Am I "over-buffered"????? H2 Buffers...

AR/M4 AR Technical Discussion
1

Guys,
Well, I recently upgraded my OEM Colt 6920 “H” buffer, to one of Grant’s “H2” buffers, which I had read (all over this forum), that the H2 buffer is a good upgrade to do on a 6920.

SO, I have not shot too many shots at all with it, and I decided to go out today and put some rounds through the gun. I had previously shot with the H2 buffer, probably 100 shots or so, with no problems.

SO, today, I started having problems with the bolt holding itself open on the last shot. EVERY time, I was dry firing on the last round, as I didn’t know the mag was empty, and the bolt was not holding open.

SO, most of this afternoon, I had just been thinking that I had too much Slip2000 lube inside the chamber area, so I THOUGHT that the excess lube MIGHT have been the cause of the bolt slipping over the mag, and not holding open… (not sure if excess lube could even possibly be the culprit, but that was my only guess).

Just a note, this 6920 is very new. It’s only got 400 shots through it, and has never had a single issue, malf, FTF, or FTE.

SO, tonight, I was just now cruising around on the net, and I came across this description for a PRI gas tube, which makes mention that a common symptom of being “over buffered” is the bolt not holding open on the last shot…
http://www.pri-mounts.com/mm5/merchant.mvc?Screen=PROD&Store_Code=PRI&Product_Code=05_0050&Category_Code=AR15_M16_ACC

Now THIS has me thinking, and I’m wondering if I am “Over-Buffered”???

Thanks for any help guys…

2

If you’re using anything other 5.56mm NATO pressure ammo you may not have enough power to make it cycle. Put the stock H buffer in it and try it again. Load 1 round per mag (with several mags) and shoot it to test for locking open when empty.

It may have been borderline leaking etc for a while and the slower cyclic rate now doesn’t allow it to run.

3

I’m not quite so sure what you mean. BTW, I am not using 5.56 NATO ammo. Mostly .223, Hornady TAP, or other .223 ammo.

When you say “not enough pressure to cycle”, do you mean that each shot might not be pushing the bolt back AS FAR as before, when the regular H buffer was installed??? I figured as long as each round was chambering, and there are no issues with ejection/loading, that the bolt was going back far enough to get behind the bolt catch??? (Maybe I’m all wrong here)…

Thanks SO MUCH for the info gotm4…

4

Not knowing how many rounds have been through your 6920 etc. this is what I mean.

By not enough pressure, what I’m referring to is gas pressure. A typical carbine gas system is running roughly 26K psi at the gas port in the barrel with 5.56mm NATO pressure ammo, lets say some .223Rem ammo runs 22-24K psi. This pressure probably isn’t accurate but lets say that it takes 9K psi to send the carrier all the way to the rear. Lets say your rifle was running 7-8K (before adding a H2 buffer) which we’ll say is just enough to allow it to eject the empty and still pick up a new round and cycle and still lock the bolt back (not all the way back but just enough with an H buffer). So then you change the dynamic and add a newer heavier buffer which then all the sudden stops locking the bolt back.

To understand the AR15/M16 you have to think of it as a dynamic system. It’s not a static system, one cannot say for certain that an H2 buffer is always best. It’s best for a lot of rifles but may not be for yours.

After checking the carrier key, gas block and for wear/leakage at the inner end of the gas tube (receiver end) I would return to the H buffer and test for function. (If you can find some M855 ammo or XM193 I would try a box through it with the H2 and see if it runs.) I’d load 1 round per mag and check all 20 round to see if it locked back everytime if it does you just need better ammo), if it still doesn’t run even with an H buffer you should rebuild it’s gas system, new gas tube, gas rings and carrier key. If you’re running a stock buffer spring it should be between 10 1/16" and 11 1/4" for a carbine stock and 11 3/4" to 13 1/2" for a rifle stock. If you’re using an ISMI, David Tubb or Wolff +10% buffer spring in it along with an H2, this can be way too strong for using .223 ammo instead of 5.56mm NATO ammo.

It’s a very dynamic system which with more wear begins to run faster and faster cyclic rates as the gas port in the barrel erodes becoming larger thus allowing more gas to reach the carrier.

This is why I’m anal with my own stuff and do this method of maintenance.

Every 3K rounds replace gas rings (3pc) and cam pin & check headspace & firing pin protusion.

Every 10K rounds replace extractor, extractor pin, extractor buffer (black one), extractor spring (gold Colt one from Brownells), ejector and ejector spring (chrome silicon) and replace Crane 0-ring. Replace gas tube & gas tube roll pin, carrier key (and stake with MOACKS). Replace buffer spring with ISMI spring. Replace hammer, disconnector, trigger, selector and takedown and pivot pin springs & check headspace.

5

I just put one of grants H2’s, an LMT M16 BCG, and McFarland Gas ring, crane o-ring, and ISMI recoil spring in my Car length gas system gun. I shot 120 rounds mostly shooting 2 rounds per mag and I was using the new wolf ammo($5 stuff).
My gun ran 100%, the extra weight of the carrier and the H2(I had a standard AR BCG and H buffer) made it run “smooth” there was less recoil making the follow up shot faster and the gun just seemed at harmony. Oh ya the gun is an RRA Entry tactical.
I doubt you are over buffed.

6

Field Testing of M-16 Rifle Magazines, ArmaLite Technical Notes

_

7

OK folks, I admit, I cheated on the final test for the Colt course. I have no Idea what a 6920 is. I deal with M16A2’s, A4’s, M4’s and Mk18’s:D

Is this the totally Civ Legal M4orgery from Colt?

Only thing I would add to what has already been posted;

If your running a FA Bolt Carrier make sure you dont have one of the funky hammers that designed to keep folks from running a FA style BCG

8

Thanks for the info guys. Just to note, this is a brand new 6920, with only 400 rds through it. Using CS ISMI Buffer spring, H2 buffer, shooting Hornady 75 gr .223 TAP ammo…
I do not own any 5.56 ammo at this time… Maybe I’ll try some 5.56 TAP…

Thanks guys…

BTW,

Does anyone care to tell me why they think that Hornady .223 FPD TAP rounds, have a POI that is 6" LEFT of any other ammo I shoot at 100yds??? Never had any ammo produce this much of a different POI, especially when it’s the windage that’s off, and not the elevation…

9

Cheater! j/k :smiley:

A 6920 is (civilian legal) the LE semi auto 16" M4 (marked LE/Gov’t use only yada yada). The only thing different than a military M4 is that it has the larger diameter trigger and hammer pins and is semi-auto and a 16.1" barrel. IIRC it as a non-notched hammer because it does have a M16 carrier.

My diagnosis is that the ammo is too weak and the rifle too new to work with both an H2 buffer and an ISMI spring. Later on when the rifle has maybe 5-10K rounds through it, it should work 100% with both at the same time.

One of these days I want to buy a Colt 6921 which is the factory 14.5" SBR, kinda the Holy Grail of civilian semi-auto M4s IMHO.

10

Mine hits about 1/2" off (at 50yds) using 75gr TAP (FPD) in my LMT 10.3" which is sighted in (for 50yds) with American Eagle 55gr .223, my barrel is 1x7 twist.

11

As M4 has said, put one round in several mags to see what you get.

TAP ammo has been known to be underpowered when compared to XM193 or M855.

Your weapon is also very new (read tight). So there coule be many possible causes here.

Generally speaking, most any 16" weapon with a carbine gas system will be able to run an H, H2 or even an H3 buffer.

I even have a Noveske 10.5 that uses an ISMI spring and an H3 buffer. Generally speaking, this combo doesn’t work together very well.

C4

12

I would tend to agree.

If you have read through my How it works thread, you can see that failure of the bolt to lock back is classic short stroking.
Either mag springs are weak or gun is under-gassed or over-buffered.

13

If you had no malfunctions what made you decide to switch to a heavier buffer? With proper maintenance, that gun should run great forever right out of the box.

Stephen

14

As you know, an AR will run just fine with a carbine buffer. The heavier buffer CAN increase reliability and also reduce felt recoil which in return leads to faster follow up shots. There is however a fine line between too much buffer and just perfect.

C4

15

Reading on the internet made him do it.
It’s obvious that a bunch of armchair commandos know more about weapons design than Colt does…

16

There are a lot of armchair commandos putting bad info out there, but running an H2 buffer isn’t one of them.

Colt 6920’s come with H buffers and have even seen a couple with H2’s.

Military issued M4’s now come with H2 buffers.

C4

17

But they still come (and run) with “cheap” USGI buffer springs.:wink:

18

Yes they do, but there are better options out there.

C4

19

I have had issue related to this topic so i will ask about it. I have a BM patrolmans carbine, with roughly around 350 rnds through it. Being it is winter, tax return time I decided to try, at the recomendations here an ISMI spring and H2 buffer. It would fully function only with American eagle 55grn. .223, korean 55grn 5.56, and ADCOM 62grn 5.56. The Wolf 55grn .223, and brit. 62 grn 5.56 would not. I changed the buffer back to carbine weight, same results. I changed to standard spring and H2 buffer, ran like a top. Reading the above maybe i will wait and try ismi spring again in a few k rounds. But was also curious as to other possibilities. Temperature, it was in the teens when i did this. May be lube gummed up. I have grease on buffer spring and kel-lube on rest. also noted that BM non mil spec receiver extension is longer than standard mil spec. Could this dimension alter performance of the spring?

20

In a couple of batches of RRA Gov Carbines and Colt M4s, H2 buffers became necessary to prevent malfunctions with certain loads in burst and automatic fire. In semi-auto, any buffer worked fine.

IMHO, I don’t find that there’s a substantial enough difference in felt recoil, or other variable, to warrant a change to the H, H2, or greater unless there’s a functional issue.

There is a point of diminishing return when you increase the mass of the buffer (and bolt carrier), and/or the resistance of the action spring. You can get too close to the functional threshold of the gun in a hurry without careful balancing of the components. Add now the economy/import/junk ammo so many use, and there’s a recipe that’s anything but “improvement.”