Anyone using any of the new BCM.....?

[bugsy714]

FYI "Receiver Extension" is the proper term for "buffer tube".

I knew that was something lol


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[prepare]

Would you mind disassembling your buffer and taking pictures? There are several different spring configurations in the patent and I wonder which they are using presently.

884ED8D0-E464-4E09-A41F-48C09D1AE3F6.jpeg

[Disciple]

Thank you!

[prepare]

This shows the modified bumper with an over travel stop to prevent the biasing spring from fully/over compressing.

BCM M2 Buffer Diagram 2.jpg

[markm]

884ED8D0-E464-4E09-A41F-48C09D1AE3F6.jpeg

That is weird.

[prepare]

That is weird.

What's weird? Same principle as the VLTOR A5 but with a larger diameter biasing spring.

[17K]

That is weird.

That is the result of having to change enough pieces significantly enough to get a separate patent from the A5.

[markm]

What's weird? Same principle as the VLTOR A5 but with a larger diameter biasing spring.

Dramatically different. I'm not big on the bias spring after weighing all the opinions here. (I'm not pulling any more out, just not convinced of its value)

[BufordTJustice]

Dramatically different. I'm not big on the bias spring after weighing all the opinions here. (I'm not pulling any more out, just not convinced of its value)

The biasing spring is what linearizes (and stabilizes) carrier velocity during the bolt-unlocking portion of rearward carrier travel. Which is the first 1/2" of carrier travel if memory serves. I'm sure Clint or Constructor or Lysander (or others) have the exact figure.

By forcing all the weights to the front of the buffer body when at-rest, this functionally increases carrier mass for every shot as opposed to a normal buffer where the weights could be in any number of arrangements of positions within the buffer body from shot to shot.

So, without the biasing spring, aggregate carrier mass during the unlocking phase (nominally 9.4oz IIRC, the mass of the bolt and cam pin are subtracted because they are longitudinally stationary until the cam pin reaches the front terminus of the cam pin track in the carrier; the bolt and cam pin are only rotating axially during this phase) can vary by the entire mass of the weight stack inside the buffer from shot-to-shot. I.e. a 4.6oz H2 carbine buffer can contribute a max of that mass to the overall mass of the carrier (during unlocking), and as little as .7oz (buffer body, roll pin, & delrin bumper), from shot to shot. So, shot #1 could see an aggregate mass of ~14oz and shot #2 could see 10.1oz total mass during unlocking, and so on. This becomes even more critical when shooting suppressed.

With the biasing spring, your carrier mass at-unlock sees very little variability, if any. I imagine Eric has done the math on what force it would take to displace the weight stack against that biasing spring.... and that not much else aside from a weapon-drop on the buttstock would cause the full weight stack to remain fully reward for any length of time. I.e., under most nominal conditions, there isn't any mass variability during unlocking.

Mass variability makes it more difficult to tune mass /or/ gas for any given gun as you've got these large variability bars at each end of your graph-of-function. As I stated above, things get even crazier with a can.

A biasing spring is certainly not mandatory for function (obviously) and speaks well of the overall design of the AR platform that it can accommodate this level of uncertainty and still function as well as it does. But it's an improvement. And the new BCM biasing spring further improves that concept with a lower K value for the biasing spring itself.


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[17K]

The biasing spring is what linearizes (and stabilizes) carrier velocity during the bolt-unlocking portion of rearward carrier travel. Which is the first 1/2" of carrier travel if memory serves. I'm sure Clint or Constructor or Lysander (or others) have the exact figure.

By forcing all the weights to the front of the buffer body when at-rest, this functionally increases carrier mass for every shot as opposed to a normal buffer where the weights could be in any number of arrangements of positions within the buffer body from shot to shot.

So, without the biasing spring, aggregate carrier mass during the unlocking phase (nominally 9.4oz IIRC, the mass of the bolt and cam pin are subtracted because they are longitudinally stationary until the cam pin reaches the front terminus of the cam pin track in the carrier; the bolt and cam pin are only rotating axially during this phase) can vary by the entire mass of the weight stack inside the buffer from shot-to-shot. I.e. a 4.6oz H2 carbine buffer can contribute a max of that mass to the overall mass of the carrier (during unlocking), and as little as .7oz (buffer body, roll pin, & delrin bumper), from shot to shot. So, shot #1 could see an aggregate mass of ~14oz and shot #2 could see 10.1oz total mass during unlocking, and so on. This becomes even more critical when shooting suppressed.

With the biasing spring, your carrier mass at-unlock sees very little variability, if any. I imagine Eric has done the math on what force it would take to displace the weight stack against that biasing spring.... and that not much else aside from a weapon-drop on the buttstock would cause the full weight stack to remain fully reward for any length of time. I.e., under most nominal conditions, there isn't any mass variability during unlocking.

Mass variability makes it more difficult to tune mass /or/ gas for any given gun as you've got these large variability bars at each end of your graph-of-function. As I stated above, things get even crazier with a can.

A biasing spring is certainly not mandatory for function (obviously) and speaks well of the overall design of the AR platform that it can accommodate this level of uncertainty and still function as well as it does. But it's an improvement. And the new BCM biasing spring further improves that concept with a lower K value for the biasing spring itself.


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What prevents recoil from placing the weights at the front of the buffer?