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AR15 Help

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ffuries
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AR15 Help

#1 Post by ffuries » Fri Jul 07, 2017 12:28 am

Took the Bushmaster AR15A2 out today, overall it shot great with no malfunctions at all.

I noticed what feels like a vibration in the butt stock, a quick Google search says this is normal. Hell on one site some one asked if the poster had ever fired a M16/AR15 as that is normal. Call me stupid but in my 23 years in the Air Force having to qualify on the M16, I never once felt a vibration in any M16 I shot.

So is this normal or not?
Is there a way to fix this?
What should I look at as the culprit?

Other than the vibration driving me nuts, fun was had by all, took out the AR15A2, AR15M4, M91, M91/30, M44, MAS36, and No4 Mk1. My nine year old GD fired her first real rifle today, my son's AR15M4 and my AR15A2, she liked the A2 the best.
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Re: AR15 Help

#2 Post by RWS » Fri Jul 07, 2017 6:23 am

CAVEAT: I can only speak for myself but I believe the vibration is not only normal, it is present in every AR I've ever shot. I also believe that it has little, if any, effect on reliability so from a functional standpoint you probably need not do anything.

The vibration comes from the spring inside the stock "ringing" but the amplitude of the vibration varies from AR to AR, being barely noticeable in some and very noticeable in others. I believe the composition of the stock material, density, thickness, etc. all play a part in determining how much vibration gets through to the outside of the stock. Also, don't discount a sub-standard spring and/or buffer being present and creating extra resonance. Lighter bolt carrier groups allow more resonance than heavy bolt carrier groups as well. Point is, the resonance is always there to a greater or lesser degree.

I also believe that once you become aware of the phenomena you can waste a shot by concentrating on the vibration rather than focusing on the target and will then be able to detect it in any AR to a greater or lesser degree. Conversely, a drill sergeant might argue that if you're aware of the vibration then you're not concentrating hard enough on the sights, but that would also be a somewhat extreme view.

So... start by changing out the action spring and possibly the buttstock spacer. Inexpensive and might solve the problem. If your buttstock spacer is aluminum, consider getting a synthetic spacer as it will transmit less resonance than an aluminum one. If not then look at the buttstock next.

This is definitely opinion but it seems to me that real USGI stocks allow less resonance to come through to the outside than aftermarket stocks. I'd bet the O.D. buttstock on your Bushmaster is aftermarket. If the vibration is really bothering you then I think at the very least you will have to borrow a buttstock from somebody and swap it out temporarily to verify that it either does or does not make a difference. I predict even a used USGI black stock will allow less vibration through than your O.D. stock.

The swap-out is quick and easy (only one screw) so it should be a simple matter to swap stocks with a buddy for a range session and see whether or not the vibration moves to his rifle. Draw a conclusion or do additional testing and proceed from there.

By the way, I have AR's with original M16 and M16A1 buttstocks. I should note that they are quieter than my A2's. I have also noticed that putting the buttstock cleaning kit in the stock recess can effectively act as a noise damper of a sort (at the expensive of a bit more weight). The original M16 buttstock did not have a cleaning kit recess and was made from a different compound than the A2 stocks. My experience has been that the M16 buttstock is the quietest of the bunch. If your 23 years of military shooting was done with M16's or M16A1's then I would not be surprised at all to find that you are only noticing vibration with the A2 profile. I'm sure you know that you can simply install a M16 or A1 stock on your rifle if you get desperate enough and you will only lose 5/8" of length. That is another viable option.

-Bob

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Re: AR15 Help

#3 Post by ffuries » Fri Jul 07, 2017 4:49 pm

RWS wrote:
Fri Jul 07, 2017 6:23 am
CAVEAT: I can only speak for myself but I believe the vibration is not only normal, it is present in every AR I've ever shot. I also believe that it has little, if any, effect on reliability so from a functional standpoint you probably need not do anything.

The vibration comes from the spring inside the stock "ringing" but the amplitude of the vibration varies from AR to AR, being barely noticeable in some and very noticeable in others. I believe the composition of the stock material, density, thickness, etc. all play a part in determining how much vibration gets through to the outside of the stock. Also, don't discount a sub-standard spring and/or buffer being present and creating extra resonance. Lighter bolt carrier groups allow more resonance than heavy bolt carrier groups as well. Point is, the resonance is always there to a greater or lesser degree.

I also believe that once you become aware of the phenomena you can waste a shot by concentrating on the vibration rather than focusing on the target and will then be able to detect it in any AR to a greater or lesser degree. Conversely, a drill sergeant might argue that if you're aware of the vibration then you're not concentrating hard enough on the sights, but that would also be a somewhat extreme view.

So... start by changing out the action spring and possibly the buttstock spacer. Inexpensive and might solve the problem. If your buttstock spacer is aluminum, consider getting a synthetic spacer as it will transmit less resonance than an aluminum one. If not then look at the buttstock next.

This is definitely opinion but it seems to me that real USGI stocks allow less resonance to come through to the outside than aftermarket stocks. I'd bet the O.D. buttstock on your Bushmaster is aftermarket. If the vibration is really bothering you then I think at the very least you will have to borrow a buttstock from somebody and swap it out temporarily to verify that it either does or does not make a difference. I predict even a used USGI black stock will allow less vibration through than your O.D. stock.

The swap-out is quick and easy (only one screw) so it should be a simple matter to swap stocks with a buddy for a range session and see whether or not the vibration moves to his rifle. Draw a conclusion or do additional testing and proceed from there.

By the way, I have AR's with original M16 and M16A1 buttstocks. I should note that they are quieter than my A2's. I have also noticed that putting the buttstock cleaning kit in the stock recess can effectively act as a noise damper of a sort (at the expensive of a bit more weight). The original M16 buttstock did not have a cleaning kit recess and was made from a different compound than the A2 stocks. My experience has been that the M16 buttstock is the quietest of the bunch. If your 23 years of military shooting was done with M16's or M16A1's then I would not be surprised at all to find that you are only noticing vibration with the A2 profile. I'm sure you know that you can simply install a M16 or A1 stock on your rifle if you get desperate enough and you will only lose 5/8" of length. That is another viable option.

-Bob
Wow thank you for the very detailed response. As far as I know I started off with M16A1's (triangular handguards, full auto etc), then to the M16A2's (round handguards and 3 round burst), before I retired our unit got M4's for the aircrew.

I know at one time we used A1's that were upgraded to A2 standards, at least that is what the CATM instructors would tell us. As for the buttstocks I remember shiny black ones, and flat black ones, as for the cleaning kit compartment, I honestly can't recall if any of the rifles had them.

Then again in the Air Force all we did was academics, then break it down for inspection, go to the range and shoot, break it down and clean it. Which is funny because every rifle I broke down for inspection was dirty.

Having never heard, felt that vibration before, I thought damn something is broken, or something is loose. Honestly as you said, after the first shot I found myself anticipating the vibration, instead of concentrating on my sight picture.

I will go through your suggestions and report back what happens, again thank you for the very detailed response and all the information.

ETA: My oldest son who just punched from the USMC, and is staying with us with his family, made an interesting observation. In the AF you use training rifles, how many thousands of rounds have gone through them, they are most likely worn out to a point, hence no sound. He was a firearms coach during his I&I tour and stated every M16 they had made that noise some worse than others.
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Re: AR15 Help

#4 Post by Rapidrob » Fri Jul 07, 2017 8:30 pm

Make sure you do not have a Carbine buffer in a rifle or other way round. It will make noise if too light/heavy for the rifle/carbine.
Check your buffer retaining latch. If the buffer has passed forward the pin latch it will be too far forward and it will "sound like a Po-go stick". I've seen this many times.
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Re: AR15 Help

#5 Post by RWS » Sat Jul 08, 2017 10:34 am

Rob makes a couple of good points. I have also (infrequently) experienced buffer detent failures, but if the failure is complete rather than intermittent then you may not be able to break the rifle open by pushing the rear takedown pin open as the buffer may be too far forward and not allow the upper assembly to rotate/move.

In this event you'll have to slide both the rear AND front takedown pins open and remove the entire upper assembly. The buffer and spring will snake forward when you remove the upper and you will be able to push it back into the stock but it won't retain the buffer in place if the detent is trashed. Easy to diagnose that one.
I know at one time we used A1's that were upgraded to A2 standards, at least that is what the CATM instructors would tell us. As for the buttstocks I remember shiny black ones, and flat black ones, as for the cleaning kit compartment, I honestly can't recall if any of the rifles had them.
I don't think the Air Force was at all unhappy with their M16 & M16A1 rifles (remember, the M16 was Curtis LeMay's darling), but when the Army and Marines upgraded to the A2 rifles the Air Force had to partially upgrade. The reason was that the military was standardizing on the M855/SS109 62 grain bullet and the 62 grainer would not reliably stabilize in the 1-in-12" rifle twists of the M16 and A1's that were designed for the 55 grain M193 cartridge. The A2 rifle uses a 1-in-7" twist that provides stability for both the M855 ball round as well as tracers, and 55-grain M193 ammo works fine with the 1-in-7" twist as well.

The Air Force chose a sensible and economic plan of upgrading at least some of their M16 and M16A1's by replacing only the upper receiver barrelled assembly and keeping their old lower receiver assemblies. This gave them new barrels as well as the ability to use the new ammo at about half the cost of new A2 rifles.

The old upper assemblies (minus bolt carrier group & charging handles that went into the new A2 uppers) were dumped on the surplus market much to the delight of many of us. I bought five of them myself, and the A1 retro rifle I built and posted about earlier is sporting one of the Air Force surplussed upper assemblies. I used another one to build a lightweight 20" rifle with a carbine buttstock. Win-win for the Air Force and me. Gratuitous pics below:
RetroAR15.JPG
AR15CrifleRWS.jpg
As to the buttstocks, the M16 rifle did not have a cleaning kit compartment but the M16A1 (and A2) rifles did have the recess. Easy to tell the difference between the M16 and A1 buttstocks because the former's rear sling swivel was moveable whereas the A1 had a fixed sling swivel. Also, the butt on the M16 was solid; the A1 had the trapdoor access to the cleaning kit recess. Shiny black was more common with M16's whereas A1's tended to be more flat black, but I think both flat and shiny versions of both exist depending on which contractor made them. I imagine that if you did not have a dedicated rifle assigned to you that you sometimes signed out a M16 and sometimes signed out a M16A1. Just speculating there.

Last observation and then I'll shut up... You note that your rifle has the HBAR barrel. I think maybe the larger mass of the HBAR allows more vibration to be transferred to the rear of the rifle and may increase the perceived vibration. Not positive about that but the "noisiest" AR I have is an A2 with the heavy barrel. I just haven't futzed around with HBAR A2's enough to know whether increased vibration is endemic to HBAR's or merely coincidental. I suspect it is but am just not positive.

-Bob

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