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.