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Rebarrel job/oversize barrel pin question: what are the limits?

4205 Views 9 Replies 5 Participants Last post by  a.stoop
I found a C93 at the gun show yesterday for a reasonable price. It had bolt gap, the bolt was in great shape, internals were good, and the sights were straight. The only drawback was cosmetic: an ugly cocking tube weld. No big deal, so I bought the rifle. The bolt gap was at .010, so I put in oversize rollers, first +2's then +4's. The +2's didn't make a difference, and I then realized that the bolt gap was being artificially caused by the bolt carrier bottoming out on the rear of the cocking handle support. The +4's increased the gap, but only to .012. I checked the barrel pin, and it came right out with a few taps of a mallet and punch. This of course will mean that the gap will most likely settle to below spec once again after firing the rifle. I was able to insert a .1995" oversize pin(which is huge at almost 5.07mm) which went in nice and tight. This would have most likely held some semblance of bolt gap, but it just seems like a better idea to pull the barrel, flip it, and start over; otherwise, the bolt carrier is eventually going to bump up against the cocking handle support once again and give a false bolt gap reading.

This thread from last month had some great information: Since I would be starting with what is already a vastly oversize barrel pin hole, however, what would be the limits as far as barrel pin diameter? Also, what size mill/drill bits would be wise to start with? Thanks in advance for any help.
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Hi GOH - saw your PM. That's not a problem at all. You're only really .002"-.003" oversized. On my very first real HK build, I converted a 93 into a 53. I was starting with a 53 kit, so at least I didn't have to fabricate much. But when I drilled the barrel pin, I followed it up with a new 5.00mm reamer, just like I'd read to use. Since it was new, I prepared it for first use like I'd always done by drilling and reaming a few holes first. Then I ran it through the hole in the trunnion. Then I tried to put the barrel pin back into place, and it pushed all the way through just with my finger. Since it was SUPPOSED to be a .001" interference fit, but I actually had a .001" clearance or so, it was about .002" too large, just about exactly what yours is.

No biggie, I just bought a hardened gage pin, cut it to length, dimpled the ends, and installed it in my press. The bolt gap didn't drop, and it still shoots just fine. I haven't shot it much, maybe 1,000 rounds or so, but most of it in full-auto. I wouldn't push it, but .002" oversized isn't going to cause a problem. As for what size bits - I don't use a reamer any more. At all. I mill completely through with a 3/16" carbide end mill, then open it up on the same center with a #9 (.1960) drill bit. I don't run it up and down, just one pass through, then back out so as not to oversize it any more. I settled on this size by measuring the diameter of factory barrel installations with gage pins and rounding down to the nearest drill bit. It results in a press fit the same as a factory installation.

You can order gage pins (they were $2 or $3 each) from jlindustrial. I ordered several just to be sure I could get the fit I wanted. I don't know where the limit would be, but .002" won't be a problem. More important is that you get the hole drilled straight and round, so careful setup in the vise is critical.
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Myself, I'd hesitate to use an .002 over pin, but that's me. I'd be afraid to crack something. Then again I havent done tons of these either.

If by "flipping" the barrel you mean pulling it, turning it, and redrilling another pin hole, I think I'd recommend against that also. The barrel has been turned down by CAI and the interference fit is practically gone. That's the problem with the C93. You don't want all the forces from firing the gun to be supported by the pin, rather you want the majority of that force to be taken by the trunion which means it needs a tight fit to the trunion. CAI defeated that by turning the barrels down, or so the internet mill says.
I've read of people taking AK barrels and welding up the OD of the barrel then turning the barrel down to get a better trunion fit so I think that would work here. If not then most folk replace the barrels. For the C93 and what it is, had I access to a lathe I'd weld it up and turn it down, reinstall it and blast away. It'd work just fine.
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Thanks very much to both of you. I've already ordered a 3/16" end mill and several gauge pins of various sizes. The idea of running the #9 drill through only once sounds like a good plan. I did pull the barrel and flip it so it would be ready to go when my end mill and pins arrive. The barrel didn't take as much force to re-seat as a factory barrel does(hkshooterusp is right on the money here), but it did require the hydraulic press, so I won't worry quite yet. Worst case scenario is that I have a $500 parts kit, but I doubt it will come to that.

Again, guys, thanks for the help. The rifle should be up and running by the weekend, and I'll let you know how it goes--Rich
GOH - following up on what hkshooterusp said - when you pressed in the barrel, how much pressure did it take? Like he says, turning down the barrels is a bad idea. It's the interference fit between the barrel and trunnion that does most of the work, the barrel pin is just for a positive stop. If the barrel went in very easily, I'd be concerned that the pin groove would wear quickly and the gap would drop too soon. Based on the value of a C93, before I took the time to build up and then turn down the barrel, I'd be inclined to get the gap set on the high side, and then weld the barrel to the trunnion in a few spots at the front of the trunnion.

It usually takes 5 tons to get the first "pop" to get an old barrel started moving when removing it, but then takes just over 1 ton to install it back into the trunnion. If it slid in too easy, you may have problems down the road, even if you get it set right now.
After reading all the info in militaryfirearms G3 trunion survey thread, in hind sight I may not have gone with a .002 oversized pin, but with that said, my C93 also needed a barrel re-press an oversize pin. My barrel to trunion fit was nice and tight but the pin channel was crap and let my gap drop significantly. I re-pressed with my 12T press to right at .020" bot gap (I suggest putting a pin in the channel that's close to help prevent over pressing the barrel on theway back in). Once the gap was achieved I reamed with .199,.201, and finished with a .202" reamer to get a perfectly concentric hole and installed . .002" oversized pin that was frozen prior to shrink slightly. Bolt gap has remained at .017(5) after nearly 800 rounds without issue.
With that said, I've never had a virgin trunion in my hand and have no way of knowing how much material reaming the hole to .202 left at critical points on the trunion
and will defer technical information to true HK builders. If the G3 trunion survey holds true to the 33/53/93 series, a pin press fit of .001" may be more desireable to avoid trunion cracking. The only other information I'd like to know would be if Century reused the 33 trunion, or a new US made part.
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When I pressed the barrel back in, it was popping/jumping, not just sliding in easily. I installed -2 rollers in the bolt and it's set right at .020, so lots of wiggle room in case there is some movement. If it won't hold, the barrel will be replaced with an RCM version, which really wouldn't hurt my feelings at all.
Rebarrel job/oversize barrel pin question UPDATE and THANK YOU

I followed advice given on the thread here: 3/16" end mill from both sides, follow with #10 and #9 drill. The hole that the original "gunsmith" cut was so mangled on one side that it needed a 5.08mm gauge pin to close it(I'm just amazed by the quality craftsmanship at Century). The new pin pressed in nice and tight, and I took it to the range this morning. Bolt gap is steady and holding .017" after this morning's shooting session. Since the bolt has -2 rollers in it, the rifle has a long life ahead of it. I'm gonna shoot it until the inside of the barrel is as smooth as Obama's crack pipe.

Thanks to those who chimed in with some great advice!!

When I pressed the barrel back in, it was popping/jumping, not just sliding in easily. I installed -2 rollers in the bolt and it's set right at .020, so lots of wiggle room in case there is some movement. If it won't hold, the barrel will be replaced with an RCM version, which really wouldn't hurt my feelings at all.
That's normal for the barrel to pop back in. And if yours did, then your interference fit sounds like it's correct. The cause is actually in your press, not the trunnion. It's because friction from a press fit is greatest when the parts are static, and it drops as the start to move. So as you build pressure in your press, the supports, press plates, and even the press frame will all deflect under the load. Then once you build enough force to overcome the friction, the friction drops as the parts start to move in relation to each other. Then all of that deflection that was forming into the supprts, press plates, and press frame is released, and as they return to their undeflected position, and you get a bigger "jump" than the press ram itself was moving.

You'll always get a bit of popping when it's pressed in. To get the smallest possible "pops", make everything as rigid as possible. I had one barrel that I must have had to press out and repress 5 times. I'd get it to .024 bolt gap every time, then the last pop would drop it to .010. So I welded stiffeners to my already very heavy press support (I have a 40 ton press), welded two pairs of press plates together, and support them with a cinder block underneath to spread the load out all along the press support. Now the "pops" are tiny - just a few thousandths with each "pop" and I can hit the desired bolt gap every time.
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