My roller-delayed blowback puzzle....

[Ignition Override]

This first HK will ship soon; a nib PTR-91.

Although my question might be mostly academic- having watched numerous videos (bolt gap check) disassembly etc- only one thing is a bit mysterious. I've also watched graphic videos of the actual movements of the components.

When the gas builds in the chamber during the bullet's travel, even though the rollers obviously delay movement of the bolt for a miilisecond or so,

...what First contacts the Bolt >>Carrier<< to force it a bit rearwards (aft)? With the bolt delayed just a tiny bit longer, there seems to be nothing on the bolt face which can exert force On the bolt carrier.
(-Before the bolt can move-) - Until the rollers disengage.

There is no gas operation (as used in our AKMs, ARs etc), we clearly know this, but on the bolt face only the firing pin has a connection between the face and the bolt carrier behind the bolt itself.

Simply curious.

 

[Dave DeLaurant]

The good news is that there is an entire book especially for you:

https://www.amazon.com/Full-Circle-Treatise-Roller-Locking/dp/0889354006



Bad news? If you don't have a copy yet, the price for one has gone crazy. It was almost one Benjamin when I bought my copy and now that looks like a bargain. I'd check with your local public library about an interlibrary loan.

There's also a good, very early Forgotten Weapons video about the development of the roller delay system at Mauser:

 

[Ethan Verity]

The back pressure on the roller bearings forces them inward, in which they pinch the pointed angles of the locking piece forcing it to the rear against the carrier. That is what initially pushes the carrier back.

 

[swg1]

Stock up on cleaning supplies. Dirtiest rifle I ever owned.

Get a brass catcher.

Good luck.

 

[Slamfire]

Your roller bolt is a high power delayed blowback. I am surprised these roller bolts are not in Chin's Vol IV Machine Guns, but if you want to understand how it works, go to Chinn’s series at Hyper books:

http://www.ibiblio.org/hyperwar/USN/ref/MG/

And firstly read Vol IV Chapter one blowbacks. At the end of it is a section on delayed blowbacks.

Mind you, Chinn does not address chamber flutes. The mechanisms he is familiar all use grease or oil to lubricate the case.

Many in the shooting community are faithful to an Army coverup over a century old, a coverup that blamed grease and oil for single heat treat 03's blowing up. They in fact are fearful of leaving case lube on the cartridges and get nasty when their belief system is challenged. This thread had to be shutdown when the insults were flying.

LUBE QUESTION???

What you see with Chinn, he is familiar with grease and oil in highpower delayed blowbacks, but not chamber flutes.

This came out of Chinn Vol IV



and so did these:







And, it is interesting that future General Hatcher understands why grease and oil must be used in highpower retarded blowback systems

Army Ordnance Magazine, March-April 1933

Automatic Firearms, Mechanical Principles used in the various types, by J. S. Hatcher. Chief Smalls Arms Division Washington DC.

Retarded Blow-back Mechanism………………………..

There is one queer thing, however, that is common to almost all blow-back and retarded blow-back guns, and that is that there is a tendency to rupture the cartridges unless they are lubricated. This is because the moment the explosion occurs the thin front end of the cartridge case swells up from the internal pressure and tightly grips the walls of the chamber. Cartridge cases are made with a strong solid brass head a thick wall near the rear end, but the wall tapers in thickness until the front end is quiet thin so that it will expand under pressure of the explosion and seal the chamber against the escape of gas to the rear. When the gun is fired the thin front section expands as intended and tightly grips the walls of the chamber, while the thick rear portion does not expand enough to produce serious friction. The same pressure that operates to expand the walls of the case laterally, also pushes back with the force of fifty thousand pounds to the square inch on the head of the cartridge, and the whole cartridge being made of elastic brass stretches to the rear and , in effect, give the breech block a sharp blow with starts it backward. The front end of the cartridge being tightly held by the friction against the walls of the chamber, and the rear end being free to move back in this manner under the internal pressure, either one of two things will happen. In the first case, the breech block and the head of the cartridge may continue to move back, tearing the cartridge in two and leaving the front end tightly stuck in the chamber; or, if the breech block is sufficiently retarded so that it does not allow a very violent backward motion, the result may simply be that the breech block moves back a short distance and the jerk of the extractor on the cartridge case stops it, and the gun will not operate.


However this difficultly can be overcome entirely by lubricating the cartridges in some way. In the Schwarzlose machine gun there is a little pump installed in the mechanism which squirts a single drop of oil into the chamber each time the breech block goes back. In the Thompson Auto-rifle there are oil-soaked pads in the magazine which contains the cartridges. In the Pedersen semiautomatic rifle the lubrication is taken care of by coating the cartridges with a light film of wax.

Blish Principle….There is no doubt that this mechanism can be made to operate as described, provided the cartridge are lubricated, …. That this type of mechanism actually opens while there is still considerable pressure in the cartridge case is evident from the fact that the gun does not operate satisfactorily unless the cartridges are lubricated.

Greased ammunition, oilers, all went on the ash heap of history when the Germans stole a Russian idea: gas lubrication through chamber flutes



Keep your chamber clean! Don't use ammunition with tar sealant around the bullet or the flutes will be clogged. A chamber brush is your friend.

 

[someguy2800]

The bolt pushes backwards against the rollers. The rollers push against the ramped surfaces on the inside of the receiver which forces the rollers inward against the bolt carrier. The bolt carrier also has two ramped surfaces so pushing inward on the bolt carrier forces it backwards.

It’s a bit like squirting a bar of soap out from between your fingers. When you do that you are creating mechanical advantage so that a short movement of your fingers creates a longer motion in in the bar of soap and it shoots out of your hand. The ramps on the receiver and carrier create a leverage so that in order for the bolt to travel backward a millimeter, the carrier must travel back several millimeters. So the bolt is never truly locked, there is just a lot of mechanical advantage holding the bolt forward for the first few millimeters.

 

[someguy2800]

Imagine in this diagram if you pinched the blue rollers with your fingers how the ramp angles would force the carrier backwards

 

[caribou]

Do not forget the weight in force that the spring behind the bolt carrier is applying, as well as the weight of the carrier in the recoil equasion.

 

[Ignition Override]

Thanks very much for so much info—-

 

[Holescreek]

This first HK will ship soon; a nib PTR-91.

Although my question might be mostly academic- having watched numerous videos (bolt gap check) disassembly etc- only one thing is a bit mysterious. I've also watched graphic videos of the actual movements of the components.

When the gas builds in the chamber during the bullet's travel, even though the rollers obviously delay movement of the bolt for a miilisecond or so,

...what First contacts the Bolt >>Carrier<< to force it a bit rearwards (aft)? With the bolt delayed just a tiny bit longer, there seems to be nothing on the bolt face which can exert force On the bolt carrier.
(-Before the bolt can move-) - Until the rollers disengage.

There is no gas operation (as used in our AKMs, ARs etc), we clearly know this, but on the bolt face only the firing pin has a connection between the face and the bolt carrier behind the bolt itself.

Simply curious.

Upon firing the gas pressure immediately expands back through the chamber flutes to counteract the internal case pressure. ( so the case doesn't stick in the chamber). The gas pressure pushes the case rearward against the bolt face. As the bolt is pushed rearward the already extended rollers are driven into the 50 degree ramps in each side of the trunnion which cause the rollers to move inward against the shoulder angles of the locking piece. The locking piece is attached to the carrier. As the rollers move inward squeezing against the locking piece shoulders the carrier gets pushed rearward at an exponential rate. Depending on the angle of the locking piece shoulder angles the carrier moves at different increments of speed. Once the carrier has moved rearward enough for the rollers to have moved completely into the bolt head the bolt carrier assembly continues rearward and trips the ejector to pivot up to kick the case out.

In these rifles the extractor doesn't actually extract the case, it just holds it against the bolt face long enough for the ejector to function.

 

[Ignition Override]

Holescreek: Wow. Thanks very much. Without the gun in mind hands opposite Youtube, it's not easy to imagine one of these being reliable, with the odd geometry of bolt and carrier.

That explanation is the only valid description I've seen, by a "long stretch". Nobody else seems to understand it, or be able explain it.
Having watched several reassembly videos (plus checking bolt gap), it is really hard to picture overall how the bolt' (via the cocking piece) interfaces with the carrier during typical shooting.

Watching guys reassemble, then sometimes finish by pulling the bolt a bit forward, before putting the entire bolt carrier group back into the rifle seems like a hidden Rubik's cube compared to my simple, imported AKMs, and even when compared to my Czechpoint VZ-58 (except for the area near the trigger group).

When my PTR arrives tomorrow (-if soon processed-), a young staffer in the store should be there; he owns a G3 derivative and might have time for a quick demo of an efficient bolt carrier dis-. and reassembly.

The graphic videos and verbal descriptions make the roller functions pretty obvious, but as to why the graphics always depict the Carrier moving rearwards Before the bolt is
---visibly moving--, never looks plausible.

 

[Ignition Override]

Iron sight cheek weld.
The chance of cheek slap with iron almost never gets mentioned, just saw it mentioned once regarding the normal butt stock.

On my VZ-58 I keep my left cheek a bit further aft than on my AKMs, but hope that the "2" aperture of a PTR/HK is clear enough without a classic AK close cheek weld. ?

David

 

[Holescreek]

The HK91/Cetme platform is very robust and reliable. Being a blowback weapon they are always dirty but will always function. The most common part to fail is the extractor spring, which lets the case fall out of the bolt before ejection. Most extractor springs fail due to improper disassembly. The guns are heavy but handle recoil very well. The recoil can easily be tamed by switching the locking piece out for one with a reduced shoulder angle. PTR has had quality problems with US made components but is said to have excellent customer service.

 

[bullseye308]

You should enjoy it. I had CETME and it was a joy. Be aware it launches brass about 40’ and with the standard brake is terribly loud.

 

[ECVMatt]

I am no expert on the roller delayed actions but I am learning. My son wanted a cold-war era battle rifle and after a lot of back and forth we settled on the CETME/G3 style rifle. I ended up with a 16" PTR and so far it has been great. After reading all of the posts about recoil I almost sprung for the improved recoil buffer and a muzzle break. After shooting a couple of hundred rounds out of it I am glad I did not. It definitely recoils and I can see why a full auto G3 would suck for accuracy but we shot all day with no problems. The recoil is more like a gentle push than a hard slam. Anyways I am happy with the regular set up and will pass on the recoil reducing additions. Our rifle has been flawless for the first 200 rounds. It does launch brass and it kind of adds to the cool factor of the gun. Accuracy is very good with our gun and after adjusting the sights we can dial hits out to 300 yards with no problems. Congrats on your new gun and I hope you enjoy it as much as we have. Here is pic from a couple of weeks ago, you can see the the bolt is still closed even after the bullet has left the barrel.

 

[Holescreek]

After reading all of the posts about recoil I almost sprung for the improved recoil buffer and a muzzle break. After shooting a couple of hundred rounds out of it I am glad I did not.
View attachment 1055341

I did a study on the rearward movement of the carrier using a gun with barrel length of 10.5 and standard full length barrel and found that the carrier doesn't come within .3" of touching the buffer. The only way I got it to come close was to run a suppressor with the standard locking piece in the gun. Too many people buy parts that don't actually do anything, the enhanced buffer is one of them.

 

[ECVMatt]

I did a study on the rearward movement of the carrier using a gun with barrel length of 10.5 and standard full length barrel and found that the carrier doesn't come within .3" of touching the buffer.

I wonder if it was incorporated to help when launching rifle-fired grenades?

 

[Holescreek]

I wonder if it was incorporated to help when launching rifle-fired grenades?

Possibly, but more than likely it is a safeguard to prevent damage in case of a weak or failed bolt locking lever spring. One thing I'm sure of is that if your carrier hits the buffer your rifle has problems.