View Full Version : Reciprocating mass and the recoil impulse
I'm curious about contradictory principles I've read regarding the felt recoil impulse of the AR system.
A lot of people recommend heavier buffers and bolt carriers to cut down on felt recoil, relying on the inertia of the greater mass to increase lock time and eat up more energy before it is transferred back into the shooter.
The heavier carrier also reduces the felt recoil impulse... The heavier buffer can also decrease felt recoil.
But then on the flip side of that coin, there are systems like the JP Low Mass Operating System (http://www.jprifles.com/1.4.7_bc.php) which aim to reduce the amount of reciprocating mass within the rifle, thereby reducing the back-and-forth impulses of operation. I know that gotm4 runs these carriers in a couple of his builds.
The trick to lower felt recoil is lowering the moving mass...
How do these principles balance out?
Bear in mind, I am not asking about reliability, lock time, etc. I know that there are a host of concerns with picking too heavy or too light of a system. I am solely asking about felt recoil.
Is one strategy better dependent on the overall weight of the gun? Is one strategy better depending on whether or not you are running a brake?
Does the buffer or the carrier matter more than the other in this equation? I.E. what if someone used a very light carrier with a very heavy buffer, or vice versa?
I'm curious how the physics all balance out here.
The M16/AR/M4 platform has felt recoil? :confused: ;)
Because I'm a bit OCD, I went digging around for more quotes to show the contradictory ideas I'm seeing:
In favor of a heavy BCG/Buffer:
In a S&W AR, I would actually use an H2 buffer. This will soften felt recoil and slow your bolt speed (making the weapon more reliable).
I like the heavier weights because it makes the rifle recoil softer and allows easier follow up shots.
I also had a 9mm buffer in my Sabre middy, it was 100% to, I put it in my son's BC M4 to improve felt recoil(9 yrs old).
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.
The heavier buffer CAN increase reliability and also reduce felt recoil which in return leads to faster follow up shots.
...I then spent 25 bucks for a 9mm buffer in one carbine and have the H3 in another...
The reason I switched is solely for the felt recoil. I can double tap and triple tap with almost no muzzle rise. On a sand bag rest the sights don't even move off the target.
Any emphasis has been added by myself.
In favor of a lighter BCG/Buffer:
...You're running a midlength with a M16 carrier which doesn't have these problems.
Ideally you only need a CAR buffer, will it run with a H? Probably. Will it run with an H2? Probably. But you'll not have any benefit from a heavy buffer and it could have problems running dirty, dry and with weak ammo and when it does work it'll have more felt recoil and muzzle lift.
All though the AR doesn't have much recoil to begin with the heavier you make the buffer and carrier the heavier felt recoil you'll have (more reciprocating weight).
...I prefer reliability over the softer recoil of say a JP LMOS system...
It'll shoot even softer with a CAR buffer or a CAR buffer minus 1 weight (replace 1 weight with an equal length piece of delrin). I got mine shooting so soft that way that I could no longer feel it lock open even off the bench or prone.
You might want to head in the opposite direction and use a LIGHTER buffer combined with a brake...
The LMOS is not recommended for tactical situations. However, where it is important to quickly place several rounds on target with great accuracy, the LMOS is tops.
Again, any emphasis has been added by myself.
It's not as if one camp is full of experts while the other is full of idiots: both Grant and Robb are known around here to have extensive knowledge on the AR system. Hence my confusion and curiosity as to how these principles balance out.
Thoughts?
Tag for "I've always wondered the same thing."
I've rationalized it to myself this way (the following is me theorizing, and not based on anything else): the light carrier has less recoil when paired with lesser powered ammo, whereas the heavier buffer has less recoil when paired with more powerful ammo. Heavy ammo with too light of a carrier would give more recoil, and light ammo with too heavy a buffer won't cycle.
But that's not a satisfactory answer based on the wording of the quotations above...and I don't even know if it's true.
I'm curious too.
Someone needs to do some scientific testing on the same day with the same gun and switch between a heavy BCG/buffer setup vs a light BCG/buffer setup.
By reviewing Brian Enos's site, it sounds like a brake is much more effective in recoil reduction than carrier weight, and a lighter carrier involves more tuning to be as reliable as standard weight. Further...
Zak says: "The primary benefits he lists are accuracy-oriented, due to increased lock time [using a Tubbs CWS additional carrier weight system]. As a side-benefit, carrier velocity is slowed and he claims the rifle recoils "softer", whatever that means. Note that just adding a weight to the gun will make it recoil "softer." The J.P. system is designed specifically so you can retain your sight picture while double-tapping targets at 3Gun ranges (0 - 300 yards). The goals of the two systems are different."
http://www.brianenos.com/forums/index.php?showtopic=16788&hl=carrier+recoil
Zak again: "Let's try a thought experiment. Clear the gun, and obtain a safe sight picture. Dry fire the gun. Now press the trigger. Press it again, and "double-tap" the trigger. Notice the gun doesn't move when you press the trigger. As long as you hold the same sight picture, the theoretical best shot-to-shot time for you is as fast as you can press the trigger in this setup. All the physics that happens when you're actually firing rounds downrange adds time. A lighter BCG helps to reduce the magnitude of the forces. A good muzzle comp is another part of the picture."
http://www.brianenos.com/forums/index.php?showtopic=14895&hl=carrier+recoil
and from George: "16 Oz. of moving mass is 16 Oz. of moving mass, no matter what you do. The buffer might have some effect on the rearward stroke, but when that 1 Lb bolt carrier slams home in the forward direction the only thing damping it is the fresh cartridge pickup. Slowing the bolt by using less gas will help but then you might notice a sluggishness in cycling that will have you waiting for the rifle in fast shooting. If there was an easy way to smooth the cycling impulse without paying the price of slowing the bolt carrier or lightening it then it would be in widespread use already. AR's aren't rocket science and the principles of equal and opposite reaction call the shots here. If you want less reciprocating mass impulse and a high cyclic speed, you gotta have less reciprocating mass (see unsprung weight and vehicular suspension technology for more about this). Otherwise you have to live with a slow cyclic rate to smooth out the starts and stops of a massive object with springs and damping appropriate to that job. As I mentioned earlier, the forward motion of the heavy bolt assembly is where most of the perceived rifle upset happens, and that's the direction it' s hard to add extra damping in. A softer spring will slow the bolt on the forward stroke but allow more rearward slamming upset. The balance of the stock spring/buffer/bolt carrier relationship has already been tweaked properly for this. The Low Mass system is also a fully tweaked system based on that bolt carrier weight."
http://www.brianenos.com/forums/index.php?showtopic=10511&hl=carrier+recoil
I still don't think the question is answered exactly...
I'm curious about contradictory principles I've read regarding the felt recoil impulse of the AR system...
How do these principles balance out?
I'm curious how the physics all balance out here.
I'm not a physicist, and I'm not an expert. I'm curious, too, and I'm guessing here...
AFAIK (again, I'm not a physicist) replacing the buffer with a heavier or lighter one won't change the recoil.
The energy going out the front of the gun is the same, and gas pressure driving rearward is the same. So, a lighter/faster buffer should have as much energy as a heavier/slower one.
However, my understanding is that the physics of recoil is only tenuously related to "felt" recoil. i.e. reducing the "real" recoil may or may not reduce the "felt" recoil.
So, the "slower" recoil of a heavier buffer may be just as much recoil energy, but if it's spread over a bit more time, it may "feel" like less.
That's my 2¢ (now worth only €0.013)
Bimmer
Robb Jensen
10-16-09, 20:57
The lighter weight changes how the recoil feels. It's still there just quite different.
All things being equal (no other things effecting/changing recoil) the lighter carrier/buffer will actually make the rifle hit your shoulder harder. But because the recoil is happening much faster it's doesn't have as much time to lift the muzzle as much and feels lighter because it's over with so much quicker and the bolt it returning forward much sooner than it normally would. This is kind of like the difference in felt recoil in 10mm and .45ACP pistols or more modern .357SIG (w/115gr-125gr) and .40S&W pistols (w/180-200gr) The 10mm / .357SIG pistols usually have a harder rearward recoil but less muzzle lift vs the slower heavier more muzzle lift recoils of the .40S&W and .45ACP. This is because the pistols slide is moving so fast and returning to battery much sooner that the muzzle has much less time to lift.
Note: Because of the already fast cyclic rate of CAR length gas systems I'd advise against the use of lightened up carriers. These should really only be used on midlength and rifle length gas systems for best results.
Combining the faster cyclic rate/less moving mass with a muzzle brake which cancels out much of felt recoil helps exponentially. You can sometimes have so much less recoil and muzzle lift that if you have a really aggressive stance/grip it can cause the muzzle to dive as the rifle fires. I've had this happen and then had to open up the ports on the brakes to lessen their effectiveness because I didn't want to modify my stance/grip.
SingleStacker45
10-16-09, 21:40
heavier buffers and springs slow down the recoil cycle and spread it over a longer time so it does not feal as sharp. This improves reliability on most guns(especialy carbine gas systems) but creates more movement of the gun which slows down rour ability to regain your sight picture for the second shot.
Mule
Most IPSC/practical shooters will have an effective muzzle brake/compensator on the muzzle of their rifle or carbine. The braking effect will keep the muzzle in line with the target and a lower reciprocating mass (i.e., the JP carrier) will help avoid muzzle dip as the carrier group goes forward.
National Match shooters may apply weights to their free-float tubes, have heavy target barrels, and add weight in their butt trap. The carrier weights (i.e., the Tubb CWS) increase unlock-extract-and eject time as gas pressure must counter the inertia of the heavier mass (helping to increase brass life and prevent blown primers in higher-charged heavy bullet target loads) and help the carrier group move slower (a body at rest tends to stay at rest) -- an advantage if the shooter is in a good position with bone and sling support. Ideally the carrier will hit the end of the receiver extension slower and return slower as it strips the round from the top of the magazine.
Thanks very much, guys. All very helpful (and fascinating) information.
I'd call it sticky material. It's the best explanation I've seen of a very interesting topic.
I'm curious too.
Someone needs to do some scientific testing on the same day with the same gun and switch between a heavy BCG/buffer setup vs a light BCG/buffer setup.
You shouldn't really need to do that. This is something that can be answered definitively on a piece of paper without ever touching a firearm.
Someone needs to do some scientific testing on the same day with the same gun and switch between a heavy BCG/buffer setup vs a light BCG/buffer setup.
+1
And I'd like to see it complicated further by using lighter and heavier bullets, i.e. 55gr M193 vs 75gr TAP.
Doing calculations on paper is a good start, but IMHO real world testing is really necessary to understand how these things change "felt" recoil.
Bimmer
So why don't you buy both carriers, load your own bullets, and do it yourself?
Sheesh.
I don't see these as contradictory. I have not been in school for 20 years or more but the heavier buffer changes the length of time that the force of the recoil is felt over which reduces the feeling of recoil. The impulse is longer
the lighter mass is a lighter recoil to start with though it may be moving slightly faster.
Someone tosses a big boulder at your shoulder vs a small pebble. You feel the boulder more. But if someone hucks a small pebble at your shoulder versus pushing your shoulder slowly with a large boulder, you may feel the pebble more painfully.
No, the principles aren't contradictory once they've been more thoroughly explained. But when people were just referring to them absent of context, it certainly seemed contradictory to me. In keeping with your analogy, I might be confused if a bunch of people said, "getting hit with pebbles hurts a lot less than getting hit with boulders," while others said the exact opposite.
Factoring intensity over time into the equation brings the context I needed to understand why people could claim "less felt recoil" on both ends on the spectrum. I wouldn't have been confused in the beginning if people said, "I like to slow down the recoil impulse and spread it over time" or "I like to get the recoil cycle over with as quickly as possible so there is less time to disrupt my sight alignment." But the absolute statements of "heavy = less felt recoil" and "light = less felt recoil" are what made me scratch my head.
I'm not saying the confusion is anyone's fault but my own - just explaining why I originally thought it was contradictory.
No, the principles aren't contradictory once they've been more thoroughly explained. But when people were just referring to them absent of context, it certainly seemed contradictory to me. In keeping with your analogy, I might be confused if a bunch of people said, "getting hit with pebbles hurts a lot less than getting hit with boulders," while others said the exact opposite.
Factoring intensity over time into the equation brings the context I needed to understand why people could claim "less felt recoil" on both ends on the spectrum. I wouldn't have been confused in the beginning if people said, "I like to slow down the recoil impulse and spread it over time" or "I like to get the recoil cycle over with as quickly as possible so there is less time to disrupt my sight alignment." But the absolute statements of "heavy = less felt recoil" and "light = less felt recoil" are what made me scratch my head.
I'm not saying the confusion is anyone's fault but my own - just explaining why I originally thought it was contradictory.
There is much more to it than simply the immediate effect of greater reciprocating mass.
There is what happens to the expanding gas, when it meets greater than normal resistance, there is the noted slower acceleration of the buffer, there is it's noted increase in inertia, there is the recoil spring's ability to convert the extra mechanical energy into heat energy, and importantly, there is the volume of space occupied within the buffer by it's weights, as well as air pressure within the receiver extension.
The result will be quite different when these variables change even a small amount.
What may seem obvious can be quite incorrect if the system is set up a certain way.
Energy is used to accelerate that mass. The same energy you will find at the end, when it reaches the extent of it's travel. This is because you are acting on the increased mass with the same amount of force you used with the lower mass. It's not like hurling a brick versus a pebble, where you must hurl that brick with more force. The force remains the same. The input energy is a constant. Think of it more like using a spring loaded punch to fire a marble into another spring attached to a scale. Swap in a ball bearing, and the added mass is compensated for by a lower velocity.
It isn't as simple as you might think... And yet it is, when you look at it properly.
There are other factors, though, that have significant effect. This isn't all there is to it, but it's a lot of it.
It's not like hurling a brick versus a pebble, where you must hurl that brick with more force. The force remains the same. The input energy is a constant.
Don't worry: that analogy didn't confuse me. I got what e-guns was saying regarding different expressions of energy: a slow, heavy push vs a fast, sharp blow. I understood that he meant the same input energy being expressed in different ways - the way that Robb above talked about different handgun rounds and their respective recoil profiles.
LittleMagic
10-24-09, 22:33
I am new to the the AR platform. So please excuse if my question has been answered.
I have a 16" upper w midlength gas system. I built the lower. Bought the CTR w milspec buffer tube, H2 buffer, and complete upper.
A few Q's:
i.) What do I need to do to lessen the muzzle rise and recoil such to get back on target quicker?
ii.) How do I id the buffer spring, as I am not sure what type it is?
iii.) Is the H2 buffer too heavy or too light?
thx in advance.
Heavy Metal
10-24-09, 22:48
K=1/2MV^2
You aren't changing the energy in the system. You are only changing V for the BCG and Buffer combined recoiling mass.
SingleStacker45
10-25-09, 19:00
I am new to the the AR platform. So please excuse if my question has been answered.
I have a 16" upper w midlength gas system. I built the lower. Bought the CTR w milspec buffer tube, H2 buffer, and complete upper.
A few Q's:
i.) What do I need to do to lessen the muzzle rise and recoil such to get back on target quicker?
ii.) How do I id the buffer spring, as I am not sure what type it is?
iii.) Is the H2 buffer too heavy or too light?
thx in advance.
In my very limited experience( I've built two rifles one was middy and one was rifle gas) the H@ is probably overkill for a middy. If it is running fine don't worry too much about it. As for muzzle rise a compensator of some type is going to do way more to help than any spring,carrier or buffer changes. You have to decide if you can tolerate the increased noise. I use a Miculek and a Ops Inc muzzel break but both are loud.
Mule
1.) muzzle brake. The FSC isn't that loud, and it also combats flash. Check out www.ar15performance.com for a new tunable brake.
2.) it's gotta be a standard carbine spring. You probably won't have a problem with it for some time, but feel free to get a Colt one from Specialized Armament for $10, and save yours as a back-up. DO a little Google and find how long the tolerance is, to measure occassionally.
3.) too heavy, but it might work fine. Use a range of ammo to test, both underpowered .223 and hi-test 5.56.
Definitely not an expert, but I took a physics class once...
The JP and Vltor systems work in different ways to do the same thing: reduce the recoil felt by the moving parts. The JP system reduces the weight of parts and lowers the gas to just what's needed to move them, where the Vltor system puts more spring behind a heavier buffer in a longer tube. Comparing the standard system to the JP system, you now have lighter components recoiling against you at the same velocity that the heavier components were (with less wasted gas). Comparing the standard system to the Vltor system, the Vltor system's heavier buffer is harder to move (greater inertia) and its longer tube houses a longer spring that stores (absorbs) more mechanical energy and exerts more force than the shorter CAR spring.
JP's system is related to momentum. A lighter object hitting your shoulder at the same velocity of heavier object will hit softer than the heavier object.
Vltor's system is related to inertia and spring length. A same weight object hitting a heavier (harder-to-move) object with more spring to resist compression (absorb energy) will hit softer than if it had hit a lighter (easier-to-move) object with less spring to resist compression (absorb energy).
-B
People shouldn't be selecting buffers based on recoil. :rolleyes:
Okay.. maybe for gaming guns... but NOT fighting guns.
And yet, a change in receiver extension, spring, and buffer was apparently shown in trials to make otherwise-stock carbines (fighting guns) more reliable. JP might've designed their system for game guns, but did Vltor? Did Mike Pannone and others before Vltor who also started using beefier springs and heavier buffers make these changes for game guns?
-B
EzGoingKev
01-15-11, 07:11
In Grant's post, Best Suppressed SBR Ever (https://www.m4carbine.net/showthread.php?t=33743), he discloses that he worked with the sizing of the gas port to give the rifle just enough gas for the action to cycle. He used an LMT enhanced bolt carrier and an H2 buffer.
What if you built an upper but optimized the gas port size to work with a lightened bolt and a lighter buffer? You would have a reduction in gas pressure from the smaller port and less reciprocating mass.
What effect would this have?
What if you built an upper but optimized the gas port size to work with a lightened bolt and a lighter buffer? You would have a reduction in gas pressure from the smaller port and less reciprocating mass.
What effect would this have?
An adjustable gas block does the same thing but allows to make adjustments if you change ammo. GP size does not let you adjust easily
With the JP system or a BCG that has been lightened and used with a lighter buffer. You keep reducing the amount of gas until the bcg will not lock back, then add a little more gas. This gives the lightest recoil impulse, light reciprocating mass and the carrier is not slamming back. smooth and light is alway better that smooth and heavy or light and harsh
To get the most out of a adjustable gas block, you need the proper size GP to begin with. If you are severely overgassed, then the gas will erode the adjustable gas block. I had a 14.5" carbine barrel that came with a GP of .099, no matter what I did, the gun was very harsh and eventually ate thru the adjustable gas block. The barrel I run now is .063 and even with the gas block turned down, I haven't had any erosion issues.
Bill Alexander
01-16-11, 10:02
Two differing approaches to arrive at the near same end point but with a lot of variation in the utility.
First one must consider what constitutes the felt recoil and the controlability of the rifle. The contribution from the cartridge recoil is usually minimum (except the big bores) and the "felt portion will consist of the acceleration of the carrier group, the bottoming out of the carrier group at the end of the buffer tube and the final stop of the carrier group against the barrel extension.
Taking the lightweight, the methodology is the minimise the reciprocating mass as far as possible. To work effectively the system timing (port position) must be well set up. The potential energy storage of the system is low so the port size must be small (usually adjustable as it must be carefully tuned to the load). Upon firing the rearward acceleration of the group does little to counteract the recoil from the projectile but equally the change of momentum when the group stops and when it runs forward again, are small so offer little disturbance in the sight picture. With the short reciprocation length of the AR, a low mass with limited spring compression to store energy reduces the energy available to strip a round from the magazine and close the breach. This linear brake effect is useful in retarding the impact of the carrier and subsequent carrier bounce but the gun has little tolerance for field contamination.
Looking at the approach of using a heavier carrier group this is more applicable to a working rifle as it detunes the system. The heavier group being thrown backwards at firing will act to move the rifle forwards away from the shooter. As long as the base rifle is reasonably timed and gassed, the energy storage of the mass/spring is greater so the group does not arrive at the bottom of the buffer tube with as much velocity. Energy is not wasted in the impact so the ability to strip the next round is enhanced. Overall the energy in the heavier system is greater but the reciprocating mass is greater so the movement of the rifle will be more than the equivelent highly tuned light carrier set up.
Bill Alexander
So with the LMOS, if you use a Tank brake vs an A2 FH, would you need to adjust the gas port for that? Other than different ammo and suppressor use, what other reasons would you need to adjust, using the LMOS system?
JP's system is designed around the adjustable gas block.
Thank you for a much more detailed explanation, Mr. Alexander.
-B
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