AR-15 Bullet Travel vs Bolt Travel

[garandman]

@skeeljc ... told ya.

 

[Doug Beach]

Have a nice night.

You too.

 

[velocette]

A bit of logic here. With a 24 " barrel and the gas port at 14", the bullet is travelling 2000 fps. In less than 1/2000 of a second +/- after it passes the gas port it will exit the barrel . Do you think that the gas system will pressurize, the bolt carrier will begin to move far enough to begin unlocking in 1/2000th of a second? In fact, all that needs happen is that the bolt carrier must get a good shove to start moving, inertia will take care of the rest.

 

[garandman]

Do you think that the gas system will pressurize, the bolt carrier will begin to move far enough to begin unlocking in 1/2000th of a second? .

Yes. It will. It must. For reasons I've repeatedly explained. Only a pressurized system cycles. And gas always follows the path of least resistance... which is the bore, not the gas port in the barrel / gas tube.

 

[skeeljc]

I suppose one could remove the gas tube and cap the gas port and see if there is a change in accuracy.

 

[garandman]

I suppose one could remove the gas tube and cap the gas port and see if there is a change in accuracy.

Best idea yet. I doubt the rifle would even cycle / unlock the bolt.

An adjustable gas system with shut off would work.

 

[Park ranger]

there is so much wrong here. But it doesn’t matter

 

[Dud]

One thing to keep in mind is that you are dealing with supersonic flows and compressible gasses.

I think the gas system is partially pressurized before the bullet passes the port because the pressure wave ahead of the bullet can't make it out of the muzzle entirely before the bullet gets to the port. It's probably not enough pressure to push the bolt forward (which is the first move the BCG makes) but I believe it is more than zero pressure.

Once the bullet leaves the barrel, the reverse happens. There is thousands of pounds of pressure in the gas system. With PV=NRT, that's a lot of gas and the only ways out are through an 0.080"ish gas port or to push the bolt forward and kick the carrier back.

One thing that will tell you exactly what is going on is that suppressed ARs run dirty and that is because the bolt moves the gas key back past the end of the gas tube before the pressure in the barrel and suppressor bleeds off. The same thing happens in blowback ARs but that's because the bolt opening is not delayed at all so GSR dumps out of the chamber.

In a properly gassed AR, the bolt carrier doesn't uncover the back of the gas tube before the bullet leaves the barrel. If it did, it would be running super dirty like a suppressed gun.

That's a long way from bottoming out the buffer in the tube.

Remember, it's really just a short stroke gas piston system but the piston is the back of the bolt.

 

[dolek]

there is so much wrong here. But it doesn’t matter

+ 1...thread should be deleted.Total BS speculation and outright BS...sad.

 

[skeeljc]

The gas tube engages the gas key for less than an inch. When the bolt carrier moves rearward less than an inch the gas pressure is vented into the action.

The reason shorter barreled AR's have shorter gas tubes is to allow enough time for the gas pressure to build to a sufficient level of pressure to operate the action. The 7 to 12 inches from the gas port to the muzzle is required to generate adequate gas pressure. Therefore the bullet must still be in the barrel when the bolt unlocks.

 

[PhilKeil]

I was surprised about the amount of gas exiting the carrier before it ever moves

I think the bullet is long gone before the carrier moves see slow mo videos below

I dont believe the best accuracy is achieved if the buffer is crashing into the end of the tube. The most accurate AR's i have shot have it balanced out perfectly or shooting as a single shot. Not to mention when the balance is right they are very smooth. I built a AR with a Shillen barrel at 100yds has given me a number of groups in the .17 range. My AR in 6 fatrat with a Kreiger is close to that if not better. But I know a great BR gun will agg in the 0's but not bad for a gas gun

 

[mike mccormick]

A couple points here....the gas unlocks the bolt first, before the carrier moves. There is still a rapidly declining amount of pressure in the barrel at that time. This residual pressure is what propels the carrier rearward. This happens at such speeds that the bullet has exited the barrel before any significant carrier movement has occurred

 

[skeeljc]

I have taken enough courses in physics, statics, dynamics and gas dynamics to figure this out but that would require too many hours of calculation. Somebody must have already figured this out but maybe they never shared it.

 

[skeeljc]

A couple points here....the gas unlocks the bolt first, before the carrier moves. There is still a rapidly declining amount of pressure in the barrel at that time. This residual pressure is what propels the carrier rearward. This happens at such speeds that the bullet has exited the barrel before any significant carrier movement has occurred

The bolt carrier must move first to unlock the bolt. The cam works only when the bolt carrier moves relative to the bolt.

 

[PhilKeil]

Dont the gas rings seal the bolt driving it back in the carrier 1st. this will unlock it then the carrier can move back to do its thing. That is why there is gas coming out of the carrier vent holes in the videos above

 

[USMCDOC]

If your buffer bottoms out in the tube, your AR is over gassed. The idea behind an adjustable gas block is to constrict the gas flow so that it moves the buffer just enough to cycle the action, and no more.

If your bolt is catching the brass before it clears the port.. it is also "over gassed"

 

[XTR]

 

[Rustytigwire]

PhilKeil
Center video at 2:25 6 7 look at the chunks of brass separate from cartridge during ejection.
The movement of gas after bullet exit of muzzle is not instant zero pressure. Gas has mass therefore takes time to get where it is going.

 

[PhilKeil]

I saw that Rustytigwire.

 

[Zero333]

The bullet is gonna exit the muzzle long before the bolt starts moving.

According to Quickload,... from the time the chamber reaches 10 % of it's max pressure to the time the bullet exits the muzzle is around 1 to 1.5 milliseconds.

so... IF I had to guess.... I'd say a 75 gr bullet would exit the muzzle of a 24" AR-15 in less than 0.002 seconds.