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Old May 8, 2021, 12:52 PM   #1
hornetguy
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AR 15 issue

Has anyone ever seen an issue with the second to last round in the magazine feeding incorrectly, jamming the action?

Mine has been doing that since I assembled it... and it does not matter which magazine I use. I've tried 20 round Magpul's, a 30 round plastic mag, and a 30 round aluminum mag.

I've noticed that when the bolt is moving rearward, it will push down that round at the very back, about 1/8", which leaves it tilted upward when the bolt comes back forward.
I don't know if that is causing the jam, or if it's another issue...

Anybody else seen this kind of thing?
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Old May 9, 2021, 04:53 AM   #2
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How and where is the round jamming? Is it getting stuck in the magazine? Is it coming out at an odd angle and hitting the receiver hi or low? Or is it going into the feed ramps and hanging up?

A pic of the jam would be very helpful..
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Old May 9, 2021, 10:11 AM   #3
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Sounds like a magazine spring and anti-tilt follower issue to me if it only happens at the end of the stack. The cartridges may look like they're being pushed down--but it could also be they are wedged in such a way they won't release easily and pivot instead. If the magazine rattles around easily in the receiver's mag well, you might want to try putting some tape on the side of the magazine where it fits into the well--just to see if perhaps the closer mag walls take away that last rounds issue. Otherwise I'd suggest trying a 5 and 10 round mag to see if the same problem happens.
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Old May 9, 2021, 10:21 AM   #4
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If you don’t have oil seeping out of the trigger pins it may be too dry. Just oil the heck out of it and see what happens. You may think it’s well lubed but you’d be surprised sometimes.
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Old May 9, 2021, 10:28 AM   #5
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I never thought to take a picture of it when it happens.
I have smoothed up the feed ramps, removing any sharp edges, and have not fired it since.
I read in an article yesterday that if the barrel ramp extension isn't properly shaped with regard to the receiver ramps it can cause this issue.
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Old May 9, 2021, 11:44 AM   #6
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What cartridge are you firing? If the bullet nose was striking the edge of the feed ramp you would probably see a D formation in the nose of the bullet.
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Old May 9, 2021, 04:02 PM   #7
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I was using mostly factory fmj's. I did try a few softpoints, though. I'll have to see if I kept any of them. Most of them were bent pretty badly.
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Old May 9, 2021, 05:15 PM   #8
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I was using mostly factory fmj's. I did try a few softpoints, though. I'll have to see if I kept any of them. Most of them were bent pretty badly.
What I meant was--5.56--6.5 Grendel, 6.8 special--etc. Was the nose bent--or the case?
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Old May 9, 2021, 05:47 PM   #9
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Not lube, not feedramp. As the stack pressure drops, bolt velocity climbs. You are seeing the affect of a very overgassed system. Not uncommon for the last few rounds to have issues especially since you already tried different magazines with the same result.
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Old May 9, 2021, 06:48 PM   #10
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sorry.... it's a 5.56

The case was bent where the bolt slammed into it when it wedged it in the chamber. It was tough getting the bolt pulled back.

What do you do with an overgassed system? I've never heard that term before.
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Old May 9, 2021, 07:28 PM   #11
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Originally Posted by hornetguy View Post
sorry.... it's a 5.56

The case was bent where the bolt slammed into it when it wedged it in the chamber. It was tough getting the bolt pulled back.

What do you do with an overgassed system? I've never heard that term before.
It is common. The vast majority of production Ar15s are overgassed 20 to 30%. That is so folks can run the steel cased Russian ammo and the manufacturers don't get calls.

You put on an adjustable gas block, load lower powered ammo, or buy a barrel with an optimized gas port diameter for length of the barrel and gas tube length. You don't ever take all the excess gas out, but taking out 25% if you are 30% overgassed will improve shootability and result in a more durable system long term.
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Old May 9, 2021, 08:31 PM   #12
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That's good information. I really just want to get this one to be reliable with "normal" 55-60gr ammo. Judging by what you've told me, I likely should look at an adjustable gas block. I suppose I could buy another barrel, but how would I know if I'm buying one with the correct size gas port? Adjustable would be easier, it seems.
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Old May 9, 2021, 09:27 PM   #13
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Correct me if I am wrong. But wont a heavier buffer help with an over gasses system? I'm guessing it has carbine buffer. I would try a heavy or "H" buffer if it does not already have one. It is not a true fix, but its a lot cheaper than an adjustable gas block and might get the gun running right. I generally prefer an H buffer anyway. I was always told, run the heaviest buffer the gun will lock open on.

Also. what length gas system do you have? I'm guessing carbine? What brand barrel?

Midway has a basic H1 (heavy) buffer avaialable for $20 right now
https://www.midwayusa.com/product/1022715133?pid=609287
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Old May 9, 2021, 11:23 PM   #14
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Originally Posted by Shadow9mm View Post
Correct me if I am wrong. But wont a heavier buffer help with an over gasses system? I'm guessing it has carbine buffer. I would try a heavy or "H" buffer if it does not already have one. It is not a true fix, but its a lot cheaper than an adjustable gas block and might get the gun running right. I generally prefer an H buffer anyway. I was always told, run the heaviest buffer the gun will lock open on.

Also. what length gas system do you have? I'm guessing carbine? What brand barrel?

Midway has a basic H1 (heavy) buffer avaialable for $20 right now
https://www.midwayusa.com/product/1022715133?pid=609287
Yes, will mitigate, not fix.

A Superlative Arms adjustable is about $100.
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Old May 10, 2021, 03:12 AM   #15
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Quote:
The case was bent where the bolt slammed into it when it wedged it in the chamber. It was tough getting the bolt pulled back.
OK--what it sounds like is a version of double-stacking, perhaps the previous case isn't being cleared fast enough or your cartridge is "popping up" in front of the carrier and then slammed into the barrel extension sideways not being properly aligned to the bolt breech face. If I understand correctly--your rifle otherwise cycles, fires, ejects normally except when it gets down to just the second to last cartridge in an unmodified 30 round only magazine?
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Old May 10, 2021, 06:52 PM   #16
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Yes, that is the case... unmodified mags, no problems until the final 2 rounds.

except, it was two different 30 rounders, and 3 different magpul 20 rounders. All had the same issue
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Old May 11, 2021, 12:32 AM   #17
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Yes, that is the case... unmodified mags, no problems until the final 2 rounds.

except, it was two different 30 rounders, and 3 different magpul 20 rounders. All had the same issue
OK, you still haven't told me whether this was a complete rifle you bought or a home-build, the reason is if you bought a complete rifle--I would simply send it to the manufacturer since doing the home modifications might cause warranty issues.

I'm just guessing since I don't have your rifle on hand--but it sounds to me like your problem is not associated with the rifle properly firing and ejecting since it works fine most of the time.

You mentioned a ledge between the lower receiver and the feed ramps of the barrel's extension--it's true that that can cause problems by catching the nose of the bullet upon feed--but generally you would be seeing damage to the bullet's nose if that were happening on a consistent basis. You can fairly easily eliminate that with a Dremel by carefully "blending" the barrel's ramps into the receiver--google for it and you should be able to find videos of what that looks like. You can "control cycle" a cartridge slowly (be sure to do this with safety on and the muzzle pointed at something you don't mind destroying if using a live cartridge) while observing where the cartridge goes and what it hits on the way into the chamber.

My money is on you have a magazine position issue which is causing a tilt in your cartridge due to low pressure in the magazine's spring/follower when most of the cartridges are expended. Again, you should be able to see if this is the issue by doing a slow cycle with your hand on the charging handle controlling the feed while observing it. I would probably pull the bolt and magazine catches and their associated parts from the lower and then re-install them and make sure they are installed correctly under proper tension and with normal clearance/functioning in the lower. Home builders often overlook the quality of a LPK--but investing in a good bolt catch is pretty important IMO. Magazines have a slot in their sides where the catch notches into them--sometimes a loose/improper fit can cause the magazine to shift up and down in the mag well. I'm guessing you get those LPK parts and magazine fit properly tuned to your receiver your feed issues will go away.

Hmmm...upon further reflection, a quick and easy solution might be to shop around and see if you can find a stiffer magazine spring and anti-tilt follower--in fact if you are really adventurous I might be tempted just out of curiosity to take one of your 30 round magazine springs and see if I can get it into the 20 round magazine just for the heck of it (though I've never done it and can't vouch for how well it would work, you might have to sacrifice the spring with a bit of trimming but they are generally pretty cheap) and test fire it and see if the second to last round curse goes away--if it does I'm guessing that would almost certainly confirm it's an issue with magazine seating to the receiver.
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Last edited by stagpanther; May 11, 2021 at 07:59 AM.
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Old May 11, 2021, 09:20 AM   #18
hornetguy
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I guess I thought I had mentioned this is a build, from psa. I bought the lower at a gun show as a stripped lower. I bought the upper fully assembled, and it came with parts to complete the lower.
I do not know what an lpk is, or what it stands for.
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Old May 11, 2021, 09:46 AM   #19
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I had one mag that wouldn't hold the last round, it would eject it out with the 2nd to last case. But It was just the mag so I don't use that mag much.
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Old May 11, 2021, 10:07 AM   #20
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I do not know what an lpk is, or what it stands for.
Lower parts kit.
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Old May 11, 2021, 10:32 AM   #21
hornetguy
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Ahh..got it. I'm pretty sure PSA does not use top of the line parts in their kits.
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Old May 11, 2021, 10:54 AM   #22
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The easiest remedy is a heavier buffer. H2 or H3. Run the heaviest you can so it still functions 100%.
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Old May 11, 2021, 11:58 AM   #23
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I'd never head that most Ar's are over gassed, good information, would that explain why my guns get incredibly dirty when running a suppressor compared to a suppressed bolt gun in 223 using the same ammunition?
I believe I have 3 different lengths of buffers in my pile of extra parts, one rifle has a standard AR stock the other a MFT adjustable length stock, both are 16" barrels, what buffers should they have for 60gr v-max bullets loaded to the max.
BTW they both function perfectly until they are to dirty to which is around 50 rds.
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Old May 11, 2021, 12:02 PM   #24
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Ahh..got it. I'm pretty sure PSA does not use top of the line parts in their kits.
No, I was not suggesting that. You have an unknown lower matched to a manufactured upper--so occasionally tolerance issues do come up. I generally work my way up from "easy fix scenarios" in a process of elimination before I venture into "gas system/buffer musical chairs." But that's just me, I'm not a professional armorer
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Old May 11, 2021, 12:45 PM   #25
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Originally Posted by Double K View Post
I'd never head that most Ar's are over gassed, good information, would that explain why my guns get incredibly dirty when running a suppressor compared to a suppressed bolt gun in 223 using the same ammunition?
I believe I have 3 different lengths of buffers in my pile of extra parts, one rifle has a standard AR stock the other a MFT adjustable length stock, both are 16" barrels, what buffers should they have for 60gr v-max bullets loaded to the max.
BTW they both function perfectly until they are to dirty to which is around 50 rds.
If you have an over-gassed AR and then suppress it, yes, you amplify the over-gassing.

But the bolt gun pushes almost all of the gas out the bore, instead of recycling some through the action like a DI system.

Most people don't want to adjust a load...but when you adjust the DI system to a load, you really should pick whatever your lowest powered load is, and tune to that. The load "gas generated" has more to do with charge weight than energy level or bullet weight.

So, given load, you can adjust the system to that load by 1. reducing gas volume into the gas tube with an adjustable gas block or 2. increasing buffer mass/spring weight.

My suppressed ARs have a carbine spring and buffer, usually gutted or low mass, low mass carrier and a standard gas block. But I use premium 16" barrels that are optimized to not need adjustable gas. When the suppressor goes on, the buffer and or spring get changed.

I am working on tuning Jerry Miculek's magnetic buffer for one of my guns. It is adjustable and tunable, so great hope it is another good solution in this area.
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