After you pull the trigger

[Failure2Stop]

Do you know why this is?

One reason that I know is gas port erosion, which is linked to the distance from the chamber to the port.

[taliv]

would have loved to see an extra line on that pressure curve graph fired with a suppressor

the date on that was '03, right? any progress made on ammo transferring heat somewhere besides the barrel in the last 5 years?

thx for posting, btw

[Outlander Systems]

This thread is screamin' for a "Sticky".

[taliv]

i'm not a mechanical/materials engineer, holiday inn, yada yada, but that statement regarding removing heat from the action being one of the functions of the brass has sort of captured my attention for the moment.

Specifically, I'm wondering how brass vs steel cased cartridges would perform relatively at that task.

according to this site
http://www.engineersedge.com/properties_of_metals.htm

The specific heat for brass is .096 compared to .110 btu/lb/*F for steel, meaning, if I recollect my high school physics correctly, that steel has about a 15% advantage in storing heat (or rather, it takes 15% more energy to heat steel to the same temp as brass)

but more interesting is the thermal conductivity, where brass is 69.33 btu/(hr-ft-F) compared to 8.11 for steel.

(no wonder it's so hard to anneal brass properly!)


my question is, don't these numbers mean the chamber would stay somewhat cooler firing steel-cased ammo than brass-cased ammo?

would that difference be worth someone manufacturing quality steel-cased ammo?

[Iraqgunz]

A little on gas port erosion. A few years back I was working at a facility that had several hundred BM carbines that were being used for training. One thing that we were noticing is that some of the weapons were having gas related mafunctions even after we replaced the rings, gas tubes, checked the carrier keys, etc...When we borescoped the barrels we saw that the gas ports were significantly enlarged and probably were the cause as the gas wasn't properly regulated.

At that point we began replacing the barrels and problem was solved.

One reason that I know is gas port erosion, which is linked to the distance from the chamber to the port.

[TooTall]

Hi all,

First time post here, but long time shooter and member of other forums.
Anyway, I came across this thread while Googling for info on gas port pressure.
Very interesting and detailed article, K.L. Davis, thanks.
I have a couple of questions regarding this article and some other related ones too.

...

Pressure from the port is regulated only by the size
of the gas port and the diameter of the barrel.

...

One other problem is that the bolt can cycle so fast
the magazine spring can not keep up with it and the
round stack is not properly aligned and forced back
into place before the bolt returns to batter --
therefore there is no new cartridge picked up and the
bolt closes on an empty chamber, this is what some
call "ghost loading", or bolt-over-base jams... this
is far worse in full auto fire as the bolt does
actually move faster in full auto than semi auto; this
is due to the fact that the top cartridge in the
magazine does not apply force to the bottom to the
bolt causing drag.

I can see how the port pressure would be affected by port diameter, but how would bore dia affect it? Is it a case of a larger bore creates a larger volume of gas to draw from? In which case, a larger bore gives greater port pressure?

Or do I have it backwards? Is it a situation where a .223 case necked up to 7mm as in the 7mm TCU has the same amount of powder, but is "spread thinner" in a larger bore? In which case, a 7mm TCU gives less pressure than a .223?

Or have I missed it completely?

Second, what do you mean by the FA being faster than the SA due to the round not applying drag to the bottom of the bolt in FA mode?
I assumed that the cycling is the same, only difference being the secondary sear does not hold back the hammer.


Next, I realize this is a M4/AR15 forum, so this next question is not really relevant, but sort of related to gas systems in general.
Does the size of the piston face make a difference? If a set amount of gas is tapped off a barrel, would a larger dia piston have more opening force than a smaller one? I would think so.

I have an interest in this stuff, so thanks to all that can offer answers.
Can anyone recommend a good technical book that explains gun design and internal ballistics?

Cheers,

Dean

[Bowser]

Makes me want to build a 20" for sure now. lol

[MistWolf]

Port size does not regulate pressure into the gas tube. It regulates flow (CFM, or Cubic Feet Per Minute) which determines how quickly the system fills and comes up to pressure.

Diameter of the piston that the gas applies force to does make a difference

[markm]

Makes me want to build a 20" for sure now. lol

Do it. Busting out the musket every now and again is a true pleasure. Nice smooth shooting with neat little piles of brass gently trinkling onto the dirt.

[glen]

I remember reading this before.

This was fundamental in helping me to understand the AR gas system. Thank you for posting this then and now.