School Me on Blowback

[Marine Cpl]

So 9mm AR's are built to be blowback. There is no hole in the barrel connected to a gasblock connected to a gas tube into the receiver. I'm not sure how it works but it does. The bolt cycles through blowback.

So how come if you shut off your gas block on a regular AR or use a barrel with no hole to make a straight pull/ bolt action, it doesn't cycle the bolt via blowback?

 

[Jem45472]

I'm no expert, and don't have a 9mm,but I'd guess the bolt is different and doesn't have the locking lugs.

The gas system on a standard ar will unlock the lugs. Somebody will correct me if I'm wrong.

 

[Snappo]

So 9mm AR's are built to be blowback. There is no hole in the barrel connected to a gasblock connected to a gas tube into the receiver. I'm not sure how it works but it does.

So how come if you shut off your gas block on a regular AR or use a barrel with no hole to make a straight pull/ bolt action, it doesn't cycle the built via blowback?

Maybe on the nine the bolt velocity from gas expansion can overcome the spring tension?
Blowback Gun Design

 

[meketrefe]

Blow back is a very simple concept. As the bullet moves down the barrel the gas expands and it looses pressure.
The same pressure is expanding the brass case against the walls and that pressure prevents the case from extracting itself.
There is also some additional pressure created by the bolt face against the base of the case that is a factor of the spring
force and the mass of the bolt group and/or slide if any.
Once the the bullet comes out in a fraction of a second the pressure comes down low enough so the brass case contracts
and then it can start to move rearwards by the pressure still left. This violent effect of the case like a firecracker is
attenuated by the bolt and the mass that with the spring is going to decelerate that case, eject it and when coming
back strip a new round from the magazine.

Makes sense?

 

[GOYABEAN]

 

[Marine Cpl]

Blow back is a very simple concept. As the bullet moves down the barrel the gas expands and it looses pressure.
The same pressure is expanding the brass case against the walls and that pressure prevents the case from extracting itself.
There is also some additional pressure created by the bolt face against the base of the case that is a factor of the spring
force and the mass of the bolt group and/or slide if any.
Once the the bullet comes out in a fraction of a second the pressure comes down low enough so the brass case contracts
and then it can start to move rearwards by the pressure still left. This violent effect of the case like a firecracker is
attenuated by the bolt and the mass that with the spring is going to decelerate that case, eject it and when coming
back strip a new round from the magazine.

Makes sense?

So why doesn't that work with a .223 and an adjustable gas block that's shut off?

 

[4excal]

Blow back is a very simple concept. As the bullet moves down the barrel the gas expands and it looses pressure.
The same pressure is expanding the brass case against the walls and that pressure prevents the case from extracting itself.
There is also some additional pressure created by the bolt face against the base of the case that is a factor of the spring
force and the mass of the bolt group and/or slide if any.
Once the the bullet comes out in a fraction of a second the pressure comes down low enough so the brass case contracts
and then it can start to move rearwards by the pressure still left. This violent effect of the case like a firecracker is
attenuated by the bolt and the mass that with the spring is going to decelerate that case, eject it and when coming
back strip a new round from the magazine.

Makes sense?

Beautifully written Mek! @meketrefe

 

[meketrefe]

So why doesn't that work with a .223 and an adjustable gas block that's shut off?

Lets say an AR15 in 223 has a locking bolt. It rotates with the help of the cam and it is locked like a Gun Safe. That is why it doesn't blow
back. The lugs stop that case from blowing back.
But lets say you chambered a 223 and used a 9mm blow back bolt/carrier without lugs in an AR15. The main issue is the difference in pressure.
The rifle works at high pressure and the case will rupture because the base (web) of the case cannot withstand the 56K-62K psi of a rifle
round. So your load will blow up and most likely break the rifle or worse.
The mass and spring that is good for a 9mm and the strength of a 9mm case for 35K psi is good enough to avoid the whole thing from blowing up. The brass cannot hold a rifle pressure w/o help from a locking system.
Alternatively there are systems called delayed blow back that can be used for rifle w/o the need for a gas system whether it is direct impingement like the AR or a piston system like an AK.
The delayed blow back works on the premise that there is an initial locking resistance by specific design and machined int he bolt system that holds the bolt during the first brutal pressrue but as the pressure goes down the lock releases itself and lets the natural block back work once the pressure has come down to a manageable level like in the case of pistol calibers.
H&K and Cetme are good examples of delayed blow back for military rifles.

 

[Marine Cpl]

Lets say an AR15 in 223 has a locking bolt. It rotates with the help of the cam and it is locked like a Gun Safe. That is why it doesn't blow
back. The lugs stop that case from blowing back.
But lets say you chambered a 223 and used a 9mm blow back bolt/carrier without lugs in an AR15. The main issue is the difference in pressure.
The rifle works at high pressure and the case will rupture because the base (web) of the case cannot withstand the 56K-62K psi of a rifle
round. So your load will blow up and most likely break the rifle or worse.

Gotcha.

 

[Jem45472]

Standard ar bolt with locking lugs, and 9mm blowback bolt

 

[meketrefe]

@Willjr75
Look what I found. I thought this new delayed blow back video explains a few of the above concepts quite well.
As you might now Savage has made a super reliable and accurate 17HMR auto-loader that doesn't blow up cases or has short stroking
problems. They have done this by introducing a little lock that opens with a tiny cam inside the carrier. Simple, light and it does the trick.

 

[Marine Cpl]

There it goes. Case closed! Thanks for learning me.

 

[meketrefe]

np. just keep in mind what in the video looks like a small ruputre in a small HMR case but in the aforementioned 223 would be total head separation if a locked bolt or some sort
of delay mechanism would not be there. Sometimes cases separate even with a bolt imagine w/o one... it would be like a little bomb and might destroy the rifle or worse.

In some cases head separation can be due to:

1) Early extraction - Poor timing and the bolt starts to pull too soon when there is still a lot of pressure in the chamber. This happens quite
often with M4 style AR15s because there is no Quality Control and people assembling from different manufactures do not really take the time
to make sure the AR is properly gassed.
2) Excessive Powder or wrong powder.
3) Defective case or a case that has been reloaded too many times and exposed to excesive headspace.
4) A bad chamber. This could be a chamber with excessive headspace or one that has the right headspace but it was curt short (Short tenon).
This is also something I have seen in AR15 kits, specially the cheapo deals. We had a member in other forum that got a bad chamber from
a PSA kit although the heedspace was ok the chamber was not well done.
5) It could be a mixture of all the above.

Case separation below. Kaboom! ...

View attachment 1760

 

[4excal]

View attachment 1760

I don't know chit about this stuff..but I do see "Fudgey the Whale"

 

[Acer-m14]

I just got really into firearms a few years ago and while i was fully cleaning my old moss 22lr .. i was like wait where is the gas port on this thing ..
after some reading OHHH its a blowback .. just a bolt and a spring .. the spring just has the right amount of force need and the recoil of the 22 brass
when it breaks free has enough force to push that bolt back for the next round to load .. ahhh science .. love it ..

 

[Marine Cpl]

One last question....

Does blowback work properly with steel cased ammo?

If I load up that Tula crap in my carbine, will it work?

 

[Acer-m14]

it may ,, i dont think the steel would expand as much as the brass so the reaction to the action is pretty much same . @meketrefe

 

[meketrefe]

a over under with drop in tubes for rifle rounds would be cool .. if not to heavy ...

One last question....

Does blowback work properly with steel cased ammo?

If I load up that Tula crap in my carbine, will it work?

the thing is some ammo is designed to originally be mass manufactured in steel cases like the 7.62x39 that is cheaper.
If the steel case is well made there should not be any difference but the issue is since steel is designed to be cheaper
manufacturing so are the primers, powders, primers and bullets. So it doesn't have to be bad but the consistency
is not going to be there and they also fail in QC so one might get many duds.
I don't think it will hurt but it might be filthy and no real guarantee the whole formula might be reliable.
It could be only a bad batch though. But this also happens with brass.
Also depending on how old it is wash the bore with some amonia solution in case they used mercury in the primers. Mostly older berdan but it is hard to know if they actually follow similar standards than the eastern manufacturers and nato.

 

[Acer-m14]

BANG .. exploding gass .. bullet goes forward .. something has to move backwards ..

 

[Jem45472]

@meketrefe What are the benefits, and why the use of the different lock up methods? Blow back is a simpler, and surely easier to manufacture method, there must be drawbacks.

 

[Acer-m14]

i say the PSI pressure thats over 22lr and 9mm .. anything else is just to much .. ready for a @meketrefe update

 

[Jem45472]

i say the PSI pressure thats over 22lr and 9mm .. anything else is just to much .. ready for a @meketrefe update

Didn't even think of that. That makes perfect sense. The recoil spring would make anything else a bear to charge.

 

[Eddie Gunks]

i say the PSI pressure thats over 22lr and 9mm .. anything else is just to much .. ready for a @meketrefe update

Uuummm??!?

Most all handguns are blowback.....even some 50 cal versions.

Weight / spring / and buffers will dampen if/when needed.

FWIW They make 40 and 45 ar in blowback.

 

[Marine Cpl]

Uuummm??!?

Most all handguns are blowback.....even some 50 cal versions.

Weight / spring / and buffers will dampen if/when needed.

FWIW They make 40 and 45 ar in blowback.

Even .357 Sig.

 

[meketrefe]

I think Acer mentioned 22LR and 9mm and he meant in terms of pressure not caliber, that is correct.
Caliber doesn't matter. Pressure is what determines if a locking or delay mechanism is needed.
Also there are handguns with gas systems that can shoot high (higher) pressure cartridges.
And I don't mean an AK, AR or whatever version but handguns that were purposely built that way
to avoid the problems and lock or delay the bolt. the H&K P7 has a ported barrel and it relies on the chamber pressure to keep the bolt locked and not just the slide mass and spring.

 

[pseudonymity]

@meketrefe What are the benefits, and why the use of the different lock up methods? Blow back is a simpler, and surely easier to manufacture method, there must be drawbacks.

I am no meketrefe, but I will take a stab at it. Blow back requires that the bolt (or whatever is the back of the chamber) contains enough resistance to keep the brass in the chamber until the pressure is low enough so the brass can contain it (think case head separation). That means some combination of bolt mass and spring pressure. Get much beyond the recoil level of a 9mm or so and the combination gets pretty unwieldy in a handgun. Rifles of course can be much heavier and still be practical, so even larger types of handgun rounds are simple enough as blowback in a rifle size package. But even a small rifle round requires a resistance that either makes the bolt the heaviest part of the rifle or makes chambering a round require gorilla strength.

So for simpler modified blowback designs you can do things like force the bolt to overcome more resistance at first (like a roller delayed G3 design), add to the resistance at first by adding the mass of the barrel (like most semi pistols) or even have the bolt and barrel move completely back together and then have the barrel return before the bolt (Auto 5 shotgun).

Gas based systems typically become lighter and require less moving mass (read: easier to keep on target) as the round fired gets larger. Mostly you can feed the pressurized gas directly onto the bolt carrier (AR direct impingement), onto some type of coupling rod that travels a smaller distance than the bolt carrier (SKS, FAL) or a rod that travels the distance with the carrier (AK, M1 Garand).

Keep in mind that the gas systems do not push on the bolt directly, the pressure is applied to the carrier which uses part of its energy to unlock the bolt by turning it or tilting it, and then carries the bolt with it for ejection, then loading and lastly locking the bolt back securely in place.

 

[meketrefe]

@pseudonymity

that's a great way to explain it. Just a couple of small things...

The actual pressure of the pistol case expanding is enough to keep the casing in place up to a point. when pressure drops and case starts to move the added pressure and mass of the bolt holds the case and delays the opening as pressure continues to drop and accelerates the action. The delay system like the roller like you well said provides an actual stop at top pressure and then it lets go repeating the same cycle as above. G3s, Cetmes and alike are dirty at the breach for that reason. They also use fluted chambers to assist with extraction.

Also, there are gas systems that push the bolt directly like the direct inpigment in the AR15. The gas injected behind the bolt is what actually propels the carrier away and in the process the cam in it makes it rotate and unlock so it can be carried away with the round that is being extracted. That is one of the reasons the AR is so balanced and accurate because there are not off center torque forces to displace the entire group that is in line with the bore. The downside is that you need a buffer tube and all that but it is a reasonable trade off. As you might know one of the challenges of short stroke pistons is how to minimize tilt and torque forces since the group was not originally designed to be propelled by a piston impact on the top of the carrier. this doesn't mean all pistons are bad but the tilt and torque forces are introduced.

 

[Acer-m14]

"I think Acer mentioned 22LR and 9mm and he meant in terms of pressure not caliber, that is correct." YEP .. @meketrefe

 

[stuck in ny]

So 9mm AR's are built to be blowback. There is no hole in the barrel connected to a gasblock connected to a gas tube into the receiver. I'm not sure how it works but it does. The bolt cycles through blowback.

So how come if you shut off your gas block on a regular AR or use a barrel with no hole to make a straight pull/ bolt action, it doesn't cycle the bolt via blowback?

To put it simply the 9mm ar has a bolt and carrier that are one piece. The ar15 in 223 is a locked breach the same as any other locked breach. (Gas pressure back on the bolt will not unlock it the carrier must be moved back first. This forced the bolt to rotate by the cam pin and the lugs and locking recesses to no longer line up; the bolt it then free to move back and cycle the weapon.

 

[nortatoga]

By that thread title I thought you were calling someone out! LOL!!