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Old 06-01-2008, 06:13 PM   #1 (permalink)
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Home-Building a CETME/HK-G3/PTR Rifle

So the latest rifle project I have officially kicked off is an HK/CETME/PTR rifle. My end goal for this build is to end up with an HK-G3 rifle, using a combination of CETME, PTR, and HK parts. I am also building off an Ohio Rapid Fire 80% flat.


The first step was to acquire some flats. I will be building this with many of the same group of guys I built FALs and AKs with over the past few years, as well as a possible few newcomers. Either way, I'm definitely excited.

My list of compliance parts will be:

US Barrel
US Muzzle device
US Receiver
US Rear Stock
US Cocking Handle

I still need 2 US parts, and I hear rumors of triggers and hammers floating around. I will keep my eyes out for one.


I will also be doing a clipped-and-pinned SEF lower. I intend to keep the SEF markings, however it will obviously be a Semi-only build.

I'll post progress as it occurs, but it will not be a continuous thread. I'll post up my experiences during this build, so take them for what they are. Your mileage may vary.

The first step (besides acquiring a CETME parts kit sans-barrel for all of 50 bucks from Centerfire way back when...) was to get an ORF flat. These were pressed off of original HK dies, so they should be identical to an HK receiver.




The next step is to press the receiver into shape. I have a bending thread posted that documents this, but I will post it here as well.





Complete with dummy bolt carrier




Start by placing your flat into the jig and centering it using the centering bolts.




Next, put the rod piece in. This is a round bar that basically forms the top radius of the bend. It is guided by slots on the front and rear of the jig. Begin to slowly apply pressure and watch as the flat begins to slowly form up.



Once pressure is applied, the ends begin to rise up (if you've done an AK, you are familiar with this) and keep applying slow gentle pressure on it, and keep both ends coming up at roughly the same pace. Every flat I bent tended to rise up on the left side a bit faster than the right, but they came out just fine.




Keep pressing until the ends meet, and then do a little more. Remember that metal has memory, and will tend to bend back the way it was. So by going a little further, once it pops back out, the ends will be much closer than they would if you simply stop as soon as they contact. Be sure to check to make sure both ends are equal, or at least close. If they are not close, you *can* fix it, but it takes a LOT of effort (I know, because one guy bending with us had his get off-center, and we had to spend a lot of time getting them back right, but the good news is, it can be done.)




Doesn't get much closer than that.

And here's the final results:

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Old 06-01-2008, 06:14 PM   #2 (permalink)
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So, the next step is to demil my CETME parts. Mainly, I'm interested in the cocking tube, trunion, and mag lock and release parts.


I start by chocking up the cocking tube and trunion into a vice. Don't go too tight, otherwise you risk deforming your tube. Notice the plug welds, in addition to the weld where the tube meets the receiver.




Using a dremel with a cutoff wheel, I cut down the length of the receiver, as shown.




Flipped it over and did the same to the other side. You want to be careful and not cut too deep. There is a sleeve inside the receiver, and if you get too carried away, you will cut this sleeve.




With those two cuts, the trunion drops away, and can be set aside.




Next, I need to remove the excess scrap receiver still welded to the cocking tube. You want to be VERY careful while cutting this off. You can very easily cut into the cocking tube's sleeve, and ruin it. You also want to pay attention where you cut. If you cut your tube too short, you can potentially have issues with bolt-gap later on. I used tape to mark where I did NOT want to cut.




Now comes the hard part--the plug welds.




I ground them down. You want to go slow here, and pay attention to the grinding tool. You can notice the metal seams as you break through the weld, so when you see the seams, stop, and begin to pry away the metal. On my kit, there were three plug welds on the side, and two on top.




Now the cocking tube is off and can be set aside.




To get the trunion out, you will have to grind as many as 6 plug welds. There should be 2 on each side, and one on the bottom. Once these welds are ground out, peel away the scrap receiver, and set your trunion aside.




You can, if you choose, press in your barrel at this step. I would recommend it, as having the trunion out of the receiver is the easiest way to do it. I will post more on the bolt gap and barrel pressing once that step is final.

I mocked up the parts to see how it's all looking. I'm not happy with the 16" barrel, and instead will be getting an 18" barrel. 18's weren't available when I ordered the 16, but I've seen them back on the PTR website, so I will save the 16" barrel for a G3K clone for a future build.





More to come... next step will be to weld the receiver... I'll follow up with pics once that has happened...
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Old 06-01-2008, 08:57 PM   #3 (permalink)
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Looks like an oversized MP5 in that last pic LOL
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Old 06-02-2008, 06:34 AM   #4 (permalink)
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Quote:
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Looks like an oversized MP5 in that last pic LOL


That's because a G3 IS an oversized MP5
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Old 06-02-2008, 07:10 AM   #5 (permalink)
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Looks good. Do you plan to Tig or Mig the receiver?
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Old 06-02-2008, 08:38 AM   #6 (permalink)
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Quote:
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Looks good. Do you plan to Tig or Mig the receiver?



TIG
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Old 06-02-2008, 01:09 PM   #7 (permalink)
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That's because a G3 IS an oversized MP5
Uhmmm, nope.

MP5 is a miniture G3
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Old 06-02-2008, 03:53 PM   #8 (permalink)
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You should machine and weld a picatinny rail on the receiver...right in front of the drum site. Make it low profile and no need for the crappy HK clamp-on shit!
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Old 06-02-2008, 04:59 PM   #9 (permalink)
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Quote:
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You should machine and weld a picatinny rail on the receiver...right in front of the drum site. Make it low profile and no need for the crappy HK clamp-on shit!


I am way ahead of you on that...
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Old 06-03-2008, 08:14 AM   #10 (permalink)
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I am way ahead of you on that...
Ok, that sounds interesting, very interesting.....

What do you have up your sleeve? I may just steal the idea for my two.
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Old 06-03-2008, 08:18 AM   #11 (permalink)
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Me too, I have 1 1/2' of Scott's steel rail. Any chance I can send it up to you to get the radius cut on the bottom?
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Old 06-03-2008, 08:47 AM   #12 (permalink)
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Wow, I will be watching this thread closely. I've got a CETME kit that needs to be built as well. Heck, I wish I had known you would have that flat bending jig, I would have bought a flat and begged to get invited to the "guys who build 68 rifles at a time club".
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Old 06-03-2008, 09:38 AM   #13 (permalink)
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Me too, I have 1 1/2' of Scott's steel rail. Any chance I can send it up to you to get the radius cut on the bottom?
What is the radius?
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Old 06-03-2008, 09:53 AM   #14 (permalink)
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Wow, I will be watching this thread closely. I've got a CETME kit that needs to be built as well. Heck, I wish I had known you would have that flat bending jig, I would have bought a flat and begged to get invited to the "guys who build 68 rifles at a time club".



Buy some flats and I'm sure we can work something out. The guy whose jig I used is over in Ft. Worth.
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Old 06-03-2008, 11:19 AM   #15 (permalink)
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What is the radius?
most likely it will be close to 7/8", but mine are not bent,yet.
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Old 06-03-2008, 01:08 PM   #16 (permalink)
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Quote:
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most likely it will be close to 7/8", but mine are not bent,yet.


It's just slightly larger than 5/8". Some people use 5/8" rod clamped to a table to draw over their receivers. It's a crude method that produces only marginal results...
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Old 06-03-2008, 02:17 PM   #17 (permalink)
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So how're you doing the bottom of your rail?
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Old 06-03-2008, 06:14 PM   #18 (permalink)
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Quote:
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Ok, that sounds interesting, very interesting.....

What do you have up your sleeve? I may just steal the idea for my two.

I've been talking to scott@rockstomper about getting a few lengths, and once I find out how much I will need, I'll be making an order.

Once the rail is in, we'll figure out how best to weld it to the receiver. It will be similar to the welding required to mount the rear sight base, so I'm not too worried about it being too difficult or anything...



Quote:
Originally Posted by afroman006 View Post
Me too, I have 1 1/2' of Scott's steel rail. Any chance I can send it up to you to get the radius cut on the bottom?

You can send it if you want, or hold onto it for build day (which I still don't know when that will be). Either way, it will get welded on when the time comes. I'm in no rush for it, though...


Quote:
Originally Posted by afroman006 View Post
So how're you doing the bottom of your rail?


Haven't figured it out yet. Once I see the rail, I'll come up with something...
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Old 06-03-2008, 06:59 PM   #19 (permalink)
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I dont care what they say. CETMEs freakin rock!
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Old 09-13-2008, 04:54 PM   #20 (permalink)
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So getting this going again...


I had originally planned on doing this during a build party among some local friends, but I'm kinda tired of waiting on them... seems like everyone is caught up with other priorities, which is totally cool with me, but I'd like to get this gun built, and so I decided to just do it on my own.


Had originally planned to TIG it, but the TIG welding was going to be done by someone else. And since I'm doing it on my own now, I had to settle for MIG. MIG's fine for this, and many people have reported good results with MIG, so I'm using what I have.


I started by clamping the magwell area closed. Almost all of this area gets cut out when doing an ATF-approved semi-build, so it's almost a waste to even weld it. I'm doing it to help hold the receiver together. I started by tacking everything together.




With the magwell area tacked, I moved to the rear. You don't want to weld this together without the stock block in place. If you do, your receiver might be too narrow at the rear. I installed the stock block, and clamped everything in place, then tacked the two ends.

With my heat settings set for sheet metal, I didn't worry about any burn-through getting on the stock block. It is such a thick piece of metal, you get absolutely no penetration when using sheet metal settings, so once the ends were tacked together, the stock block simply popped out with a hit from a hammer.




The stock block needed a bit of sanding on the bottom. You can see how the holes don't perfectly line up. Personally, I'd rather have a block with no holes cut in it, but this works just as well. I sanded the bottom to try and get the holes a bit closer, but didn't make it perfect. That won't be a problem, because the holes for the stock pins need to be drilled out anyway, so I figured the amount it is off won't hurt it, since it will all be drilled out a bit larger anyway.




tacked everything in place before doing final welding.




Receiver welded together. I blew through the end a bit, which I figured would happen.. lol. But I'm not worried about that. Once the trunion is installed, I will fill that area with weld. It kinda looks like the receiver was welded with a huge gap in the ends. This isn't the case, though. The ends were welded completely flush. I installed the front trunion before welding, to make sure everything fit. Then I removed the trunion before doing the final weld.




The rear stock block was a bit tricky. To weld it in, I set the heat settings all the way up. I hit the block with the weld, let the puddle form, then walked it over to the edge of the receiver and let the puddles pool together. If you fuck up here, you will blow a huge hole right through your receiver. And those are never fun to fix.

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Old 09-13-2008, 06:17 PM   #21 (permalink)
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Next up is the cocking tube




I drilled 3 holes on each side for the cocking tube sleeve. This is the same way it is done on the G3/CETME rifles from the factory, so I figured I'd do it this way too. Some people only weld the tube where it meets the receiver. I think the factory also puts two welds on top, but I didn't do those... mainly because I forgot to drill holes for them




Then weld it on.

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Old 09-13-2008, 07:10 PM   #22 (permalink)
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Very nice build so far.
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Old 09-14-2008, 11:03 AM   #23 (permalink)
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So next up is the semi-auto shelf for the receiver. This piece is meant to block the installation of a full-auto lower receiver. It must be wide enough so that a full-auto lower receiver cannot be installed. It also blocks the pin hole for the full-auto lower receiver.


I started by using a 3/8" piece of steel, cut to 20mm x 22mm (yeah I use both systems for measuring... )

In hind-sight, I would rather use a 1/2" thick piece, but this will work just as well. I'm using my Century CETME as a go-by for this, and the 3/8" is what they used.





Then measure and mark for a chamfer.




I ended up going a bit deeper on the chamfer than originally marked. It didn't look deep enough compared to my CETME.




Then measure and mark the receiver where it will be cut out.




Then cut it out.




Cut it just enough to allow the semi-auto shelf to slide in tightly.




And it is wide enough to clearly block an un-modified full-auto lower receiver.




So next up is the paddle mag release switch. I can see why companies don't include the paddle mag release on their builds, as it is kinda a bitch to do. On my semi-auto shelf, I have measured and marked the area that needs to be removed to make clearance for the paddle.




And this is where you guys with your fancy milling equipment can go fawk yourselves with my dremel tool
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Old 09-14-2008, 11:04 AM   #24 (permalink)
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Rather than try to dremel out the whole opening, I drilled it out first.



Then dremeled and hand-filed it to make it square.




The ears of the lower receiver must be widened to fit on the semi-auto shelf. Some people simply cut these off, but I want a clipped-and-pinned style build, so I just sanded them down with a dremel until it fit.

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Old 09-14-2008, 02:24 PM   #25 (permalink)
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And so, putting the receiver aside for now, I moved on to the barrel and trunion.


These are PTR 91 barrels that came from PTR inc. One is 16", the other 18". I am using the 18" for this build.




Press the trunion onto the barrel. This was a fun experience. After pressing in the barrel, and checking my bolt gap, the barrel needed to press in further. So I pressed it in further. Then, after checking the gap again, I needed to press my barrel back out a bit. So I pressed it out a bit. Then, after checking the bolt gap again, I needed to press the barrel back IN again. This went on a few more times before I decided to throw a stack of pennies into the trunion. These stopped the barrel from jumping too far. Seemed to work pretty good...




I was happy with a .015 bolt gap. Spec is anywhere from .010 to .020 if I'm remembering correctly. I set the bolt gap before installing the trunion into the receiver. I've read alot of people doing it this way with no ill effects, but to be safe, I will check it again with it in the receiver before pinning the barrel and welding the trunion.

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