Another one done

Though it's been done for months, I never got around to posting pictured of my M16A1 build.  To recap, the lower is a standard 80%, the barrel is a JSE 20" Lightweight profile, the sling is GI surplus, and every single other part is original Colt M16A1(with FCG parts modified for semi-auto only).  The new lower was painted with fake worn pain, everything else has it's original worn Colt finish.  I'm still want to find a "correct" 20rd Colt magazine, but this original 30 rounder will work for now.

Are you mocking me?

I finally have enough pieces that I can mock up my PSG-None.  None of the 3D prints are glued together yet. Even though I'm likely going to paint all the 3D printed parts, I printed them in black so that if/when they get scratched, the scratch won't show as much(except for the buttpad which is red because that's the only color of Ninjaflex TPU I have).

More faking it while making it

Because this is a low dollar range toy, I didn't want to buy a high end match barrel that cost more than all the other parts of this project combined(and I'm not a good enough shot for a barrel like that anyway).  The barrel I'm using is an 18" CETME barrel from Apex.  It is a very nice barrel, well machined and reasonably priced.  There is one problem with it though, it's 18" and not the 25.6" of the PSG-1, and it's too thin compared to the heavy barrel contour of the PSG-1.  Because of that, it looks obviously too short and too thin with my 3D printed PSG-1 style handguard.

Don't worry though, I have a plan for that.  The muzzle on the Apex barrel is threaded 5/8-24, a standard size for 30 caliber muzzle brakes and flash hiders.  So I decided that I would just make a barrel extension that threaded on.  In addition to being an extension, it's also a sleeve that slips over the barrel until it's under the handguard to make it appear that the whole barrel is heavier.  I used some 3/4" 1144 stressproof steel bar(because that's what I had handy), drilled all the way through it with a really long 11/32" drill bit(three actually, short, medium, and long, and it took me all afternoon to drill on the lathe), then counterbored it to fit over the barrel, with a smaller step 5" down for the threads, and tapped it with an $11 e-bay 5/8-24 tap.  I had my doubts about the e-bay tap because it was so cheap, but it cut great and still looks like new.  I had to get creative with the tap holder because the tap is 4" long and the threads are 5" down in the hole, and I ended up sacrificing a thin wall 12 point socket for it.  Here's where the extension sits on the barrel, if you could see a cutaway of the extension, you'd see about an inch of threads in line the barrel's muzzle.

With the extension in place, the overall barrel is the same length as the PSG-1, and looks much better with the handguard.

I also put a recessed crown on the barrel extension instead of the flat crown on the PSG-1, mostly because I don't trust myself not to ding it up.  11/32 is only .035" bigger than .308, so it's not too noticeably bigger at a casual glance, and hopefully it won't affect accuracy.  I also don't know for certain that the drill didn't wander, but being cut on the lathe all from one direction hopefully it will be close enough.

A barrel of fun

One of the things I've been avoiding the most on my CETME build is pressing the barrel into the trunion.  I actually bought my press specifically for this one thing, and I still put off it as long as I could.  The reason I wasn't looking forward to it is because it has to be very precisely located, and hydraulic presses aren't really known as precision equipment.  Unlike a normal gun where we'd need headspace gauges to check our clearance, HK and CETME roller delayed guns are measured by checking the bolt gap.  If the barrel is pressed in too far or not far enough the gun won't be safe to shoot, and the go-no go range is about .015"(about the thickness of 5 sheets of notebook paper). 

Since my lathe is up and running, I turned a brass piece to use for pressing the barrel.  Here it is ready to press the barrel.  There is a nub on the end that fits into the chamber to keep it centered.

I covered the barrel in anti-sieze and pressed the barrel most of the way using the brass piece, stopping and checking often.

Once it was most of the way there, I switched to using the bolt assembly it's self to press it the rest of the way.  My press plate has a hole close to the edge to give the bolt carrier some room.  I also put .020" shims between the bolt head and the bolt carrier to make sure I maintain my bolt gap.  My brass pusher piece has a hole in the center to clear the firing pin while pressing on the bolt carrier.

There was still a lot of pressing-checking-pressing-checking, etc.  Once I got close I put the trunion in the receiver and checked the gap.  Outside of the receiver the bolt head can wiggle a bit and throw off the reading, so it should always be checked with everything assembled.  The bolt gap spec for CETME and G3 based rifles is .004"-.020".  New builds tend to "settle" a little bit after firing and the bolt gap will close up some, so you want to shoot for the high end of the spec.  If you miss the spec or you shoot enough that you actually wear it out, over and undersize rollers are available, but it's best to try to get it in spec.  I ended up right at .019".

You can see all my welds here in their disappointingly sloppy Mig welded glory.  The trunion, receiver, and reinforcement rail are also all welded together at once through the hole that was drilled in the reinforcement rail.  While I was at it, I also drilled the holes for plug welding the cocking tube.

 

 

I left the barrel pin hole undrilled until the the trunion was welded into the receiver in case things moved a little while welding(and nothing did).  Normally, machining something like this is a pain because the existing hole in the trunion will want to make the drill bit wander as it's trying to drill through the barrel.  CETMEs and HKs are a bit different though.  The trunions on these things are HARD.  They will just laugh at a standard drill bit.  Because of this, you can use the trunion as a drill guide to drill the softer barrel.  The barrel pin is 5mm exactly.  Instead of drilling and reaming the barrel for the pin, the easy thing to do is to drill it with a #9 drill bit.  A #9 drill is just .0008" under 5mm, and makes for a good press fit without reaming. 

 

I'll just shelf this one for later.

Since using the stock front grip pin on my CETME makes it a machine gun by default, I am going the standard route for semi-auto HKs and replacing the front grip pin with a shelf welded on the receiver so that an unmodified full auto grip cannot be installed, with a matching shelf welded into the grip assembly.  Conveniently, this is the same method HK used on the real PSG-1 so I won't have to put a fake pin in like a lot of people do.  Since it was always intended to be a semi-auto marksman's rifle, the PSG-1 was set up from the get go with a semi shelf already installed.  Since I'm cheap, I decided to make my own shelf on the mill even though they are commercially available.  To locate the shelf on the receiver, I printed up a locating jig that is available on Thingiverse.  

The grip assembly needs to have a matching shelf welded into it.  I made one out of a piece of angle iron I had laying around.  I also made sure that the modified trigger box fit with the shelf in place before I welded everything.  In hindsight, I should probably have milled one that looks nicer, but it works fine and didn't take long to make.

With those pieces welded in, I could start fitting things together, and it's finally starting to look like something.