PTR "delayed blowback roller-lock platform?"

[Payload]

Recently I started looking into PTR and their rifles after I heard that they were leaving their home state in protest of new gun laws. When I got to their web page and found they were making G3/HK91 style rifles I was intrigued. Delays have always been a point of interest for me. But when I read the technical descriptions of their guns I found something completely out of place. They claim their action is a "Delayed Blowback Roller-Lock System." (See for yourself http://ptr91.com/products/PTR91F?id=1) Locks and delays are not the same thing and both a Roller Delay (like the G3/HK91) and Roller Lock (like the CZ 52) exist. Thinking they may have modified the design I shot them an E-mail.

"Hello, over the last few months I've been seeing an increasing number of your rifles at local gun stores and been hearing great things about them. Delayed actions have always been a favorite of mine. There is however one point I am confused on. On your web page it describes the weapons' action as "Delayed Blowback Roller-Lock System." I have always been under the impression that a Roller Lock and a Roller Delay were two distinct methods of operation. Do your weapons use the HK91 style action or have they been modified in some way, possibly pushing them closer to the CZ 52?"

Here's what I got back two days latter...

"Hi Patrick,
Thanks for getting in touch with us.

Our rifles use the same operating system as the HK 91. It is in fact a delayed blowback roller-lock platform."

Okay, I understand that their guns use a roller delay. But what's with the off the wall terminology? I recall seeing a few other companies over the years that have had similar cases. Sometimes I get it when an action is truly bizarre like the AN-94's "Blowback Shifted Pulse" (translated Russian and possibly not considered to be the full description of the weapons action.) Got any theories on this one?

 

[TheGloriousTachikoma]

it's the same case as the difference between referring to a semi-auto as an automatic and an autoloader. Same exact thing, just a little difference in terminology.

 

[DPris]

No need to theorize.
They use the same action as the H&K 91, and that's all you need to know, really.
Denis

 

[caribou]

theres a difference between roller locked and roller delayed blowback, but delayed blowback roller locked is a bit weird.

Roller locks are usually unlocked by a seperate pice, usually driven by gas in the form of a piston and/ or bolt carrier.It uses gas to operate and rollers to lock the bolt to the barrel untill 'unlocking' occures via the mechanisim. The bullet has left the barrel while the bolt and barrel are still locked together.

A delayed blowback that uses the case head thrust to operate the mechanisim, which is truly not locked, but its timing is delayed as it uses mechanical and weight advantage to allow bolt to be delayed. This delay is the time is when the rear ward thrust is over comming the mechanical/weight advantage for the cartridge, and its the time that the cartridge case is just getting unseated and yet the chambers still sealed untill the bullet is well clear of the barrel and pressures are dropping rapidly. Still,the bolt and cartridge are sent back by case thrust alone, and not a truely locked siuation.


But a roller delayed blowback would be more descriptive

 

[stubbicatt]

Well said, Caribou.

 

[SilentScream]

The correct term for this system is "Roller delayed blowback"
The rollers do not lock the bolt in battery say like an AR15, M1 or FAL. They simply delay the rearward inertia impulse long enough through mechanical disadvantage to allow the bullet to leave the barrel and the pressure in the chamber to drop off to safe levels.

 

[RustHunter87]

As far as i know its exactly the same as an HK bolt, and to say it doesn't lock seems untrue to me.

Its a multi piece bolt and when it slam forward the locking piece pushes the rollers out and Locks the front portion of the bolt in to the trunnion, after the round is fired Inertia is transferred to the heavy mass of the bolt carrier which moves back taking the locking piece with it, that allows the rollers to retreat and the whole bolt moves back as one.
Its quite simple really.

http://www.militaryfirearm.com/Foru...r-Blowback-Understanding-Actions-Gap-and-Ammo

This link has some good pics and the forum much more friendly than HKpro.

 

[dogmush]

ts a multi piece bolt and when it slam forward the locking piece pushes the rollers out and Locks the front portion of the bolt in to the trunnion, after the round is fired Inertia is transferred to the heavy mass of the bolt carrier which moves back taking the locking piece with it, that allows the rollers to retreat and the whole bolt moves back as one.
Its quite simple really.

Not quite. Remember, the bolt head and the bolt carrier aren't touching each other. (unless it's a CAI rifle, then they might be)

After firing the bolt head moves back, and the rollers are pushed in against the locking piece by the trunnion. This action (Trunnion pushing rollers, rollers pushing locking piece, locking piece pushing bolt carrier) takes long enough that the bolt head is still (mostly) in place when the bullet leaves the barrel.

Once the rollers are retracted and the bolt carrier is moving then inertia takes over to complete the rearward movement.

 

[briansmithwins]

theres a difference between roller locked and roller delayed blowback, but delayed blowback roller locked is a bit weird.

That's a understatement.

The MG42 (for example) is a roller locked machine gun that's recoil operated the recoiling barrel unlocks the action and provides the energy to operate the firing cycle.

In the HK designs the operating force comes from the pressure the cartridge head exerts on the bolt. As soon as the bullet starts moving down the bore, the case starts moving back.

But bolt carrier is under a great mechanical disadvantage that keeps the bolt closed long enough that the cartridge case doesn't rupture.

Totally agree that the HK system is never technically 'locked'.

BSW

 

[Ian]

I did a video on two German WWII prototypes, one roller locked and one roller-delayed blowback. I think it does a good job explaining the difference between the two systems:

https://www.youtube.com/watch?v=WEPwmYcCPFs

 

[Payload]

It's still bugs me that they are mixing two distinctly different methods of operation. They've been manufacturing these for years. It comes off as either unprofessional or as the "safety-scissors-and-glue" approach. "We don't expect you to understand what a delay is so we'll just tell you it's a lock in a manner that it can't legally backfire on us."

 

[kBob]

Toe-May- Toe, Tah-maht- oh, Toe-May-Toe, Tah-maht-oh, lets call the whole thing off.......

just for giggles put some thought into the HK 91 firing pin and what it does besides make the gun go bang.......

-kBob

 

[Sol]

If I recall correctly, the AN-94 has a gas system like an AK. The uniqueness to it is that it uses a cog and cable system to cycle some of the parts for THEORIZED recoil mitigation due to it's 2 round burst cyclic rate.

 

[Slamfire]

The video that Ian referenced, :Last Ditch Innovation: The Development of the Gerat 06 and Gerat 06H Rifles
http://www.youtube.com/watch?v=WEPwmYcCPFs

is one of the best I have seen on this topic.

Breaking the friction between case and chamber has aggravated Gun Designers since the very first semi auto matic mechanisms.

From Arms and the Man 1 Nov 1920

“John Thompson’s surprise Party” by Capt E. C. Crossman

General John Thompson

So the submachine gun was fitted out with felt oil pads in the rear of the receiver housing, over which or by which the bolt slides at each stroke. The “gun crew” has nothing to do with the oiling beyond squirting some more into the pads every 500 rounds or so. …

The locking bolt is the working out of a theory which I have followed for some years with much interest. Some years ago, trying to blow up a Winchester Model 1886, one of the best and most popular of the lever action type, I found much to my surprise that the bolt would unlock, and would back out of the length of the shell in firing. The conditions permitting this, were, the removal of the lever catch on the tang, and second the oiling of the cse, combined with an action pretty old and worn smooth and free of extra friction.

The gun did not blow open, it did not open under heavy pressure, it was entirely safe so far as that is concerned. It unlocked when the pressure was nearly off, and the bolt slid gently open, taking the fired case with it.

I wrote up this peculiarity, and a Navy Officer, Commander Blish, called on me, and discussed the matter. It developed that he had found by experimental work, combined to a few cases of inexplicable opening of the breech bolts of big guns, firing blank cartridges that any arm which has a bolt cammed entirely shut, as by a locking bolt of the type the M1886 referred to, is likely to cam itself open again when the breech pressure falls enough to “unlock” the friction surfaces the locking bolt in its grooves in the receiver and breech bolt.

The Thompson-Blish arrangement is in neither class. Nothing moves while the gas pressure is high, the breech is immovably locked. Not until the pressure falls does the bolt cam down the locking bolt, and start backward.
It is apparent that friction is the cause the locking tendency, and therefore, as oil makes friction uniform it may be, as pointed out, that the oiling system, helpful as it is to certainty of function, is a necessity.

Thanks to Google books for putting this on the internet.


Prior to chamber flutes, the early fielded delayed blowback highpower actions used oilers.

Schwarzlose:


Breda M1930:


Nambu:





Col Chin gives the reasons why:




The memory of oilers has disappeared from the shooting community as the HK rifles since WWII and fluted chambers put messy oilers on the ash heap of history. In fact you will run into angry, vocal groups of people who insist that it is dangerous if the case does not "grip" the chamber. The fact is, without the friction breaking action of those chamber flutes the roller bolt actions would not function. It is just that simple.





I don't understand why fluted chambers are not on all semi auto rifle chambers as the less breech friction the more reliable the mechanism.

XTRAXN is fluting the chambers on the AR15 type rifles: A great idea.






XTRAXN came up with a catchy name for what they are doing, so that will be just another aggravation for those who want consistent terminology.

But then, cartridge names don't make sense either. The last time cartridge names made sense was in the Blackpowder era, caliber, powder charge and bullet weight. 45/120-500. That told everything you needed to know.

 

[Payload]

AN-94 does not mitigate recoil... It delays the recoil of the first shot until the second shot has been fired. So you get two rounds off before taking what I can only imagine is one rough kick. The G11 did something similar.

Something that's always bugged me about the flutes. I get how they could equalize pressure between the cartridge and chamber walls, but I don't get how they would do anything to help the cartridge once it starts to withdraw. On a tapered cartridge I can see it increasing the working surface on which the gas pressure acts but I don't see this having too great an effect for a variety or reasons. What would it do to a straight walled cartridge? Providing space for the cartridge to fire form into sounds like it would not only increase the "weld" of the cartridge to the chamber (ever seen how they make semi-trailer flat beds?) but also increase the surface area and friction. Everything about the idea sounds counter intuitive as far as improving extraction. Yes I've seen the evidence of case head separations and such, but I would sooner suggest that the cartridge is extracting too fast. How and Oiler would do the same job as flutes is completely beyond me.

 

[briansmithwins]

Friction depends solely on the force and coefficient of friction and is independent of surface area, as counter intuitive as that is.

The flutes equalize the pressure outside the cartridge with the pressure inside, reducing the force.

BSW

 

[stubbicatt]

It isn't really a question of equalizing pressure, it is more that the gas transmitted by the flutes into the chamber is itself a lubricant, preventing cases from gripping the walls under the high pressures of firing. Once the rollers "unlock" the rearward thrust of the cartridge against the bolt face continues to allow extraction and subsequently, ejection.

I've heard it said that on the HK91 one does not even need an extractor if one fires the cartridge, the case simply blows out of the chamber as the kinetic energy imparted to it forces it rearward. The extractor is useful in extracting unfired cartridges, and in holding a spent cartridge against the bolt face so that the ejector can kick it out of the action.

I was surprised to find those flutes in the chamber of my SVT 40 too.

 

[Slamfire]

Something that's always bugged me about the flutes. I get how they could equalize pressure between the cartridge and chamber walls, but I don't get how they would do anything to help the cartridge once it starts to withdraw. On a tapered cartridge I can see it increasing the working surface on which the gas pressure acts but I don't see this having too great an effect for a variety or reasons. What would it do to a straight walled cartridge? Providing space for the cartridge to fire form into sounds like it would not only increase the "weld" of the cartridge to the chamber (ever seen how they make semi-trailer flat beds?) but also increase the surface area and friction. Everything about the idea sounds counter intuitive as far as improving extraction. Yes I've seen the evidence of case head separations and such, but I would sooner suggest that the cartridge is extracting too fast. How and Oiler would do the same job as flutes is completely beyond me.

The amount of analysis that goes into building a successful semi automatic mechanism is beyond the comprehension of most. You just have to work on sophisticated engineering programs and see teams of mechanical engineers working on a single item, to appreciate the complexity of the task.

These come from AMCP 706-260 which can be downloaded at DTIC.

Pressure curves are carefully measured.



Cycle times are carefully calculated.



That little inset in the pressure diagram is vitally important to a semi automatic gun designer. He has to design his weapon so it unlocks before pressure is zero, but not above the rupture strength of the case sidewalls. Lets say the weapon unlocks below 650 psia, the residual pressure in the barrel helps push the case out of the chamber and keeps the mechanism moving, thus increasing the time useful energy is available to use. All of this is highly influenced by the mass of the mechanism, spring constants, and of course, the pressure in the barrel. Designers have to balance all of that out and it is not a simple effort.

There is friction between the case and the chamber and that resistance to movement is bad for reliable extraction. Breaking the friction between case and chamber with either a bottlenecked or straight walled case is going to do the same thing, get that case moving once unlock occurs.

These are oiler mechanisms to be found in LTC Chin’s Book of the Machine Gun

 

[Malamute]

I've read about them in the past, thought I had it figured out, but it seems contradictory. I think its just magic.

However they work, they seem to work very well. I had an H-K many years ago, it never ever missed a beat, and I shot some very dirty, nasty corrosive german ammo in it. Was nice buying ammo for $2/box retail though.