few friends are putting together a few rifles, and while aesthetically different, they are mechanically identical. Two of the rifles are complete and I took mine out yesterday and had an issue with short stroking on certain steel cased ammunition from herters. Today we narrowed it down to just one of the lots of ammo (I ordered 62gr fmj, 55gr fmj, and 62gr hp, and it would short stroke at least once or twice per 20 rounds with the 55gr).

Fine, I even gave a box to someone I saw at HSC of the 55gr stuff, and it seemed to run fine, although I noticed their bolt didn't lock back on the last round. The ammunition also feels underpowered. The 62gr fmj stuff I have runs fine, and any hotter brass loads of I have tried all cycle the rifle perfectly. I should note we are using normal buffers and a mid-length gas system on 14.5 inch barrels (with pinned flash hiders), identical setup to my old AR.

My main concern is at least once per mag a round will hang up during the feeding process. It seems the case neck is getting grabbed by the barrel extension at the top of the feed ramps. The hollow points failed to feed at least 50% of the time, sometimes apprently catching on the front of the magazine, which makes me wonder if the bolt isn't catching the back of the round low enough and possibly causing it to nose dive.

We are using AXTS A-Dac-F ambi bolt catch lowers (superbly machined), Spikes uppers and FA BCGs, and Daniel Defense 14.5 midlength lightweight CHF barrels. I am hoping it is a feed ramp problem but fear it may be a lower problem.

Any insight?

Mike
 
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A 14.5 mid length can be a soft shooting rifle as they are typically not over-gassed. I would try full power XM193 or XM855 (full power 5.56) and see how it goes.
 
One question... you say "normal" buffers. A2 stock with a rifle buffer or adjustable with a carbine buffer? If it's an adjustable there are options besides the carbine buffer that folks have had good luck with in 14.5 mids.
 
Im running a standard weight carbine buffer and spring in a mil-spec receiver extension. name brand loads have no problem cycling the rifle. This isn't my first mid-length. I don't feel this rifle is under-gassed, just that I do have some junk ammo.

The issue I am having now is feeding issues on full power ammo, it is getting caught up in the feed ramps, this occurs on both rifles. I really should shoot more rounds through them to confirm the problem, but considering my last rifle had NO problems this is a little concerning. I may polish the feed ramps and see if that helps.

Mike
 
Gotcha... Pardon my poor reading comprehension on your first post [grin]

I take it you've tried a few different mags and the problem exists across them?

If the mags are good I'd inspect the feed ramps carefully and polish them up a little as needed.
 
I looked at the feed ramps before I barreled the upper and they weren't obviously burred. I'll take a look at them tomorrow. The barrels are Daniel Defense CHF, they weren't cheap, and I got them a couple weeks apart so I am not sure if they are from the same batch. I'm definitely not enjoying any of these problems.

The other thing is the bolts seemed snug in the carrier, more-so than I am used to seeing. I'm confident it's all put together right, this isn't my first rodeo. I'm starting to second guess whether or not the feeding failures we had with the good ammo was with the same mag or not.

Mike
 
Probably back to the range with a few known good mags.

If you have another BCG I'd bring that too. Maybe a commercial semi one just for comparison.
 
The thing is it is two mechanically identical rifles having identical problems. The only difference are the sights and the rail system. Same uppers, lowers, LPKs, barrels, BCGs, etc. I'm really sitting here thinking that the last couple failures experienced with the good ammo may have been with the same mag. It was a nicer mag but I suppose anything is possible. Ugh just sitting here driving myself crazy. Can't wait to break them open tomorrow and clean them up, see if I notice anything out of the ordinary.

Mike
 
They're obviously not mechanically identical if one ran ammo X and the other doesn't.

That said, this isn't something that I would lose sleep over, unless you demand that your rifle operate with what is basically turds formed into rifle ammunition. Wolf 55 gr is basically the weakest stuff out there save for maybe UMC or Ultramax.

-Mike
 
They're obviously not mechanically identical if one ran ammo X and the other doesn't.

That said, this isn't something that I would lose sleep over, unless you demand that your rifle operate with what is basically turds formed into rifle ammunition. Wolf 55 gr is basically the weakest stuff out there save for maybe UMC or Ultramax.

-Mike
Definitely soft... Have you checked the gas rings to ensure they are not lined up? Not the same barrel(DD 14.5 carbine length here) but I found I could not run the standard semi auto carbine buffer that you get out of the box with an adjustable stock. H2 has now run flawlessly for more than 10k rounds after initially seeing similar short stroking as you describe.

I ran some reeeealysoft ammo yesterday 2100 fps (new load, hornady steered me wrong). Still ran flalwlessly with the H2.
 
They are mechanically identical... the two rifles we built are having the EXACT SAME issues with ammo and feeding. I am now questioning more and more whether or not we used the same mag at the end of the day yesterday when we experienced the feeding issues with the good ammo (the only time the brass cased had FTFs, in both rifles).

Wouldn't I want a lighter buffer to run junk ammo? I have an extra standard weight carbine buffer at home, and I am considering lightening it so I can use it with the junk herters ammo. Or by making it heavier will it increase the dwell time and allow more pressure to develop before it unlocks, thus doing so more reliably?

Mike
 
They are mechanically identical... the two rifles we built are having the EXACT SAME issues with ammo and feeding. I am now questioning more and more whether or not we used the same mag at the end of the day yesterday when we experienced the feeding issues with the good ammo (the only time the brass cased had FTFs, in both rifles).

Wouldn't I want a lighter buffer to run junk ammo? I have an extra standard weight carbine buffer at home, and I am considering lightening it so I can use it with the junk herters ammo. Or by making it heavier will it increase the dwell time and allow more pressure to develop before it unlocks, thus doing so more reliably?

Mike
You are dealing with a spring, inertia, and dynamic gas pressure so the solution is not always immediately intuitive. My debug consisted less of trying to compute out the math of the situation and more of realizing that is pretty common from my understanding that an M4xx now uses H2 buffer for the same setup I have. So, I purchased one and found that all of my problems went away.
 
They are mechanically identical... the two rifles we built are having the EXACT SAME issues with ammo and feeding. I am now questioning more and more whether or not we used the same mag at the end of the day yesterday when we experienced the feeding issues with the good ammo (the only time the brass cased had FTFs, in both rifles).

Wouldn't I want a lighter buffer to run junk ammo? I have an extra standard weight carbine buffer at home, and I am considering lightening it so I can use it with the junk herters ammo. Or by making it heavier will it increase the dwell time and allow more pressure to develop before it unlocks, thus doing so more reliably?

Mike

I'll take a stab at this but I'm no AR expert.

You've already got a FA BCG, so you've already added weight to the reciprocating part of the action. I wouldn't think a heavy buffer is necessary on top of that. Are you sure the spring is appropriate to the BCG and buffer you have?

I also wonder if the gas port in the barrel is sized right, though Daniel Defense seems very unlikely to screw that up.

I could be wrong but it seems that a DI AR can only be made to handle a certain range of pressures and maybe the stuff you used was just on the bottom line of that range. Another way of saying that is that there's a reason many piston guns let you adjust the gas port size.

Regardless, if you figure this out, please post back as it's a very interesting topic.
 
They are mechanically identical... the two rifles we built are having the EXACT SAME issues with ammo and feeding.

Sorry, massive reading comprehension failure on my part. I thought you were comparing the rifle you built to the one you sold off earlier.

I am now questioning more and more whether or not we used the same mag at the end of the day yesterday when we experienced the feeding issues with the good ammo (the only time the brass cased had FTFs, in both rifles).

I've seen bad mags jam up even well made rifles, so that's something worth tracking.

I think what cekim is talking about (the buffer) is worth looking into. It seems like there are 4 or 5 different buffers floating around out there.

-Mike
 
I could be wrong but it seems that a DI AR can only be made to handle a certain range of pressures and maybe the stuff you used was just on the bottom line of that range.
From observation, that range is more than wide enough in correctly configured gun to handle wolf ammo.

I have run ammo well outside factory specs (on the low side) without issue on multiple guns - provided they are correctly configured.
 
I feel confident I can attribute the short stroking to bad ammo, because it only happens with specific ammo. It functions flawlessly with hotter brass loads like prvi, remington, etc. I am running a standard (non heavy) weight buffer with the mid-length gas system. I would be interested in trying the uppers on different lowers, especially with regard to the feeding issues, which I feel are more the problem. These were built to be guns which could be relied upon, and there is clearly at least one component at fault with both of them, rather one component central to both rifles.

As I mentioned my old AR ran mainly on cheaper steel cased stuff, and never had a single failure.

Mike
 
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I've seen bad mags jam up even well made rifles, so that's something worth tracking.
yep.
drgrant said:
I think what cekim is talking about (the buffer) is worth looking into. It seems like there are 4 or 5 different buffers floating around out there.
I've tried:
1. Standard carbine buffer
2. hydraulic buffer (better than standard, but still short stroked - magazine catch on last round would often catch BCG instead of bolt face, particularly when dirty)
3. H2

I have an H3 as well, but the H2 worked so I stopped experimenting. The mid-length gas system might require still more iteration to find the right one...
 
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First make sure you are using a mag that you know is 100%. Number them so you can keep track

Make sure the gas block is centered over the gas port. If you are using a D-ring, the block may need to be just off the shoulder of the barrel.

Try a lighter buffer, it may increase bcg speed and travel distance
 
Wouldn't I want a lighter buffer to help with short stroking?

Mike
Frankly, what you will find if you start looking into "which buffer is correct" on the 'net is that the right answer is that "it depends" on everything else on the gun being in spec and the ammo that you use. The standard buffer _should_ be correct with a midlength on the 14.5 from my understanding, but you will find people all over the map (as I did) on which worked for them. That's why I ordered the H2 and H3 to experiment. Both because the H2 was the standard issue for the M4 (14.5 carbine gas), but also because the heavier/damped hydraulic worked "better", I figured I would go heavier first.

One thing that I did not think has been asked is whether you have confirmed the alignment of the gas block? (EDIT: looks like moto got to it while I was typing)

If your gas block can be easily removed, then you will see carbon collecting around the gas hole that outlines the gas block location. It would be good to confirm that you don't have any blockage there. If you have confirmed there is no binding and no blockage, I'd get a range of buffers and a fist full of ammo and run it until it is happy... (high speed camera doesn't hurt, but I didn't have one).

p.s. just to be clear, before you start down the road of buffer madness, address all the other issues first...
 
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The gas blocks are taper pinned from the factory. The short stroking seemed to 100% of the time be with that one lot of ammo. We had ZERO failures with all other ammo including the 62gr Herters russian imported stuff.

We did however have failures to fully feed, it wasn't that it wasn't cycling all the way, it would pick up the round and the round would catch going in. I'm going to try to get a few pictures up but my cell phone camera is garbage.

I also took some measurements of the lower while cleaning and wanted to run it by you guys. The rear feed lips are sitting 10.5mm above the back of the lower receiver, and the front divot (cut down portion) of the mag sits between 3.5 and 4.5mm above the line if the magazine, depending on whether or not you are pulling back on or pushing the mag forward. This is the same accross multiple metal mags

The cuts for the M4 feed ramps (which blend nicely with what is cut into the barrel extension) start between 2.5-3mm above the bottom of the upper. The uppers and lowers fit together snugly (nicest fit I have ever seen).

I may try some better magazines that I've used much more and see what results I have.

Hopefully I'll get some pictures up of possible problem areas for you guys to go over soon.

Mike
 
The gas blocks are taper pinned from the factory. The short stroking seemed to 100% of the time be with that one lot of ammo. We had ZERO failures with all other ammo including the 62gr Herters russian imported stuff.

We did however have failures to fully feed, it wasn't that it wasn't cycling all the way, it would pick up the round and the round would catch going in. I'm going to try to get a few pictures up but my cell phone camera is garbage.

I also took some measurements of the lower while cleaning and wanted to run it by you guys. The rear feed lips are sitting 10.5mm above the back of the lower receiver, and the front divot (cut down portion) of the mag sits between 3.5 and 4.5mm above the line if the magazine, depending on whether or not you are pulling back on or pushing the mag forward. This is the same accross multiple metal mags

The cuts for the M4 feed ramps (which blend nicely with what is cut into the barrel extension) start between 2.5-3mm above the bottom of the upper. The uppers and lowers fit together snugly (nicest fit I have ever seen).

I may try some better magazines that I've used much more and see what results I have.

Hopefully I'll get some pictures up of possible problem areas for you guys to go over soon.

Mike

What type of magazines were these?

I have a Robinson XCR and I've discovered, thanks to it, that a lot of USGI 20 rounders point the round slightly down. By contrast, all the USGI thirty rounders point the tip slightly up.

With these bad 20 rounders, the rifle will strip a round and jam it below the feed ramp, thereby pushing the bullet back into the case to the point where the powder falls out.

So, in short, I'd pick some known good magazines to test with.
 
30 rounders with green followers.

I have some mags I KNOW work so I'll bring those with me next time. I may polish the feed ramps just because, hopefully I don't have to remove the barrel to do so.

Mike
 
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I'd try another trip with numbered mags and see how it goes before touching the ramps.

Getting a semi carrier and an H2 buffer in the bag to play around with can't hurt.

Looks like a DD Lite handguard... Very nice... I have one myself on a middy [grin]
 
Yes I love the setup, the balance on the rifle is great, even with the light and vfg.

On my next trip I will bring numbered mags, and I also want to try messing with some of the weights in one of my extra buffers, so I'll bring a punch and I'll cut some foam blocks to replace the steel weights in the buffer. I'd like to try running it with the weak ammo on another lower, and then just with another buffer and spring. I don't feel like it's under-gassed, before finally assembly I blew air through the barrel and air freely flowed from the gas block.

If anyone makes regular trips to a range in the Boston area or HSC with various weight buffers, let me know via pm.

Mike
 
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I've only had good experience with tula though. Good to know though, lol. I figure worst case scenario, if it is just the ammo, I get to use it to practice my malfunction drills.

Mike
 
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