Buffers & Springs

[zeus100]

The 16” length vs the 14.5” length matters.
Barrel diameter at the gas port matters but 6920, m4 and m4a1 are all the same barrel diameter at the gas port.
0.625” barrels run smaller gas ports than 0.75” barrels.
0.875” and 0.936” barrels run bigger gas ports than 0.75” barrels.
It’s not ONLY the gas port size that restricts the gas but also the LENGTH of the gas port in the barrel which is why different barrel diameters at the gas port behave differently.

you are saying the difference in barrel thickness is another delta of gas timing?? now thars some good cunt hairs!!

[zeus100]

While the bolt is moving forward, the gun isn’t perfectly static.

hands r up..u got me...dang physics LAWS...

[556Cliff]

The 16” length vs the 14.5” length matters.
Barrel diameter at the gas port matters but 6920, m4 and m4a1 are all the same barrel diameter at the gas port.
0.625” barrels run smaller gas ports than 0.75” barrels.
0.875” and 0.936” barrels run bigger gas ports than 0.75” barrels.
It’s not ONLY the gas port size that restricts the gas but also the LENGTH of the gas port in the barrel which is why different barrel diameters at the gas port behave differently.

Hey Randall! I didn't know you still posted here! I had you reprofile feed ramps on a Brownells 20" A1 barrel for me to M4 profile at the very start of the year and I wanted to say you did an excellent job! Probably the nicest and smoothest looking set of feed ramps on any of the ARs I have.

I just put that barrel and upper together within this last week and sighted in the iron sights on it a couple of days ago. Just need to get the ACOG for it now.

[556Cliff]

Double post.

[lysander]

you are saying the difference in barrel thickness is another delta of gas timing?? now thars some good cunt hairs!!

Not that small an influence.

The gas port is essentially a small pipe with a diameter of the port, and a length equal to the wall thickness.

Flow through a pipe is highly dependent on the length of the pipe. The fluid constricts as it enters the pipe, reducing the maximum possible mass flow and and as it expands back up the pressure drops. The longer the pipe, both more the mass flow and the pressure drop. In an AR, the thicker the barrel wall, the lower the pressure in the gas tube, and the lower the mass flow. The speed is little effected, but the lower pressure and mass flow mean it takes longer for the carrier cavity to pressurize, and reduces the carrier's acceleration. This means unlocking will take longer.

[zeus100]

Not that small an influence.

The gas port is essentially a small pipe with a diameter of the port, and a length equal to the wall thickness.

Flow through a pipe is highly dependent on the length of the pipe. The fluid constricts as it enters the pipe, reducing the maximum possible mass flow and and as it expands back up the pressure drops. The longer the pipe, both more the mass flow and the pressure drop. In an AR, the thicker the barrel wall, the lower the pressure in the gas tube, and the lower the mass flow. The speed is little effected, but the lower pressure and mass flow mean it takes longer for the carrier cavity to pressurize, and reduces the carrier's acceleration. This means unlocking will take longer.

indeed, as a plumber and electrician i get the fluid dynamics of pipe size and length. ty for the illustrations. All my barrels are .75 diameter, so I think the wall thickness is not an issue. I do have a .625 on the shelf to build yet. thanks for the notable consideration.

I like how the buffer and spring thread got hijacked by the gas port..LOL

[Disciple]

3) Biasing springs - They completely reverse the intended purpose of the weights in the buffer. The whole point of the loose weights in the buffer, is that during the forward acceleration of the bolt during counter-recoil, the weights will, observing Newton’s Laws, slide to the back of the buffer body tube. Then when the bolt impacts the barrel extension, and begins to rebound off it, the buffer weights impact the front of the buffer body and the rebound is cancelled out. The physics of this can be demonstrated by Newton’s Cradle.

Biasing the weights forward means the weights will impact at the same time as everything else, and the carrier will rebound. Fortunately, is only an issue in full automatic fire, and even then is highly depended on ammunition and how the gun is held during firing, so most people won’t notice the problem.

I think we've talked about this before but the bias spring in the VLTOR buffer is very soft. I don't think the weights stay at the front (muzzle-end) of the buffer under acceleration from the action spring.

[prepare]

Any idea what the approximate amount of force the buffer strikes the end of the buffer tube is?

[zeus100]

Any idea what the approximate amount of force the buffer strikes the end of the buffer tube is?

I have been wondering the same. Does this infer that the design of mil spec carbine set ups is intended to fully compress the spring? my rubber end of the buffer protrudes about 3mm past the fully compressed spring, so is buffer on buffer tube impact intended and ok?
In pursuit of a lower than normal mass recoil set up, I consider this would likely increase speed and transfer of energy at the end of the tube

[1168]

The answer to the amount of force is “it depends”. Mainly on mass of the buffer, bolt carrier velocity, and spring rate. In a competition setup where avoiding sight movement during recoil is a top priority, light buffers are used, and the gas is turned down in combination with that, to minimize the impact in the bottom of the receiver extension, and at the end of the forward stroke returning to battery. This works well in a competition setting, where a gun may get shot a shitpile and get quite dirty, but won’t ingest sand or mud, and the ammo and gun can be tailored for each other. The competition gun is also unlikely to get limpwristed. The “tactical” gun needs to work under broader circumstances, and it is not necessary for you to be able to double-click the mouse with only one sight picture at 50 yards. Thus a buffer with more weight and more gas to drive it.

No, the spring is not meant to be “fully compressed” (solid height) in normal operation.

Yes, the bumper of the buffer should strike the bottom of the receiver extension, and it should not be made of rubber. A constant recoil system could exist, but would require a longer system, otherwise the gas key is likely to strike the bridge. The difference between “doesn’t lock back” and “strikes bottom of receiver extension” is what, like a quarter inch?