AR-15 Bullet Travel vs Bolt Travel

[Doug Beach]

Gas guns have at least two significant problems achieving consistent precision, that don’t exist in precision bolt actions.
1. The gas system is attached to the barrel, providing a point of contact and subsequent harmonics that are not present in a floated barrel.
2. There is movement of the action, prior to bullet exit (the bolt doesn’t open, but things have commenced to move). Any variation in this movement from shot to shot will have a detrimental effect.

Service rifle scores & records in the M14 & M16 era show that these rifles can be very precise indeed, but fall short of a precision bolt gun.

 

[garandman]

Gas guns have at least two significant problems achieving consistent precision, that don’t exist in precision bolt actions.
1. The gas system is attached to the barrel, providing a point of contact and subsequent harmonics that are not present in a floated barrel.
2. There is movement of the action, prior to bullet exit (the bolt doesn’t open, but things have commenced to move). Any variation in this movement from shot to shot will have a detrimental effect.

Service rifle scores & records in the M14 & M16 era show that these rifles can be very precise indeed, but fall short of a precision bolt gun.

Yup. It amazes me this is controversial, and not universally known / accepted. That this discussion is even happenning.

 

[Park ranger]

Brandon green doesn’t seem to be too concerned about any of this. He will out shoot most of us with an M16A2 even if we had a 6.5 wizbang and a 100-600 x520 March scope.

 

[Doug Beach]

Brandon green doesn’t seem to be too concerned about any of this. He will out shoot most of us with an M16A2 even if we had a 6.5 wizbang and a 100-600 x520 March scope.

We talk a lot about the arrows, on this site. Not so much about the archers.

 

[skeeljc]

Gas guns have at least two significant problems achieving consistent precision, that don’t exist in precision bolt actions.
1. The gas system is attached to the barrel, providing a point of contact and subsequent harmonics that are not present in a floated barrel.
2. There is movement of the action, prior to bullet exit (the bolt doesn’t open, but things have commenced to move). Any variation in this movement from shot to shot will have a detrimental effect.

Service rifle scores & records in the M14 & M16 era show that these rifles can be very precise indeed, but fall short of a precision bolt gun.

Here are a couple more to add to the list:
1. The fit of the upper receiver to the lower receiver can allow movement between the two.
2. The bolt carrier fit to the upper receiver is always a little loose allowing it to move in the receiver.
3. Trigger lock time is much greater in an AR-15 than a bolt gun.

 

[rammac]

Yup. It amazes me this is controversial, and not universally known / accepted. That this discussion is even happenning.

It amazes me how wrong you are and that nobody has proven it yet. I haven't found similar time information for an AR yet but if I do I will update this post. But in general, all U.S. military designed gas operated rifles will NOT open the bolt until after the bullet leaves the barrel. ALL of the action movement is after the bullet is several calibers past the muzzle. ALL of the bolt movement is directly created by momentum, not gas pressure. Gas pressure simply applies the energy that creates the inertia that starts the action's movement.

Data Item ..........................................................M1 Rifle ..............................M14 Rifle
Type of gas system .............................................gas impingement ...................gas cut-off and expansion
Location of gas port from muzzle ..........................1.5 “ .....................................8.0 “
Gas port diameter ...............................................0.0793 “ ................................0.0768 “
Barrel length ......................................................24 “ ......................................22 “
End of hammer fall ..............................................0 milliseconds ........................0 milliseconds
Bullet passes gas port .......................................1.31 milliseconds ...................1.01 milliseconds
Bullet clears muzzle ...........................................1.36 milliseconds ...................1.25 milliseconds
Initiation of operating rod recoil ...........................1.58 milliseconds ...................1.53 milliseconds
End of operating rod dwell ...................................2.89 milliseconds ..................3.57 milliseconds
Completion of bolt unlocking ...............................4.07 milliseconds ..................5.00 milliseconds
Amount of bullet travel at start of op rod recoil ......7.2 “ past the muzzle .............9.2 “ past the muzzle
Position of bullet at end of op rod dwell .............50.2 “ past the muzzle ..........76.1 “ past the muzzle
Position of bullet at completion of bolt unlocking .....88.9 “ past the muzzle ...........123.0 “ past the muzzle
Average bullet velocity ........................................2735 feet per second .............2733 feet per second

Reference: Springfield Armory Technical Note SA-TN11-1094 dated 16 Dec 1957

 

[urbanrifleman]

It amazes me how wrong you are and that nobody has proven it yet. I haven't found similar time information for an AR yet but if I do I will update this post. But in general, all U.S. military designed gas operated rifles will NOT open the bolt until after the bullet leaves the barrel. ALL of the action movement is after the bullet is several calibers past the muzzle. ALL of the bolt movement is directly created by momentum, not gas pressure. Gas pressure simply applies the energy that creates the inertia that starts the action's movement.

Data Item ..........................................................M1 Rifle ..............................M14 Rifle
Type of gas system .............................................gas impingement ...................gas cut-off and expansion
Location of gas port from muzzle ..........................1.5 “ .....................................8.0 “
Gas port diameter ...............................................0.0793 “ ................................0.0768 “
Barrel length ......................................................24 “ ......................................22 “
End of hammer fall ..............................................0 milliseconds ........................0 milliseconds
Bullet passes gas port .......................................1.31 milliseconds ...................1.01 milliseconds
Bullet clears muzzle ...........................................1.36 milliseconds ...................1.25 milliseconds
Initiation of operating rod recoil ...........................1.58 milliseconds ...................1.53 milliseconds
End of operating rod dwell ...................................2.89 milliseconds ..................3.57 milliseconds
Completion of bolt unlocking ...............................4.07 milliseconds ..................5.00 milliseconds
Amount of bullet travel at start of op rod recoil ......7.2 “ past the muzzle .............9.2 “ past the muzzle
Position of bullet at end of op rod dwell .............50.2 “ past the muzzle ..........76.1 “ past the muzzle
Position of bullet at completion of bolt unlocking .....88.9 “ past the muzzle ...........123.0 “ past the muzzle
Average bullet velocity ........................................2735 feet per second .............2733 feet per second

Reference: Springfield Armory Technical Note SA-TN11-1094 dated 16 Dec 1957

Cool info. Sweet.

 

[garandman]

It amazes me how wrong you are and that nobody has proven it yet. I haven't found similar time information for an AR yet but if I do I will update this post. But in general, all U.S. military designed gas operated rifles will NOT open the bolt until after the bullet leaves the barrel.

For one thing, an op rod is an *entirely* different operating system than DI gas impingement. Your cool info is totally irrelevant.

Secondly.... You should read my posts more carefully. I'm not gonna re-re-re-re- post them. Until you do.... you have a great day, OK there?

 

[Dud]

Not really, direct impingement is just a short stroke gas piston with some plumbing added. An op-rod is just a weird long stroke gas piston.

I would hazard to guess that the AR15 does better than the M14 in most regards. Everything is lighter and coaxial.

It's not a perfect example but I think it's pretty easy to make an AR15 more accurate than the most accurate Mini14 that you can make. The same goes for an LR308 vs an M1A/M14.

 

[garandman]

This thread is a botched abortion. I'm out. Argue it amongst yourselves.

 

[Dud]

There is agreement amongst ourselves...

 

[Fast14riot]

Skip ahead to 10:00 for the sequence of operation.

 

[garandman]

Skip ahead to 10:00 for the sequence of operation.

Not even gonna waste my time explaining why that video proves me right....

 

[jb]

if the bullet is still in the barrel when the bolt starts to open how can a grand work?

 

[urbanrifleman]

It's pretty simple math in the case of direct impingement. The longer the gas system relative to the end of the barrel the longer the DELAY before the gas hits the bolt. If the gas system is 20 inches then the gas has to travel BACK 20 inches before the pulse hits the bolt. The gas has to travel 40 inches.

If the barrel were only 22 inches for example, it is pretty obvious that bullet would be long gone before that pulse got back to the reciever.

And this is well known.

 

[msinc]

This is an interesting thread to me. Those that don't "approve" of it have a remedy....go away!!! Just like if you feel no one is getting your point, maybe it's time to stop trying.
Consider the gas trap Garand. These second gen attempts to make a viable semi-auto came about because; 1. they didn't have a way to prove or disprove at the time {late 30's} exactly what was happening when and, 2. They knew the gun functioned, but believed that too much gas was getting bled off and costing velocity, which at the time wasn't an easy task to accurately measure either.
Somehow prior to the war they realized that the bullet was long gone and velocity did not degrade due to having a gas port in the barrel.
I believe that whatever movement might be present due to gas beginning to pressurize the operating system immediately following the bullet passing by the port has to pale in comparison to: A. trigger movement and pressure caused by breaking the sear, B. the ridiculously large heavy hammer swinging in a 2" arc to slam the back of an unsupported {in the rear} bolt.
Now consider that a 700 with a speed lock type spring and striker has the bullet several inches out of the muzzle before the primer is hit in a stock 700 assuming exact same time of sear break. Now compare this to an AR or Garand with it's pathetic lock time and the target must be getting hit by the time the hammer makes contact with the striker. We're talking milliseconds, and accuracy affect in either case depends alot on how still and secure the guns can be held.
There are a lot of ways to say the same thing but sound different in this thread and Guffey, the undisputed word game king, would have had a field day!!!

 

[rammac]

For one thing, an op rod is an *entirely* different operating system than DI gas impingement. Your cool info is totally irrelevant.

Secondly.... You should read my posts more carefully. I'm not gonna re-re-re-re- post them. Until you do.... you have a great day, OK there?

No, the M14 and M16 are both actually considered piston systems even though the M16 has acquired the improper identification of a "gas impingement" system. Both rifles have a piston, an actual piston shaped part in the M14 and the bolt carrier in the M16. In so far as physics goes, there is no difference between the systems other than the M14 having a connecting rod to transfer energy to the bolt rather than a gas tube, gas key, and bolt carrier. A piston is simply a part that gasses push against to convert gas pressure in to physical work, usually measured in some kind of pounds of force. The moving parts, once they start to move, will continue to move due to momentum until some counter force (usually friction or spring pressure in this case) slows and eventually stops their movement.

In the case of the M16 the gasses push on the bolt carrier and it converts the gas pressure to work, that is why it is considered a piston. The bolt carrier receives gas from the gas tube/gas key and that gas expands inside the chamber between the carrier and the tail-side of the bolt. The carrier starts to move (since the bolt is locked) backwards and acts like a piston. As it moves backwards it rotates the cam pin which, when rotated fully, unlocks the bolt and pulls it rearward.

Even if these were completely different systems my data isn't irrelevant because the military uses the same theories to design all of their gas operated systems, namely that they wont open the bolt until the bullet leaves the bore. When the bullet is in the bore you will have gas pressures that vary from about 60,000 PSI down to around 30,000 PSI, depending on the rifle, and if the bolt were to open at those pressures you would loose body parts pretty quickly. Most gas operated rifles vent excess gasses before the bolt opens because even when the bullet leaves the bore there will still be pressures of several hundred pounds inside.

By the way, your comment about that video makes no sense to me, what is it that you think it proves? I found it interesting that in that video at around 10:25 they show the bullet moving past the muzzle before the gasses even get to the bolt carrier and the bolt opening long after the bullet has left the muzzle.

 

[garandman]

@rammac ... thanx for a detailed reply. But I'm done, here. Experience has taught be to quit trying this discussion in the public forum. If you wish to know what I meant about the video, I'll invite you to IM me, and I'll explain privately. That way I don't bug you or anyone else further, here.

Thanx again.

 

[PDS]

Now consider that a 700 with a speed lock type spring and striker has the bullet several inches out of the muzzle before the primer is hit in a stock 700 assuming exact same time of sear break. Now compare this to an AR or Garand with it's pathetic lock time and the target must be getting hit by the time the hammer makes contact with the striker.

I'm having a hard time understanding this but then my lock time is not as quick as it used to be.

 

[Shawnba67]

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.

He should put it an inch past the muzzle! Tell people its a Creed and that was only way to keep the gun from voting democrat!