A simple calculation to help find the best buffer/spring combo

AR/M4 AR Technical Discussion
1

For my first build I went to great lengths to find an ideal buffer and spring configuration for my gun. I wanted my build to be a sweet shooter, so I started out with a 14.5" middy for its purported benefits over a carbine length gas system in combination with an M16 BCG. To tune the buffer and spring to the rest of the gun I turned to the forums but I was discouraged by the trial-and-error approach that is commonly advised. I didn’t want to buy a bunch of extra parts and I wanted an answer that would satisfy my perfectionist side.

Due to the lack of hard numbers out there I had to find a way to mathematically represent different combinations of buffer weight and spring tension in order to find an optimal compromise between reliability and reduced recoil. Now, I am by no means an engineer or an expert so take all this with a grain of salt.

To start with, I dug up a chart of technical info on the forums that states that a Colt GI carbine spring imparts 6.040 lbs of force when compressed to 6.902". A standard H buffer weighs 3.8 oz. Since both of these components work in combination with each other I decided to use the product of the measurements as a unit of comparison. In order to do so I multiplied the spring weight by 16 so that the measurement would be in oz. So 6.040 x 16 = 96.64 and when multiplied by the buffer weight of 3.8 oz, we get a product of 367 units. Units of what you ask? **** if I know; like I said, I’m not an engineer. Think of this number as a unit of comparison.

So now that we have a number for the intended setup for a 14.5 middy we need to find a number for the point at which a setup will induce short cycling with the weakest ammo that you intend to use. According to various anecdotal reports, a Sprinco blue spring with an H2 buffer is where we will commonly start to see short stroking.

To get the info needed for this, I emailed Sprinco to find out how much heavier their blue spring is than stock. I got a prompt response of 17% (thanks Alan). This info coincided with the numbers on the chart that I dug up earlier so at this point I was starting to feel pretty good. So using our previous formula, a Sprinco blue with H2 buffer gets us 548 units.

By now, I assume that we are both on the same page, so in the interest of brevity I am going to present to you the information just as it appears on my notepad.

GI spring H buffer 367* intended weight

Sprinco blue H2 short cycles wolf 548
GI spring H2 does not 444
Failure point is somewhere in the middle at 496 (presumed)

15% below failure rate is ideal
(421)

367* 26% below failure rate of 496
405 18% below failure rate
429 14% below failure rate
452 9% below failure rate

GI spring H 367* intended weight
GI spring ST-T2 406
GI spring H2 444

Blue spring C 357
Blue spring H 452
Blue spring ST-T2 500

JP spring H 393
JP spring ST-T2 434

The setup I arrived at is the JP tuned and polished extra power buffer spring (7% stronger than USGI) and the Spikes ST-T2 buffer. This setup is a comfortable margin below the failure rate while still being 15% heavier than the stock setup. As a basis for comparison, M193 ammo is very roughly 10% “hotter” than bottom of the barrel (think wolf) .223 ammo. As an added benefit, both of these components offer a reduction in cycling noise, and the ST-T2 does not rattle when shaken like a standard buffer would. I should also mention that my rifle functions perfectly with whatever I feed it.

So there you have it. I realize the unscientificness of this study, so feel free to pick it apart.

2

Holy overthinking Batman!

A brief search here would’ve lead you to using a standard H buffer (I prefer an H2 in my 14.5" w/ my ammo choice) or an A5H3. The trial & error has already been done and all that info is here at your fingertips. It’s still dependent on an individual’s ammo selection so there’s varying setups.
I’m afraid your “calculations” may have lead you to the worst option based on the opinions of many here. There’s only a very small following for the Spikes set up. The majority of experts here steer away from it.

But hey, if it works for you, roll with it.

3

Wow. That’s a lot of mental masturbation, but nice work. I just have a bunch of blue Sprincos and a variety of buffers. Buffers are cheap. I use increasingly heavier weights until the rifle stops reliably cycling a given ammo. Then I use the next lighter buffer. 99.9% of the time, I think the choices are between an H or an H2, and maybe an H3, depending largely on the ammo (.223 vs 5.56) and the barrel length.

4

I like that you put the thought into it, but you don’t account for gas port size at all.

5

Nothing wrong with being anal.

6

The premise is flawed. The combination we are looking for is not spring rate x buffer weight. Using that formula, we would use lighter springs with heavier buffers and heavier springs with lighter buffers.

What we are looking for is a balance between spring rate and reciprocating mass which includes the BCG

7

As others have pointed out, you don’t have enough information (what ammo, gas port size - in other words, what force you’re trying to counter with the recoil spring/buffer) to perform your calculation.

From my T&E (trial and error :)) using various systems over the years, i came up with my own answer that works for everything reliably: vltor A5.

8

I agree with that. I have three A5-equipped rifles and all cycle flawlessly, in addition to mitigating the recoil impulse to at least some degree. No drama, no fiddling, no calculating, just wrench in the stock A5 from VLTOR and good to go.

9

That’s the route I’ve been taking also. I still have some rifles that are using an H or H2 but all my recent builds are A5 equipped.

10

I think you made it more complicated than necessary and as stated above didn’t get all of the variables correct. I recommend less math and more VLTOR A5.

11

I like the op’s thinking! BTW, your unit would be oz squared since you multiplied oz x oz.

12

With all other variables being the same, I have found that ambient temperature is one of the most influential factors on reliability. Guns like to run in warm and hot conditions, but everything changes when it gets really cold.

This is why military guns are gassed so hard, and work extremely well in the cold, while often exhibiting harsh recoiling characteristics in warmer temps. It’s as if the AR15 TDP guns were built to run optimally in extreme cold, when using 5.56 NATO ammunition.

13

Good point, and something the game guys who reload know well. Sometimes in the winter they don’t make their power factor.

Somewhere out there is temperature range the M-16/M-4 was designed to operate in. I bet a SME on here knows.

Some info:

http://firearmshistory.blogspot.com/2012/11/the-effect-of-temperature-on-ammunition.html

If someone has a copy of this doc, please post it:
Frankford Arsenal, “Study of the Temperature Effects on the Ballistic Performance of 5.56 mm Ammunition.”

14

The idea was/is to make it work within the parameters that they are presented with to achieve. If it fails to work at colder temps, then they would have to make it so it did so function, as long as it did not induce failures at warmer temps outside their “margin rate of failures”.

15

Would A5 vltor be the best set up for BCM BFH 14.5 ?
I’m
Looking at these parts now to go with my Mega billet lower and SD-E trigger and my 14.5 BCM BFH upper
THANKZ

Sent from my rocket ship using
My cell’y

16

Wow, you’re a lot smarter than me. I just went A5, bought a quality rifle with an properly sized gas port and forgot about it. Boring reliability.

17

I think mitigating recoil impulse is more important to a gamer rifle than an SD rifle. My priority is reliability across a wide range of ammo loadings and temperatures (we get yearly 100 degree temp swing from 90’s down to sub zero temps). Standard carbine spring and an H1 to help keep the BCG closed during lockup, and both my 16" middy’s run 100%; the 20" gets a standard rifle buffer. Sure they may have a smidge more recoil impulse than a finely tuned buffer and gas port, but I know it’ll run on damn near any ammo in any temp.

18

Mitigating recoil and operational span can be mutually exclusive. That does not mean that the two cannot overlap, they can, in many cases.
Going from one end of the spectrum to another in function may have better charactoristics than another depending on the user’s priority. I would tend to choose to prefer a system that has a possible wider span in use over recoil characteristics.