AR-15 Bullet Travel vs Bolt Travel

[Nick Caprinolo]

Bullet leaves the barrel at 2700 FPS. It takes .0004 of a second to travel one foot. I'm no ballistic expert nor am I a mathematician, but it seams to me the bullet is leaving the barrel before the bolt even thinks about moving. Gas pressure is 50,000 PSI. By the time inertia of the bolt starts to move the bullet is a good way down range.

I've shot two, one hole three shot groups, with my cheap old AR15, 24 inch $170 barrel and expect to shoot a few more. I don't think that's possible if the bolt starts to move prior to the bullet leaving the barrel.

One of the above videos shows the bullet leaving the barrel before any recoil is started. Sometimes common sense trumps science.

Now I know there will be a lot of folks telling me I am wrong, maybe saying how stupid my theory is but that's the way I see it.

 

[Dud]

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

The gas actually pushes the bolt forward first (because it weighs less). This is why gas piston ARs can get jacked up, they have no gas to hold the bolt forward so the the cam pin can drag on the upper receiver.

It looks simple but every piece is doing things that are not obvious at first glance.

 

[skeeljc]

The first video PhilKeil posted (AR-15 firing in Slow Motion) shows the BCG travels rearward at 13 fps during the first inch of travel. Assuming a 24" barrel with a 12" gas port location and a 3000 fps muzzle velocity the bullet will take 0.000444 seconds to travel from the gas port to the muzzle. During that time the BCG will only move 0.06933 inches. This assumes the BCG accelerates instantly to 13 fps which is not the case. So the BCG will move less than 0.069 inches when the bullet exits the muzzle.

It is therefore my assumption that the movement of the BCG rearward has no effect on accuracy. I am betting the bolt is not unlocked when the bullet exits the muzzle.

 

[ebb]

It was posted that when the bullet leaves the muzzle pressure drops instantly. Nothing happens in this world instantly. If this statement were true how can a muzzle brake do any good. After the bullet leaves the barrel gases continue like the blast from a jet engine and add to felt recoil, the muzzle brake redirects the blast so it doesn't impact your shoulder.

 

[Dusty Stevens]

 

[PhilKeil]

The first video PhilKeil posted (AR-15 firing in Slow Motion) shows the BCG travels rearward at 13 fps during the first inch of travel. Assuming a 24" barrel with a 12" gas port location and a 3000 fps muzzle velocity the bullet will take 0.000444 seconds to travel from the gas port to the muzzle. During that time the BCG will only move 0.06933 inches. This assumes the BCG accelerates instantly to 13 fps which is not the case. So the BCG will move less than 0.069 inches when the bullet exits the muzzle.

It is therefore my assumption that the movement of the BCG rearward has no effect on accuracy. I am betting the bolt is not unlocked when the bullet exits the muzzle.

Please also keep in mind the pressure has to drop a little before the unlocking begins. If not the case would be still stuck to the chamber walls. I have seen systems setup and built wrong and bolt trying to unlock when the pressure is too high and ripping the rim completely of the case and the case getting stuck. To change systems the older H and K in 308 ( cant remember the model) had fluted chambers to allow the high pressure gas to flow backwards and assist in blowing out the case as well as reduce the amount of surface area for the case to grab upon firing.
Good luck in your endeavor. The ar 15 is a very accurate and very flexible system and quite frankly kinda genius. My only complaint is that the limiting factor is the magazine. Keeping to mag length is what limits you but not much if you work at it.
Now for another discussion what if you could run a AR 10 platform with a smaller cal bolt head and feed from a mag the longer heavy bullets seated/throated out long. Now the mag would not limit you.

 

[garandman]

It is therefore my assumption that the movement of the BCG rearward has no effect on accuracy. I am betting the bolt is not unlocked when the bullet exits the muzzle.

If that's so, why do ppl who are serious about accuracy eschew the semi auto?

 

[urbanrifleman]

If that's so, why do ppl who are serious about accuracy eschew the semi auto?

Serious about accuracy. Hmmm.

For one thing you can push the pressure much higher in a bolt gun. Much higher. And the pressure is captured in the mechanism. A semi simply can't take the stresses of a bolt gun.

Two, cycling the bolt causes the rifle to move and jump around.

Three, much harder on brass. Like a billion times harder.

Etc etc

Like a said, David Tubb dropped the semi to invent the Tubb Gun based on a semi and never once mentioned "it wasn't accurate enough" and he's pretty "serious" about accuracy.

Truthfully??? They could invent a semi tomorrow that would outshoot a BAT or a Panda and these "serious accuracy" guys wouldn't shoot it, because they shoot BATS and Pandas. Because they always have... and they would verbally fist fight anyone would said they weren't the best.

 

[Drop Port]

View attachment 1141843

What do you think he is thinking ?

Does gas leave the rectum before the lips start moving?

 

[garandman]

Serious about accuracy. Hmmm.


Truthfully??? They could invent a semi tomorrow that would outshoot a BAT or a Panda ....

If they could... why haven't they? They don't like money? They are being mean and holding out on us? Fact is... They can't. They never will. ***PART*** of that is due to the gas system, which is inherently less accurate than a bolt gun.

That's all I'm saying.

 

[dfrosch]

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

An adjustable gas system with shut off would work.

There was a guy here and on 68forums that used a Bear Creek side-charging action. He would buy barrels without a port and use the upper like a straight pull bolt gun. Last I remember, he had a 22 Nosler barrel that shot very good.

Just to screw with people at the range, he'd put a gas bock and tube anywhere on the barrel. He posted a pic with the gas block about an inch or two from the muzzle.

 

[msinc]

One thing that hasn't/isn't being considered here in regards to semi-gas gun vs. bolt gun accuracy is lock time.......consider the time it takes in milliseconds for the striker in a typical bolt gun to fall straight line directly .250" from the sear to the primer. Now consider how long it takes the hammer of an AR to swing in it's arc from sear position to eventually finally hit the striker {firing pin} and sent it another .250" or so {whatever it is, who cares at this point??} to finally eventually hit the primer. I really believe this substantial difference in time has way more to do with how accurate a gas gun can be than what happens to the rest of it after the bullet has left the muzzle.

 

[garandman]

There was a guy here and on 68forums that used a Bear Creek side-charging action. He would buy barrels without a port and use the upper like a straight pull bolt gun. Last I remember, he had a 22 Nosler barrel that shot very good.

Just to screw with people at the range, he'd put a gas bock and tube anywhere on the barrel. He posted a pic with the gas block about an inch or two from the muzzle.

That's prolly the best way to see how much the gas system affects accuracy. It'll have the same lock time as a functional semi, but w/o a functioning gas system, the best potential accuracy would be realized.

 

[dfrosch]

That's prolly the best way to see how much the gas system affects accuracy. It'll have the same lock time as a functional semi, but w/o a functioning gas system, the best potential accuracy would be realized.

http://forum.accurateshooter.com/threads/22-nosler-no-gas-ar.3974526/#post-37454386

 

[garandman]

Lastly.... (last attempt) once the bullet leaves the muzzle... the gas system will depressurize exponentially faster than mechanical parts (locking mechanism, buffer etc) will move. Logically, *some* movement occurs B4 the bullet leaves the barrel, while the system is still pressurized, creating enuf momentum to cycle the firearm *after* the bullet exits the muzzle.

How much? I doubt its even possible to know. I don't really care. Cuz I don't use my semis for extreme accuracy. Its " other people's problems."

 

[rwj]

Just to screw with people at the range, he'd put a gas bock and tube anywhere on the barrel. He posted a pic with the gas block about an inch or two from the muzzle.

Barrel tuner!

 

[Depart123]

This has been one of the most retarded threads that I've seen in a while.

Youtube had the answer within easy reach the whole time.

 

[skeeljc]

All I really needed was the bolt speed. It was buried in one of the You Tube videos. I guess I should have asked a better question.

 

[Dud]

Lastly.... (last attempt) once the bullet leaves the muzzle... the gas system will depressurize exponentially faster than mechanical parts (locking mechanism, buffer etc) will move. Logically, *some* movement occurs B4 the bullet leaves the barrel, while the system is still pressurized, creating enuf momentum to cycle the firearm *after* the bullet exits the muzzle.

How much? I doubt its even possible to know. I don't really care. Cuz I don't use my semis for extreme accuracy. Its " other people's problems."

Yes, some movement occurs but beside the gas key it is on the same Axis as the bullet so it does not introduce any bending forces, left/right or up/down. Maybe the bolt head unlocking introduces a bit of twist (unless it counters the rifling twist) but that's about it.

The lock time is an issue but Remington got it to zero with their electronic primers. I don't see match shooters resurrecting that system. There is a point where it is good enough and an AR is close to that.

 

[garandman]

Maybe the bolt head unlocking introduces a bit of twist (unless it counters the rifling twist) but that's about it.

In my accuracy guns, I'm going for zero movement.

Everyone else is free to do whatever they want.