Fake Gas Block on the 20" 450 B Upper?

[Texas Sheepdawg]

Being a water guy and an old smelly firefighter, let me
Throw these thoughts in.
Water does not compress.
Air and gases do.
Then there is tube diameter.
Port diameter, (hole in barrel),
Apposing spring load on the carrier.
Then there is friction loss and turbulence.
How do these factors change
As the tube gets longer -vs- moving the gas block?
But seems to me there would be a measurable
Timing difference from the time the bullet passes the
Gas port up until it leaves the barrel.

[crewdog]

Hi Gang,

I just decided to build my 1st AR in 450 and ordered the 20" from Midway "On Sale" (if they have not oversold like last week*%$#@!)!!
So..... I'm in the learning stages and need info please!
1) What is the diameter of the 450 Barrel?
2) I'm an Iron Sight" Guy so just what are my options with this Fake Gas Block that looks like they "Sawed Off"" the AR sight posts?!
3) Any sugestions on sights and/or a replacement gas block with rails!?
4) Where can I get the latest reloading info and why has all the data I've seen so list 44mag powders like 296 instead of slower rifle powders?
Thanks & Merry Christmas

[Siringo]

This gets into a lot of physics. There are many variables -- like velocity of the projectile, port pressure, size of port, length of gas tube, weight of carrier, dwell time to unlock, ect.

Here is my take on this:

If you move the gas port 4 inches further down the barrel and assuming the bullet is traveling 1500 fps between those two points (actually it is still accelerating), the time to transverse that 4 inches distance is .00022 seconds. I do not know what the gas pressure is at that point, but lets just say it is 10,000 psi, the sudden release of the gas in the empty unpressurized gas tube is much, much more than 1500 fps -- remember at that point it is not pushing on anything and is unrestricted until it hits the carrier key. For arguments sake -- let say the gas expands/escapes at a 4,000 fps (NRA Fact Book) That means in 4 inches, the gas would transverse that distance in .000083 seconds. In that amount of time, at projectile traveling at 1500 fps would have covered about 1.5 inches.

So -- in a nut shell, 4 inches of extra gas tube does not equate to moving the gas port down the barrel 4 inches. There are many other factors to consider.

NOTE: MY NUMBERS AND CALCULATIONS ARE ONLY MEANT TO BE ONLY REPRESENTATIVE AND COULD BE INCORRECT OR INACCURATE.

[wildcatter]

This gets into a lot of physics. There are many variables -- like velocity of the projectile, port pressure, size of port, length of gas tube, weight of carrier, dwell time to unlock, ect.

Here is my take on this:

If you move the gas port 4 inches further down the barrel and assuming the bullet is traveling 1500 fps between those two points (actually it is still accelerating), the time to transverse that 4 inches distance is .00022 seconds. I do not know what the gas pressure is at that point, but lets just say it is 10,000 psi, the sudden release of the gas in the empty unpressurized gas tube is much, much more than 1500 fps -- remember at that point it is not pushing on anything and is unrestricted until it hits the carrier key. For arguments sake -- let say the gas expands/escapes at a 4,000 fps (NRA Fact Book) That means in 4 inches, the gas would transverse that distance in .000083 seconds. In that amount of time, at projectile traveling at 1500 fps would have covered about 1.5 inches.

So -- in a nut shell, 4 inches of extra gas tube does not equate to moving the gas port down the barrel 4 inches. There are many other factors to consider.

NOTE: MY NUMBERS AND CALCULATIONS ARE ONLY MEANT TO BE ONLY REPRESENTATIVE AND COULD BE INCORRECT OR INACCURATE. Ding, Ding, Ding, I was afraid I'd be too vague, and picking apart my thesis would be all to easy, which is why I wanted some literary license. I could have been the Mad Scientist and then I'd really have the Mob Howling. All the observations are true, to a point. However, compressed or not, the moment the gases start to move into the tube, other gases are moving out and onto the carrier key, at the same moment. Which is why no extra length of gas tube can change the timing..t

[BD1]

I'm no AR15 expert, but I'm gonna make an attempt at putting this in fairly simple terms as I understand it.

I think the use of "timing" in the terminology is leading to some confusion. The "timing" has next to nothing to due with how far the gas has to travel before it impinges on the bolt, or how long exactly it takes to get there. The "timing" has to due with what the pressure in the barrel is at the point that the bullet heel passes the gas port. When this happens, the pressure needs to be low enough that the brass case has relaxed back from the chamber walls, and at the same time there needs to be enough pressure left to do the work of unlocking the bolt and driving the bolt and carrier back against the force of the buffer spring. When the bullet exits the barrel, the pressure drops off to near ambient almost immediately. So the port can't be all the way at the end of the barrel or the bolt will short stoke. There needs to be some barrel travel left to cover the time that the bolt and carrier move rearward.

The pressure curve produced by a faster powder, (generally used with lighter bullets), peaks earlier in the bullets travel, and then drops off more quickly, requiring a gas port closer to the chamber. The pressure curve of a slower powder, (generally used with heavier bullets), builds more slowly and may not drop off enough for the case to release from the chamber walls until the bullet is near the muzzle end of the barrel, requiring the port to be closer to the muzzle. To get the action to operate reliably requires the coordination of barrel length, port location and powder speed. Valves, the length of the gas tube and the diameter of the port can be used to tweak the system, but if the basic relationships are not optimized to the particular application, you're fighting an uphill battle to get reliable operation.

My own opinion is that we'll wind up with more than one port, and some simple method to switch between them.

BD

[Texas Sheepdawg]

I would love to have an AR engineer weigh in on this thread.
Everyone has great points and observations. And it seems the more
We get into this discussion, there are more questions.
I find the science of gas operated and recoil operated auto-loading
Firearms fascinating. I have a Remington Model 11 12 gauge shotgun.
It is a recoil operated auto-loader. This design/shotgun was first
Made in 1909. The barrel recoils back into the receiver to eject
the spent shell and Cycle into the chamber the next shell. It's COOL!

[Hoot]

This is a great thread!

Being able to share both opinion and knowledge among you peers, in a civil discussion, is what justifies the existence of forums like this.

Tim, have you developed enough pressure curve samples to put together a chart like this one I stumbled upon, except for the 450b instead of the .223?
Include a trace for factory 250gr and one of your heavy, slow, experimental loads to show how their pressure buildup actually differs. It sure would help "remove the scales from our eyes".



Thanks,

Hoot

[wildcatter]

Yup Hoot, I got'em, but my computer that contains such data has gone on the fritz. Hopefully today we'll have it running, supposedly the only thing we need now is a particular driver.

Listen, Thank you for your observation of our civility. Because we are such gentlemen here, we are actually getting much more work done and having fun all the while.

TxSheep dog, I am a Ballistics Engineer and I'm trying not to muddle the waters with high end hifalutin, ever more confusing, explanations, but I gotta tell ya, BD has made a valiant stab at it and has done a better job of it than I.

Lemme try again. Move the gas block up and down the barrel changes when the bolt opens. At all those locations various size holes are needed to energize the system and talk into it into opening, at the right moment. Everything at that point is a factor, port size, location, bullet weight, powder burn rate, powder amount, everything else. There is no easy route here and no pat answers either. You'd think you could model the results and expect performance, but alas, that's not to be. Experimentation is the only final route. You don't know what want, until you need it, or as my retired GM Hot-Rod Experimental Engineer Father-in-Law says,.."Model everything you want, but on Race Day, we'll need something else". In other words, all the lab goodies can ever replace, good solid road work..t

[Texas Sheepdawg]

I've been on some forums in my time and I
Gotta say this one is awesome and educational.
I have literally spent more time reading here than
I do the news paper, facebook, and Fox News.
.....guess I'm a 450 Bushmaster junkie now....

[wildcatter]

This is a great thread!

Being able to share both opinion and knowledge among you peers, in a civil discussion, is what justifies the existence of forums like this.

Tim, have you developed enough pressure curve samples to put together a chart like this one I stumbled upon, except for the 450b instead of the .223?
Include a trace for factory 250gr and one of your heavy, slow, experimental loads to show how their pressure buildup actually differs. It sure would help "remove the scales from our eyes".

[ http://ar15barrels.com/gfx/223plot.gif ]

Thanks,

Hoot

Just a little something to consider.

In looking at your diagram, which is a GI generated data, there is what is called.."Common gas system locations showing peek gas port pressure". What it does not show is how much gas (in PSI) is being generated at those locations or how big the gas port is, two things, which are vital to know, if you want the system to function well..t