I have heard this a few times about the correlation between barrel weight and what weight buffer you should be using....the heavier the barrel, the lighter the buffer???? I think that is what I heard??? I need to clarify

As I understand it, the buffer weight is dictated more by the gas length and if you are using a suppressor.

Some of my barrels are heavier than others but I typically use a buffer than came from the OEM with the exception of a couple of my rifles.

Thanks,

Bronc

Barrel weight has absolutely nothing to do with buffer weight.

H2 buffer was spec'd for the M4A1 (heavy) barrel for reasons. "Carrier bounce" or "slap ".

The heavier the mass of the barrel, the more elastic it is for the bolt carrier. So the heavier the barrel, the more the carrier will bounce back off the barrel extension, causing a brief time of the unlocking of the bolt after the auto sear releases the hammer. By the time the bolt locks again and the hammer settles on the firing pin, there is not enough inertia on the firing pin to detonate the primer. This is known as "bolt carrier bounce".


Eta: Meant to type H2, not H.

This is well more into a gassing issue than a profile related issue.

Buffer system tuning is based off gas port, other reciprocating mass, dwell time, and ammunition used.  Not barrel profile.

The M4A1 is spec’d with an H2 buffer because the more massive SOCOM barrel causes more bolt bounce. Doesn’t have anything to do with gas.

M855A1 pressure bump/ increased carrier velocity/ increased barrel mass = bolt bounce.


Take a steel hammer, throw a smack on a piece of 3" steel shafting.

Now try the same on a section of 6".

H2 buffer intent is to slow carrier speed and prevent bounce after the carrier impacts the extension.

Bronc

Before this dives down a rabbit hole.....

Basically, when tuning is the intent, the reciprocating mass and gas drive should be matched.

Gas drive is what accelerates your bolt carrier. By increasing the reciprocating mass you are lowering the carrier velocity and allowing more time for feeding and extraction.

GP diameter is a major factor in tuning.

Bolt bounce is a full auto issue only.

Happens in semi as well as auto, but yes, issues arise in auto fire regarding the unlocking of the bolt after the auto sear releases the hammer.

This.

Guys,

As Tig said, this may go off on tangents....

The reason I am asking is I heard on two or three different videos that barrel weight determines the buffer weight.

And I have been trying to figure this one out.  I am a machinery engineer by degree, retired now....and dealt with reciprocating mass in our big refienry recip compressors, mass can hurt and help...

I can see bolt bounce as an issue, thinking about the Newton pendelum toy, when the bolt is going forward and lands on a heavier mass, I can see bolt bounce occurring.  

But anyway, the comments are good.  

This^ Colt ships there M4A1 Socom barreled rifles with H2 buffer & all government profile barrels with an H buffer.

It has.
As said multiple times the m4a1 requires a heavier buffer compared to the govt profile m4.

The same reason a rifle buffer is heavier than a carbine one

It’s something related to FA only but bolt bounce is a thing

Wrong.  Barrel weight is the reason for the H2 in the M4A1.

By "is not an issue", I mean that bolt bounce does not effect operation or reliability of semi-automatics, and therefore you don't need to worry about it.

And, the issue is not having the bolt unlocked, they don't bounce that much, only about 0.07" maximum*.  The issue is the hammer fall strikes the firing pin while it is still supported by the carrier and reduces the energy delivered to the primer.

"ARL . . found that light primer strikes in that weapon were most often caused by the bolt carrier rebounding from the barrel extension while the hammer was falling.  This rebounding caused the rear flange of the firing pin to contact the bolt carrier instead of the shoulder on the rear of the bolt.  The firing pin was then prevented from protruding far enough through the bolt face into the primer."

-"Bolt Carrier Bounce Measurements for a 5.56mm M4A1 Carbine"

___________
* That distance is is the actual straight distance of the locking cam track.  Apparently, the bounce does not have enough energy to rotate the bolt, and stalls when it hits the cam path.

This is not true.  The H2 buffer development was independent from the heavy barrel.

Problems with the light barrel were known since 1996 and the heavier barrel was being designed.

The issue with bolt carrier bounce in the M4A1 was first identified in 1998.

In 1999, the the Army was experimenting with several ideas, the leading two were using 5 smaller tungsten weights with aluminum spacers, or tungsten shot; the target weight for the new buffer was to be about 5.0 oz.

In 2001 the new heavy "SOCOM" barrel is introduced.

In 2002, the Army chooses three weights of the same size as currently in production, only differing in material, in a two tungsten/one steel combination, with a total weight of 4.6 oz.  All testing for the heavy buffer was done with the light barrel.

This is where you saw it OP

Wrong.

Barrel weight is one of the key determining factors for choosing the proper buffer weight.

OP heavier barrel = Heavier buffer.

This

So much wrong info in this thread, but keep parroting it...

Weight of the barrel has nothing to do with it.

Gas length
Port size
Carrier weight in case of lightweight carriers
Spring rate

ALL HAVE MORE TO DO WITH THE RELATION TO BUFFER WIGHT THAN A PENCIL BARREL OR HEAVY BARREL IN THE SAME EXACT SIZE.

If anybody has any actual tech references to studies/data regarding this, I'd be interested.

Not this. Tig is correct.

Dont hold your breath.  Some members cant seem to stop spreading incorrect info.

Take a mass produced usgi M4/m4a1 and all the variables above are almost the same.

You’re left with barrel profile and it does make a difference.
Saying it has nothing to do is just incorrect.

@coldblue may have some info

I agree. The military doesn't care about 'tuning' an AR with buffers to counteract port size or gas length, that's a hobbyist thing. If the military only had semi auto rifles, I'm betting they would be designed around using Carbine or H buffers only. The military would have no reason to use more than an H buffer, unless they're trying to reduce rounds per minute or adjusting the timing or reducing bolt bounce in full auto. And yes, a heavier barrel or heavier rifle causes more bolt bounce in full auto and a heavier buffer is used to improve reliability (in full auto). Full auto introduces timing issues not seen in semi auto. H2 and H3 buffers were not designed for 'tuning' semi auto ARs.

I started this brew-ha-ha and really thought hard about asking the question....as I noted in my original post, I had heard the barrel weight and buffer weight on some videos on Youtube, now when I am watching these videos I am usually working on something or doing paper work....so my attention is not 100%, but this is what I heard...

Everyone makes good points and does not look like it will 100% nailed down here....anyway a lot of good points.

What I can say, all of my rifles run great with the buffers from the OEM, I did bump up the weight on my SBR and also on a DD V7SLW.  The only rifle that has given me fits since day one is my Larue 762 OBR.  This rifle is a Dr Jeckel and Mr Hyde when running with and without a suppressor.  It went back to Larue twice and I had to buy a XH buffer from Slash to make it function.  I think it is ironed out now....damn accurate rifle and really fun to shoot.  

Bronc

I remember reading an article about the development of the M4A1 and it mentioned that the buffer weight was raised because the heavier barrel/heavier rifle caused a timing issue in full auto (bolt bounce). No luck finding it now though.

I always heard it as the H2 was used because of full auto instead of burst. I'm no engineer, but a few ounces of barrel weight causing a significant change in bolt bounce honestly sounds laughable.

Bolt bounce can, and does happen in semi autos. It's just not an issue because pulling the trigger that quick is next to impossible.

Okay, some physics to set the stage:

When two objects collide, with a perfectly elastic collision, energy and momentum are conserved.

{[ma x (ua)^2]\2} + {[mb x (ub)^2]\2} = {[ma x (va)^2]\2} + {[mb x (vb)^2]\2}

and

(ma x ua) + (mb x ub) = (ma x va) + (mb x vb)

with:

ma the mass of object A
mb the mass of object B
ua the initial velocity of object A
ub the initial velocity of object B
va the final velocity of object A
vb the final velocity of object B

Solving these two equations yields:

|va - vb| = 1
|ua - ub|

Now, we define the coefficient of restitution, how close the collision is to being perfectly elastic, as:




Putting this back into our first two equations and solving again we can get a simple formula to both resulting velocities:



Now, let's look at this as to increasing the mass of the barrel.  The barrel is attached to the rifle, so anythning that tries to move the barrel will also have to move the rest of the rifle, minus the mass of the bolt and bolt carrier assembly and buffer.

So if we define

ma as the mass of the bolt and bolt carrier group with buffer
mb as the mass of the rest of the rifle

we can calculate the change in bolt carrier velocity (va) due to and increase of 0.31 pounds (the difference between a "M4" and "SOCOM" barrel) in mb

So, if our regular profile barrel in our M4 weighs 7.0 pounds, our SOCOM barreled M4 will weigh 7.31 pounds or a 4.5% increase. And, if we assume a Cr of 0.5 for steel-on-steel, typical bolt closing velocities of 8 to 10 fps, and typical bolt/carrier/buffer weights of 1.3 pounds we will see a little less than 0.5% increase in carrier bounce velocity.

0.5% is hardly enough to warrant an change in buffer weight.

 It seems so simple when you put it like that.  ??

I forgot to mention this applies to a rifle suspended on strings and allowed move without restraint.  If you are actually holding the rifle, a portion of your body mass must be added to the mb, and reduce the percent increase of bounce velocity.

This is backup by the fact that holding the rifle loosely (firing from the hip or one-handed) shows an increase in bolt bounce malfunctions versus a tight grip or firing from a rigid mount.

Well, that's a bit  brain blinding.


Let me rephrase things....


Any tech references related to the testing/development in regards to the M4A1 (Model 921HB ). Articles/test reports/etc.

I know that Phase II of the PIP explored developing a new bolt carrier but over six months of testing revealed that the current bolt carrier assembly outperformed the competing designs, especially in the areas of reliability, durability, and high-temp and low-temp tests.

The heavier bbl profile was chosen for better heat dissipation and heavier buffer chosen to combat bolt bounce experienced during FA fire.

I was waiting for the conservation of KE and momentum to be posted....going back to my college physics and Mechanical Engineering classes...now I need some Tylenol  

Colt says heavy bbls need H2, end of story.

The M4 PIP, Phase I was started in 2007, and Phase II a year or so later. The heavy barrel was introduced in 2001 and the heavy buffer 2002, well before the PIP was initiated.  The goal of Phase I was simply to bring all the M4s and M4A1s in service up to the SOCOM barrel standards, as well as ensure things like the buffer, extractor spring and disconnect spring were the latest versions.

Disregard.

Discussion has gone far enough off of topic......

"Abbreviated Report for the Technical Feasibility Test of the Carbine, 5.56mm, M4A1", Gregory Brewer, Light Weapons System Division Armament and Ammunition Testing Directorate, September 1994.  Dates of test: January to September 1994.

This reports notes that the M4A1 met all test criteria except the 6000 round endurance test due to excessive failures to fire.  It recommended that tests be conducted to determine and eliminate the failure to fire stoppages before the M4A1 be fielded.


"Abbreviated Report for the Initial Acceptance Tests for the Carbine System, 5.56mm, M4A1", Larry Hubbard, Light Weapons System Division Armament and Ammunition Testing Directorate, December 1996.  Dates of test: January 1995 to December 1996

This report noted that a total of 40,000 rounds were fired with 12 Class I stoppages and no Class II or III stoppages, and concluded that the M4A1 being delivered were satisfactory for issue.

At this point, the buffer was still the standard, three steel weigh, buffer, P/N 9390023, and the barrel was the lightweight profile, P/N 9390009.


"Bolt Carrier Bounce Measurements for a 5.56mm M4A1 Carbine", Brosseau and McLaughlin, Army Research Laboratory, March 1999.

Due to excessive light primer strikes in acceptance testing of M4A1s, ways to reduce bolt carrier bounce are investigated.  A 5 ounce buffer with fire "Mallory" weights is recommended.


January 2001, the heavy barrel, P/N 12991849 is approved for the M4A1.  Barrel, P/N 9390009, is still the only barrel approved for the M4

January 2003, the "H2" Buffer, P/N 13004468, is standardized for both the M4 and M4A1, superseding Buffer, P/N 9390023.

The 1 May 1991 issue of TM 9-1005-319-23&P with change 5, dated 9 April 1997, list only one barrel for both the M4 and M4A1, P/N 9390009, and only one buffer for both the M4 and M4A1, P/N 9390023.

The 28 November 2008 issue of TM 9-1005-319-23&P lists heavy barrel, P/N 12991850, for the M4A1 and the light barrel, P/N 9390009, for the M4 or M4A1.

Cool.  I didn’t know all that started with the light barrel.  

First I saw of the H2 I was told it’s for the heavy barrel because it causes more bounce.

lysanderxiii

Thank you for posting this. Seems that I've got some reading to do.

And suppressors that are becoming more common outside the sf community add even more weight and speed to the equation

I know MK18s can use H3 if necessary.

This thread caused me to pop open my two 6920s I use and they both have H2 buffers.  I don’t remember swapping’em out but I’ve had ‘em for a loong time.

I know my BCM SOCOM had an H2 because I put it there.

They all run fine