Over Gassed FTE?

[mrmackc]

47 minutes ago, 308kiwi said:

Ditto to what 98Z5V said.

I would go as far as to say that unless some component involved in the initial extraction of the case from the chamber, eg extractor, has failed then your extraction issues are due to the bolt groups initial unlocking/extraction being too violent.

Adding extra weight into the system, heavy buffer, simply delays the rearward travel of the BCG by a few milliseconds allowing the case to relieve itself from the chamber a little more and the gas pressures to decrease ever so slightly, it is a solution but you need to fix the cause, not the symptom.

Sort your gas system first and you'll likely find your problems will go away.

Quite true, one of the oddities of the gas impengiment system is that the gas does not have a mechanical linkage, such as the other gas operating systems do.   In this system in order to slow the effect of combustion pressure from the bore, the very high pressure must be controlled by orifice size of the gas port and the relative low volume of the gas tube,  distance from the chamber to the gas outlet port, gas key and the bolt.  The bolt rings also provides some delay in bolt unlocking (the key to successful extraction) the recoil spring pressure on the bolt is minimal compared to the other factors. Mainly the recoil spring energy absorption is as it's name implies, to absorb the rearward movement and enable deceleration of the BCG, which is important to extraction and ejection. All of these factors must occur in a finely orchestrated fashion, in my humble opinion.

 

[gdonley308]

12 minutes ago, mrmackc said:

Quite true, one of the oddities of the gas impengiment system is that the gas does not have a mechanical linkage, such as the other gas operating systems do.   In this system in order to slow the effect of combustion pressure from the bore, the very high pressure must be controlled by orifice size of the gas port and the relative low volume of the gas tube,  distance from the chamber to the gas outlet port, gas key and the bolt.  The bolt rings also provides some delay in bolt unlocking (the key to successful extraction) the recoil spring pressure on the bolt is minimal compared to the other factors. Mainly the recoil spring energy absorption is as it's name implies, to absorb the rearward movement and enable deceleration of the BCG, which is important to extraction and ejection. All of these factors must occur in a finely orchestrated fashion, in my humble opinion.

 

Well, you people are very helpful!! I have decided, when I get Home, to chase the threads in the adjustment port, buy a new adjustment screw and a locking screw. Then I’ll go through the steps Armalite recommends for tuning. Screw the gas regulation screw all the way in then back it off 3 turns. I’ll use the Federal 168gr Gold Match to tune it with. The reason I wanted a front mount adjustment screw was because the factory screw wasn’t directly accessible through the KeyMod hand hard. Decided I’ll just relieve that one hole in the hand hard slightly to allow a good straight access to the screw. From the general consensus here, seems like an over gassed issue. 

[308kiwi]

And if you really want some bedtime reading that's guaranteed to put you to sleep have a read of the info on the linked page, it deals with calculating pressure/flow/volume/velocity drops across orifices. (it deals with fluid flow but the same calculations can be extrapolated to get an idea of the drops through a gas port).

And when you get your head around all the info, then bear in mind that by fitting an adjustable gas block you are compounding the calculations as you now have 2 orifices to deal with, the gas port in the barrel and the adjustment screw in the gas block, not to mention the leakage via the gas block/gas tube junction, the gas tube/carrier key junction etc etc etc.

So, is the diameter of the gas port in the barrel just some random diameter hole that we, armed with our trusty Black and Decker and a cheap drill set, should be playing with.?

https://neutrium.net/fluid_flow/calculation-of-flow-through-nozzles-and-orifices/

 

[mrmackc]

11 hours ago, 308kiwi said:

And if you really want some bedtime reading that's guaranteed to put you to sleep have a read of the info on the linked page, it deals with calculating pressure/flow/volume/velocity drops across orifices. (it deals with fluid flow but the same calculations can be extrapolated to get an idea of the drops through a gas port).

And when you get your head around all the info, then bear in mind that by fitting an adjustable gas block you are compounding the calculations as you now have 2 orifices to deal with, the gas port in the barrel and the adjustment screw in the gas block, not to mention the leakage via the gas block/gas tube junction, the gas tube/carrier key junction etc etc etc.

So, is the diameter of the gas port in the barrel just some random diameter hole that we, armed with our trusty Black and Decker and a cheap drill set, should be playing with.?

https://neutrium.net/fluid_flow/calculation-of-flow-through-nozzles-and-orifices/

 

At the time I made the post I didn't get into the physics of gasses enough to include the temperature factor of the cooling effect of the expansion of the gas as it loses pressure and it's temperature change, or it's volumetric change of passing from a high pressure to the low pressure side of the orifice, so there is also that factor in the D.I. operating system.  I believe most of us know the factor as being "Refrigeration".

Maybe we should do some thinking about the "Bigger is Better"  ideology  when doing any orifice changing in our AR/LR308 pew-pew sticks.

Edited May 8, 2018 by mrmackc

[COBrien]

1 hour ago, mrmackc said:

At the time I made the post I didn't get into the physics of gasses enough to include the temperature factor of the cooling effect of the expansion of the gas as it loses pressure and it's temperature change, or it's volumetric change of passing from a high pressure to the low pressure side of the orifice, so there is also that factor in the D.I. operating system.  I believe most of us know the factor as being "Refrigeration".

Maybe we should do some thinking about the "Bigger is Better"  ideology  when doing any orifice changing in our AR/LR308 pew-pew sticks.

Wait, so you're saying I can use my LR-308 to keep my beer cold?

[mrmackc]

28 minutes ago, COBrien said:

Wait, so you're saying I can use my LR-308 to keep my beer cold?

You could think about it.

[308kiwi]

1 hour ago, mrmackc said:

You could think about it.

?well you could consider the gas port as a Thermal Expansion Orifice, or a 'TX' valve, the key component to any air conditioning system.

Interesting that so many unrelated things, an AR', your AC and your beer fridge, work on the same principle don't you think?

 

[mrmackc]

Physics is Physics, grasshopper.

[98Z5V]

11 hours ago, mrmackc said:

Maybe we should do some thinking about the "Bigger is Better"  ideology  when doing any orifice changing in our AR/LR308 pew-pew sticks.

I will never hesitate to tell someone - openly - that they need to drill up their gas port, when they've got a weakass gas system, and sluggish performance.  No gas block in the world is gonna fix that symptom. Superlight buffers are only going to compromise - and complicate - the system, just to get it to function with a too-small gas port. Too often these days, any monkey in the business can get a barrel made, with a too-small gas port on it, and mass-sell it cheap.  The people pumping out these cheap barrels don't understand what it takes - and they don't care.   When "bigger is better" is the solution - it's really the solution.

Yeah, recoil system needs to be considered, gas block, sure.  That's exactly why I always tell everyone to build a functional weapon first (no fancy shiit), and make sure it works.  After that, just go crazy and let your hair down, with all the fancy adjustable gasblocks and low-mass this and that, silent-captured whatever.  Change one thing at a time, and function-test your weapon.  If it still works, get freaky with your next adventure in making the baddest 308AR on the planet...

Don't complicate shiit with complicated shiit.  Build it simple, and make sure it works...

Once you have a simple functional weapon, it'll usually work with whatever ammo you throw at it, or have the chance to pick up... 

Edited May 9, 2018 by 98Z5V

[gdonley308]

17 minutes ago, 98Z5V said:

I will never hesitate to tell someone - openly - that they need to drill up their gas port, when they've got a weakass gas system, and sluggish performance.  No gas block in the world is gonna fix that symptom. Superlight buffers are only going to compromise - and complicate - the system, just to get it to function with a too-small gas port. Too often these days, any monkey in the business can get a barrel made, with a too-small gas port on it, and mass-sell it cheap.  The people pumping out these cheap barrels don't understand what it takes - and they don't care.   When "bigger is better" is the solution - it's really the solution.

Yeah, recoil system needs to be considered, gas block, sure.  That's exactly why I always tell everyone to build a functional weapon first (no fancy shiit), and make sure it works.  After that, just go crazy and let your hair down, with all the fancy adjustable gasblocks and low-mass this and that, silent-captured whatever.  Change one thing at a time, and function-test your weapon.  If it still works, get freaky with your next adventure in making the baddest 308AR on the planet...

Don't complicate shiit with complicated shiit.  Build it simple, and make sure it works...

Once you have a simple functional weapon, it'll usually work with whatever ammo you throw at it, or have the chance to pick up... 

This is a factory new rifle. From what everyone on here is saying, it seems over gassed. This would coincide with Armalite’s litterateur that says the rifle ships with the gas block adjusted wide open. I’m going to buy another Allen screw for adjustment and another for locking. Armalite says for tuning, screw in the adjustment all the way then back out 3 turns. Conduct the one round function test. Adjust until the bolt just locks back, then back off another 1/4 turn and lock in place. I will relieve the one hole in the hand guard to give clear access to the adjustment screw. 

[98Z5V]

3 minutes ago, gdonley308 said:

This is a factory new rifle. From what everyone on here is saying, it seems over gassed. This would coincide with Armalite’s litterateur that says the rifle ships with the gas block adjusted wide open. I’m going to buy another Allen screw for adjustment and another for locking. Armalite says for tuning, screw in the adjustment all the way then back out 3 turns. Conduct the one round function test. Adjust until the bolt just locks back, then back off another 1/4 turn and lock in place. I will relieve the one hole in the hand guard to give clear access to the adjustment screw. 

Don't buy stainless hardware...  No stainless set screws... 

[308kiwi]

2 hours ago, 98Z5V said:

I will never hesitate to tell someone - openly - that they need to drill up their gas port, when they've got a weakass gas system, and sluggish performance.  No gas block in the world is gonna fix that symptom. Superlight buffers are only going to compromise - and complicate - the system, just to get it to function with a too-small gas port. Too often these days, any monkey in the business can get a barrel made, with a too-small gas port on it, and mass-sell it cheap.  The people pumping out these cheap barrels don't understand what it takes - and they don't care.   When "bigger is better" is the solution - it's really the solution.

 

Agree completely, seems like every man and his dog are making barrels and all other manner of neato AR go fast bits, like you've said so many times, there aint no std on the large frame AR's, seems like the specs/std's are slowly being shifted further and further +/- on every front and on every sized AR, extraction/ejection issues are not limited to the 308's, there are plenty of posts on the 'other' forums re the same issues with the smaller family of AR's.

OH and I concur, don't go buying stainless set screws etc, recipe for disaster.

[sketch]

go figure i need a refresher on stainless ? why not stainless screws .

[308kiwi]

1 hour ago, sketch said:

go figure i need a refresher on stainless ? why not stainless screws .

Stainless tends to gall, pick up on itself, very easily, especially when screwed into an unlike material, add to that it being in a location exposed to high temp gases composed of who knows what and who knows how corrosive, stainless is only what it's name suggests, it stains less, it still oxidises just as carbon steel does, just not as much and not as quickly.

Sure carbon steel screws in the same environment are exposed to the same gases and temperatures but two like materials in contact with each other tend to be more chemically stable than two unlike metals.

I spend some of my spare time threading barrels for suppressors for a local gunshop, carbon steel barrels take a thread SO much easier than stainless ones, cutting threads in stainless takes considerably more care and more precise feed rates for a nice finish, galling is a big problem, the material you are removing with the threading cutter tends to gall onto the tool. Aluminium does the same, threading aluminium I use a specific cutting fluid designed for aluminium and make twice as many passes with the tool  otherwise it just picks up and makes a mess. I also have to advance the cutting tool on every pass so that it is only cutting on the front face where as with carbon steel the tool can cut both faces of the thread at the same time without issue.

Stainless, and aluminium for that matter, is a freakin PITA.

 

 

[sketch]

i learn something new! thanks 

[98Z5V]

56 minutes ago, 308kiwi said:

Stainless tends to gall, pick up on itself, very easily, especially when screwed into an unlike material,

Stainless, and aluminium for that matter, is a freakin PITA.

 

 

Yep.  Right on the money. 

[gdonley308]

9 hours ago, 308kiwi said:

Stainless tends to gall, pick up on itself, very easily, especially when screwed into an unlike material, add to that it being in a location exposed to high temp gases composed of who knows what and who knows how corrosive, stainless is only what it's name suggests, it stains less, it still oxidises just as carbon steel does, just not as much and not as quickly.

Sure carbon steel screws in the same environment are exposed to the same gases and temperatures but two like materials in contact with each other tend to be more chemically stable than two unlike metals.

I spend some of my spare time threading barrels for suppressors for a local gunshop, carbon steel barrels take a thread SO much easier than stainless ones, cutting threads in stainless takes considerably more care and more precise feed rates for a nice finish, galling is a big problem, the material you are removing with the threading cutter tends to gall onto the tool. Aluminium does the same, threading aluminium I use a specific cutting fluid designed for aluminium and make twice as many passes with the tool  otherwise it just picks up and makes a mess. I also have to advance the cutting tool on every pass so that it is only cutting on the front face where as with carbon steel the tool can cut both faces of the thread at the same time without issue.

Stainless, and aluminium for that matter, is a freakin PITA.

 

 

Yes, absolutely correct. I have over 4yrs machinist background and know how stainless can gall. I have sourced some ARP grade 8 steel set screws. I’ll be using these for my gas block. I’m at work in Texas for the next 3 weeks. I’m looking forward to getting home and tuning the gas system! I’m happy to here this is an easy fix, with the rifle being over gassed.

[392heminut]

Kiwi pretty much nailed it on the stainless properties. I used to do a lot of hand checkered frontstraps on 1911's and a 30 lpi checkering file would last through eight or ten carbon steel receivers easily but 3 or 4 stainless receivers would ruin a file. The best word I know to describe stainless steel is 'gummy'!

Stainless steel is carbon steel with chromium added to it to create the stainless properties. There are different grades of stainless with some grades having enough chromium in the mix that the metal won't attract a magnet!

Edited May 10, 2018 by 392heminut

[gdonley308]

UPDATE:! 

I got back from work 05/30. First I want to thank the members of this board for their help. I bought a $0.48 8-32 Allen screw and cleaned up the gas block threads with a tap. I then followed Armalites instructions and screwed the gas adjustment all the way in, then backed it out 2 1/2 turns. At that setting the rifle ran perfect! No FTE, no hiccups of any kind with both Federal 168gr Gold Metal Match and UMC 150gr FMJ. Yep, the rifle was over gassed, even has less recoil as well. 

This is 10rds of the Federal GM at 100yrds. The other rounds are 5.56 from an iron sight 6920.

[jtallen83]

Good deal, got yourself a shooter 

[98Z5V]

Way to go, gdonley!!!  I love seeing updates like this, I really do. 

***Side note - just imagine how much we could fix here if we were "real gunsmiths"!!!...***

[Bigfoot]

Nice shooting there.

[shepp]

1 hour ago, 98Z5V said:

Way to go, gdonley!!!  I love seeing updates like this, I really do. 

***Side note - just imagine how much we could fix here if we were "real gunsmiths"!!!...***

???

[gdonley308]

Thanks guys!! Who needs gunsmith’s when we can figure it out. 

[shepp]

1 hour ago, gdonley308 said:

Thanks guys!! Who needs gunsmith’s when we can figure it out. 

Kinda a joke from the other night ?