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Discussion Starter · #1 · (Edited)
I apologize for the delay in the upload of this segment. I was on vacation two weeks ago and last week I worked over on a special project for my company.

Not to be remiss toward my obligations on this series of threads, I have conducted quite a bit of research these past few weeks.
I would like to restate that the motivation behind this thread is to discover if the PTR-91 is battle ready. ‘Is the PTR-91 an effective & capable firearm?’

Looking first at its construction seems to be a good place to start. To investigate the materials and techniques used by PTR in their GI series and compare them to the original HK G3/91, as well as other 7.62mm NATO rifles re: ‘FN-FAL, M14 & AR-10’.

Initial information was provided to me by Chris Petersen, Customer Service Manager at PTR-Industries. The previous threads focused on the barrel and the trunnion. This thread will focus on the receiver.

According to Mr. Petersen the PTR-91 receiver is made from a sheet of 16 gauge, 1018 grade steel.
1018 grade is a mild carbon steel and initially I was a bit put back by this information. I was already aware that several US made AKM receivers were of 4140 grade steel and I was fully expecting the same from PTR. However, the real test is comparing the PTR receiver to the original G3/91.

Now, I have spent hours-on-hours searching the internet for technical data on the G3 and the CETME. And to my own dismay I have been unable to find anything. I’ve used dozens of different keyword combinations, including German & Spanish language keywords, and I must report that I found nothing of substance.
I did consider for a moment that the G3/91 technical data is a closely guarded secret, privy only to a select few. But it may also be that the information is out there and I simply failed to find it. In any case, I cannot make a direct comparison of this important component between the PTR-91 and original G3.

This stands in stark contrast to the M14, which belongs to the public domain. The original complete technical drawings for the M14 are available to everyone, for free. These drawings contain all the blue prints and technical information needed to manufacture the M14 to its original specs. But, it appears that no such public access is given to the designs of the G3, or the FAL for that matter.

However, in my search for technical information I did come across a few bits and pieces, a few clues as to the durability of the PTR receiver.

The first clue is on page 8 of the HK G3 Armorer’s Manual. There, can be found the section “Removing dents in the receiver and cocking lever housing”, where is shown the receiver straightening mandrels. The manual proscribes the use of a plastic hammer in removing dents from the receiver.

A plastic hammer? To remove dents from the receiver of a firearm that was designed for the nuclear holocaust that was to be WW3?


Photo shows the bottom front section of the receiver. You can see the arc welds that attach the trunnion and receiver together as well as the weld seam that bonds the receiver edges.

Auto part Cylinder Muffler





Now if you have any experience working with 4140 chrome-moly steel, you know that it’s some tough stuff and a plastic hammer is usually not the first tool applied when trying to shape it. Not that a big enough plastic hammer can’t be employed, ‘I have an Estwing 45oz polyurethane hammer that will knock a dent in just about anything, given a hard enough blow’. But, that’s not the proper way. A plastic hammer is used on more gentle materials, like, mild steel.

The fact that HK produced ‘straightening mandrels’ to begin with, I suspect, was that their G3 receivers were made from comparatively mild steel.


Photo shows arc weld seam that attaches the semi-auto ‘shelf’. Its hand welded and visually appears to show good penetration between the shelf, seam and receiver.

Revolver




Looking more closely at the properties of 1018 steel, I found it’s tensile and shear strength to be considerably higher than 7075 aluminum alloy, (used in the construction of so many AR-10 receivers), albeit, the AR-10 receiver is thicker than the G3.





Pressing as hard as I can with my thumb and finger, the mag well area is stout. And becomes even stronger with magazine installed.






Also, I discovered that 1018 steel has good ‘work hardening’ properties, and this is where a big difference begins to develop when comparing the PTR receiver to the AKM.







Side view of the magazine well, *Note the raised rib which physically reinforces the mag well area and work hardens the steel.

Font Metal










The weld seam that attaches the rear of the trunnion to the left side of receiver. It’s also hand welded and shows good penetration. *Note the deeply stamped, (and work hardened), carrier guide rail.













This photo of the rear of receiver reveals the retaining pin guides that are pressed in and then welded into place. They are seated through the entire width of the receiver and are reinforced with a bracket. When the rifle is fully assembled with back plate and pins this area of the receiver becomes virtually indestructible.

Auto part










Underside of receiver shows pin guides are pressed flush with outer surface of receiver and then welded to the reinforcement bracket.

Technology Electronic device




Looking at the AKM receiver you will see that it is a very basic & simple piece. There aren’t a lot of stamping operations going into it.

2 AKM receivers, side-by-side.

Technology Mobile phone accessories



Now compare that to the PTR’s receiver, with its much more complex geometry and deeply stamped guide rails. The PTR has many more stamping operations to it; much more work is being done. The design of the G3/91 receiver hardens the steel as its being formed in the press.







Rear edge of receiver shows its complex geometry. There are virtually no flat surfaces, every area has a formed shape, every area of the steel is either work hardened or reinforced.

Revolver





Searching for more clues, I began to look at the various firearms forums. If PTR receivers were soft then deformation, (receiver stretching and cracking), would occur and this failure would certainly find its way into threads posted by other PTR owners. My search not only revealed no such occurrences, but, I actually found multiple praise for the PTR series among the many owners of registered sear packs , for automatic fire. Certainly these guys are firing thousands of rounds through their full auto PTR’s with no complaints about soft receivers. (Not to say there haven’t been any complaints at all, because there are some, we all know of the issues that some PTR models have had). But it appears that the overwhelming consensus, especially from the machinegun guys, is that the PTR-91 is a good host gun for a registered full auto sear pack.



OEM back plate installed onto PTR receiver. *Note the relatively tight tolerances between these two components but the pins & back plate are still easy to remove.

Photography Gadget




So then, despite being unable to compare the PTR receiver directly to the original G3 I am reasonably certain that the 1018 grade steel receiver of the PTR-91 GI has more than enough strength & durability to withstand the rigors of just about any imaginable scenario.
Up next we will examine the internal parts of the PTR-91 GI, , ,

NDS
 

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Germans are the ultimate engineers and scientists on the planet. The original US space program would have gone nowhere fast without expat German scientists. PTR has had a few teething issues in the past but I'm hoping they can continue to produce near or equal to the design parameters of these awesome war tools. Enjoy your detailed posts, sir, keep them coming
 

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Nice article, so of course I had to inspect my older JLD receiver. It has extra reinforcements on the 2 tubes that hold the 2 rear pins. While I can't figure out how to post those photos here, I could e-mail those photos to someone who could post them here. When I purchased my JLD gun 8 or so years ago, there was a real HK91 next to it. I compared them closely, & to me the JLD gun was nicer. While it's not a HK value wise, it seemed the people who built it put more effort into the quality. Of course back then, JLD was still a relatively new company, so they tried harder. GARY [email protected]
 

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Discussion Starter · #7 ·
Nice article, so of course I had to inspect my older JLD receiver. It has extra reinforcements on the 2 tubes that hold the 2 rear pins. While I can't figure out how to post those photos here, I could e-mail those photos to someone who could post them here. When I purchased my JLD gun 8 or so years ago, there was a real HK91 next to it. I compared them closely, & to me the JLD gun was nicer. While it's not a HK value wise, it seemed the people who built it put more effort into the quality. Of course back then, JLD was still a relatively new company, so they tried harder. GARY [email protected]
I remember when the JLD guns first hit the gun stores and I couldn't believe how well made they were. Back then the price for one was $750. LOL.
 

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In 2007 I bought an FMP kit from Apex and a JLD PTR91 receiver off Gunbroker to build my semi G3 rifle. Like the OP, I contacted the receiver manufacturer, then JLD Enterprises, to find out the type of steel used in their receivers. I was told they were 1095 mild carbon steel at that time. IIRC, I was told this was the same as the original HK receivers. If one thinks about it, the stamped and welded receiver only acts as a channel to guide the bolt carrier as it cycles. No super strength needed there, as the barrel, trunnion, rollers, bolt and bolt carrier provide the lock-up to contain the cartridge upon firing.
 

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Discussion Starter · #9 ·
In 2007 I bought an FMP kit from Apex and a JLD PTR91 receiver off Gunbroker to build my semi G3 rifle. Like the OP, I contacted the receiver manufacturer, then JLD Enterprises, to find out the type of steel used in their receivers. I was told they were 1095 mild carbon steel at that time. IIRC, I was told this was the same as the original HK receivers. If one thinks about it, the stamped and welded receiver only acts as a channel to guide the bolt carrier as it cycles. No super strength needed there, as the barrel, trunnion, rollers, bolt and bolt carrier provide the lock-up to contain the cartridge upon firing.
Interesting !

For reference,

AISI 1018 LOW/MILD CARBON STEEL
Tensile Strength, Ultimate - 63800 psi
Tensile Strength, Yield - 53700 psi
Shear Modulus - 11600 ksi
Hardness, Rockwell B - 71



AISI 1095 Carbon Steel
Tensile strength - 99400 psi
Yield strength - 76100 psi
Shear modulus - 11600 ksi
Rockwell B - 92

Sourced from AZOM . com
 
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