Need some schooling on "blowback" actions

[gunnutery]

Wasn't sure if this should be in auto handguns or general since some rifles use blowback too.

My specific question relates to what actually creates the blowback force? Is it from the initial explotion in the cartridge or is it based on compression of gas as the bullet travels down the barrel?

My next question is about delayed blowback: what's the difference between that and straight blowback? Is it more mechanically based?

 

[Sam1911]

You'll probably hate me for saying "wiki" but look how much writing we save, and isn't amazing how much gun stuff has made it into that resource?

http://en.wikipedia.org/wiki/Blowback_(firearms)

I think there are a few errors, or at least contentious points, but boy...there's a lot of explanation there, too.

 

[gunnutery]

Usually people jump all over me when I suggest wikipedia.

I'll check it out, thanks.

 

[Nom de Forum]

Usually people jump all over me when I suggest wikipedia.

I'll check it out, thanks.

There are four types of "Blowback" operation.

Simple Blowback - Sten

Advanced Primer Ignition Blowback - Steyr MPi81

Delayed Blowback -H&K MP5

Blowback with a locked breech - Hispano Suiza HS404 20mm cannon

If you Google these you should find definitions and more firearm types.

 

[JSH1]

Usually people jump all over me when I suggest Wikipedia.

I can't imagine why. Wikipedia is a fantastic resource.

 

[230RN]

I wrote this up a while ago for someone, so I might as well post it here. The pic is of my Ruger Standard .22 semiautomatic pistol, which is blowback operated. The bolt, or "breech" of the gun is the third item from the top. The little gizmo on top is the firing pin and the second gizmo from the top is the recoil spring.

The "bolt" is the equivalent of the slide on most semiautomatic pistols, except Ruger put it inside instead of outside. It's built more like a rifle action than a handgun action but the principles are the same.

Quoth Terry (personal correspondence ca 1998, shortened version):

When the cartridge goes off, pressure acts in all directions, including against the wall of the case, the base of the bullet, and the "back" of the cartridge case, including whatever's holding the cartridge case in, meaning the breech or breechblock of the gun. (Or the "slide".)

If the breech weighed as much as the bullet, it would come back with the same velocity as the bullet and be just as dangerous.

But the heavier the breechblock, the slower it will come back. If it's heavy enough, it won't come back until the bullet exits the barrel and the pressure has dropped to safe levels.

The recoil spring also helps in holding the breech closed during firing.

This is called "blowback" action. Nothing else besides the weight of the beechbolt holds everything together and sealed up until the bullet exits and the pressure has dropped.

There is a careful balancing act between bullet weight, pressures, barrel length, spring "weight," and breechblock weight in designing a blowback action.

Example:

The breech (breechblock, "slide," or 'bolt") on my Ruger standard .22 weighs 2,035.5 grains (about a quarter-pound), while the .22 bullet itself weighs only 40 grains. This difference in weight (actually, "mass," but let's not get into that) is what keeps the breech sealed by the cartridge case until the bullet gets out the barrel.

By way of further example, the military .30-06 cartridge with a 150 gr bullet (and the very high pressure involved) would have to have a breechblock weighing 27 pounds if it were fired in a blowback action.

Terry, 230RN

 

[twofifty]

good example

27 pounds! very cool

 

[stressed]

230RN is there a calculator you can use to determine what bolt weight would be sufficent for different rounds?

 

[230RN]

Well, as I mentioned, there are a number of variables besides bolt weight. The "27 pounds" I got from Hatcher's Notebook, where he discussed the types of actions, and this was probably calculated out by U.S. Ordnance Department engineers back in the 1930s.

Hatcher's Notebook has examples of how a "first approximation" may be made just from the weights and velocities (and hence the recoil velocities and momentum) involved, but this does not include a bunch of other necessary variables.

I strongly recommend Hatcher's Notebook for anyone, including the beginning shooter. Besides having intricate technical details, it is highly readable, and although it was written in the late 1940s, it is highly entertaining to boot. It's one of those books you can pick up, open to almost any page at random, and be immediately interested in the subject of that page.

In addition, I think about 80% of the questions asked on gun boards can be answered directly from this book.

Wanna know what happens when you put a can of gunpowder on a fire?

Wanna know how far a shotgun pellet will travel?

Wanna know what happens when a whole arsenal (like Picatinny Arsenal) blows up?

Consult Hatcher's Notebook.

Really, get a copy. (It's even online somewhere.)

Terry, 230RN

REFs:
http://carteach0.blogspot.com/2012/03/book-review-hatchers-notebook-from.html
http://www.midwayusa.com/product/710604/hatchers-notebook-book-by-julian-hatcher

 

[tyeo098]

Are you building something?

 

[gunnutery]

Thanks Sam, Nom de Forum and 230RN. Very informative. My initial question was based on not knowing if so much barrel length was needed. Then I realized that I didn't know that much about blowback. I knew weight/mass and springs were needed, but also knew there were different types that I hadn't wrapped my head around yet. Thanks.

Are you building something?

No, but I did start the linked thread about building a PCC, though I'm not currently building any.
http://www.thehighroad.org/showthread.php?t=743066

 

[barnbwt]

Though the nitty gritty of all his assumptions isn't necessarily completely valid (namely his assumptions regarding pressure curve effects on the bolt thrust), the Orion's Hammer blowback explanation and calculation page has a very thorough rundown of the concept.

In short;
-Chamber pressures pushes on the bullet and bolt equally, but the bolt head usually has more area and receives more force (barrel receives the difference plus some friction with the bullet, minus some friction with the case)

In long;
-Momentum of the bullet and bolt are essentially equal (there's some minor variance since the barrel is blown forward ever so slightly at first, and if a hammer is used, its momentum must be added to the bolt's)
-Bolt thrusts are so much higher than return spring/buffer forces, that they have an infinitesimal impact on delaying the breech (though the mass of increasingly heavy springs plays a small part), but play a huge roll in determining the rate of cycle and bolt travel length
-A too-light bolt will allow brass to bulge, tear case heads or extractor grooves, and even allow case rupture under pressure. Most designs have the ejection port large and very near the breech so a case rupture (most often due to out of battery detonation in an open-bolt design) is not contained and causes little damage to the gun or shooter
-Nearly all designs ever built to be blowbacks have far heavier bolts than mathematically needed, both for margin, and to keep cycle speeds down to make the guns controllable

Delayed blowback uses mechanical leverage to magnify bolt mass displacement due to case head displacement, with the effect of increasing the momentum needed at the bolt face to open the action. Since the leverage can run the gamut from 1:1 all the way to infinity:1 based on geometry, a vast range of cartridges can be accommodated (7.5 Swiss to 9mm Luger, unless H&K ever made a 50cal or larger version of their CETME clone).

When the ratio is at infinity:1, you have a locked breech; the case head cannot budge the bolt mass no matter how it tries, and instead the entire (slide+barrel) assembly receives momentum as the bullet is fired. This has the benefits of negating the problem of case rupture from a too-light bolt, as well as slowing down the cycle speed of the recoil operation by adding the mass of the slide and barrel to that of the bolt, and finally by "trapping" and nullifying any extra force delivered to the moving part of the action because of a wider case head (it instead sees only the bore area, and its momentum is therefore equal to the bullet's recoil --thus "recoil" operated)

TCB

 

[MedWheeler]

230RN writes:

..the military .30-06 cartridge with a 150 gr bullet (and the very high pressure involved) would have to have a breechblock weighing 27 pounds if it were fired in a blowback action.

Like on a Hi-Point pistol!

(I can rag on 'em; I have one, too!)

 

[Tirod]

I once owned a stainless Detonics Pocket Pal, which was a blowback auto pistol in 9mm. That is beyond the normal limit in caliber - too much force involved. How they did it was another form of delayed blowback, previously used in submachine guns. The chamber is shaped as a cone to allow the mouth of the cartridge to expand slightly, and as the slide retracts, the case is "resized" back to it's original diameter, which creates the friction needed to delay the operation.

The followup point is that most self loading actions use a portion of chamber pressure to cycle the action - the gas pressure on the cartridge forces it against the bolt face and it helps propel the bolt into it's loading cycle. That also happens early enough in the pressure cycle that gas residue escapes past the cartridge into the action. Those of use who owned HK's in the day are well versed in seeing dirty brass and filthy bolts. So do blow back .22's, and guess what - AR15's. The bolt does NOT get dirty because of the gas tube, it gets dirty because the barrel exhausts residue past the brass. The proof is readily available - wipe the brass off with a white cloth and you see it. Another proof the cartridge base is pushing the bolt face is that the cam pin attempts to rotate the bolt closed and gets pressed against one side of the upper pin channel, creating a lot of friction. That is addressed in the military by the instruction to heavily lube the channel in the upper receiver. The aftermarket responded with a roller cam pin.

Bolts on M1's, M14's, FNFAL's, AK-47s, you name it all get dirty. Only the locked manual action guns eject clean brass. Self loading actions all "crap where they eat" regardless of whether the bolt is locked or not. Tear down a M17 grenade launcher, M2, M60, M249, you get a handful of dirty bolt. Recoil, roller delayed, piston, or othewise, the timing of extraction allows it.

 

[tyeo098]

The chamber is shaped as a cone to allow the mouth of the cartridge to expand slightly, and as the slide retracts, the case is "resized" back to it's original diameter, which creates the friction needed to delay the operation.

That's genius.

 

[Jim Watson]

There is straight blowback, dead common and simple.
There is locked breech, the Browning tilt-barrel recoil operation being the most common in pistols, although there are some "prop up" actions with locking blocks like Beretta and rotating barrels, most recently also from Beretta. Other mechanics are seen to use recoil in rifles, machine guns, and shotguns. There are a very few gas operated pistols like Desert Eagle and Wildey and many many gas operated rifles and shotguns.

Of delayed blowback there is a great variety of approaches.
Tirod mentions the Detonics Pocket which works the brass, in common with the old Mann ring grooved chamber, also seen in the experimental High Standard T3 and the Seecamp pocket pistol.
Remington is bringing back the Pedersen delay of the PA51 pocket pistol and PA53 Navy .45 in the swoopy looking R51.
And there are lots more. You can usually recognize a delayed blowback rifle or machine gun by its case lubrication, from wax on the ammo for the Pedersen rifle to the actual oil pump in a Schwartzlose machine gun.