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Thread: Installing a barrel to the trunion

  1. #1
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    Default Installing a barrel to the trunion

    Whats the trick on installing the barrel to the trunion and getting the retainer pin hole lined up correctly?

    Any help would be appreciated

    Thanks

  2. #2
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    There's no trick - you don't even try to do it. It is virtually impossible. No matter how closely you align the rotation, it always twists a slight bit when it starts going in.

    If you're installing a 3-lug mp5 barrel, rotate the barrel 120 degrees from the original slot, press it in, then redrill a new hole.

    If you're doing a 93 or 91 barrel, you could go 120 degrees, or 180 degrees.

    Before drilling the new hole, you have to start it with an endmill so the hole isn't starting on a curved surface. Drill it through at 3/16", then ream it so it's about .002" smaller than your barrel pin. The standard barrel pins are .1970", so a .1950"-.1955" hole should be about right.

    If you screw it up and make the hole a thou or two too large, all is not lost. Just order some hardened gage pins that will give you a .0015"-.002" interference fit. Grind them to length, chamfer the edge, and press it in. You can even dimple the ends with a dremel and nobody would ever know the difference.


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  3. #3
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    Quote Originally Posted by trek45 View Post
    I always assumed it a) but I only have one 93 so it could be a very convenient fluke. This is also why I prefer German HK93s to clones - I assumed they would all match up to the .22 kits.
    Pure fluke, I've got two G-3 kits. Neither one lines up with the rifling (from chamber adapter to rifling) and both are different to each other. Doesn't mean squat, no more than a 22 bullet entering a rifled bore, it will just continue out. What it WILL do is flabbergast any CSI investigators because of double the rifling and not being evenly spaced around the projectile.:D:D (I'm sure they could figure it out eventually):D:D
    45acp... turning human garbage into useful fertilizer since 1911

    Auf gebeginning to fixin der gun mit grosse busen of der schpritzin lubrication allen over derfukin platz, mit das squirtin and squeezin oil in allen fukinholes.
    Ja, und racken und strokin das bolt as if gecrazygoing, yanken dat handle bak und forth, zwei hunnert times, ja?
    Ist neue, und its needing some brekkin in.
    Zo, BREKKIN ZEFUKIN THING IN!


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  4. #4
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    Quote Originally Posted by trek45 View Post
    The grooves in the barrel of my 93 line up perfectly with the grooves in my 93/33 .22 kit's chamber adapter. It looks like HK pressed the barrel so it would line up with the .22 kit and create continuous grooves from the chamber adapter to the 93 barrel.

    So either:
    a) they managed to get that barrel in there without any rotation
    b) my .22 kit's chamber adapter randomly lines up with the barrel grooves
    c) the amount of barrel rotation during pressing is significant for lining up the pin hole, but insignificant for the chamber adapter groove matchup

    I always assumed it a) but I only have one 93 so it could be a very convenient fluke. This is also why I prefer German HK93s to clones - I assumed they would all match up to the .22 kits.
    I didn't think the OP was asking a question about a .22 conversion kit. It sounded to me like he was asking about assembling the actual barrel into an empty trunion, not a barrel sleeve or chamber adapter.

  5. #5
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    Quote Originally Posted by trek45 View Post
    My point was that it seemed to me that HK was able to press a barrel without having it turn. So I figured there must be a way.
    But they don't do it that way. HK drills the cross hole AFTER the barrel is installed to the correct depth that results in a proper bolt gap.

  6. #6
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    Quote Originally Posted by trek45 View Post
    I don't think I'm making myself clear. What I'm saying is that I believe that HK was able to install 93 barrels without having them turn during pressing. What makes me think this is that the 93 .22 kits' rifling lines up perfectly with the rifling in the 93 barrel. I believe this was intentional on HK's part. I have three 93 .22 kits, and I just inserted each one into my 93 and the grooves all lined up perfectly with the rifling in the barrel. Then I examined the barrel inserts for my two G3 .22 kits, and the rifling in each of those is oriented differently.

    So it looks like (sample size 3) HK made sure that the 93/33 .22 kits lined up with the rifling in the 93s. They didn't care about the rifling orientation in the G3 kits because it doesn't mate to anything.

    This is all somewhat tangential to the original question, except to say that I believe there is a way to prevent the barrel from rotating, and I believe HK did this with the 93 barrels so they would line up with the .22 kits. Yes, I know, this is based on looking at one rifle and three .22 kits, but the alignment is perfect and all the 93 .22 kit chamber adapters are oriented identically.


    But rotating the barrel seems like a good option, unless it's a 93 and you want it to mate up precisely with the .22 kits.
    No, you're making yourself perfectly clear. I just know that they don't precut the slot like in the pulled barrel that the OP has, and then try to align it with the holes in the trunion.

    It's possible that the .22 kits are all made identically, which is possible if there is some orienting feature on the chamber adapter. And the orientation of the one 93 barrel you're checking them on just happened to be aligned with the rifling in the kits. If the kits are all identically rifled with respect to some orienting feature on the chamber adapter, then you could likely check every kit ever made on that one particular rifle, and they would line up. Let's just say that they did actually try to get the rifling in some particular orientation, just for the conversion kits. They could get them within 1/2 degree or so, and that would be good enough for the purposes of rifling that visually they would look perfectly aligned. But that wouldn't work for a barrel pin. It's got to be perfect or something would either break, or move when the pin is pressed in. More than likely, the weakest object would break, so the pin would shatter, or the holes through the trunion would break out. It's already a .002" interference fit, 1/2 degree off would make an additional .004" misalignment on each end of the trunnion.

    The last time I was at Urbach's shop I was asking him if he'd ever tried pressing barrels from the front and realigning the barrel pin grooves. This would prevent having to reinstall the triple frames and would be a big time saver. He said he tried a bunch of different methods, and could never get them lined up good enough to reinsert the pins. Every now and then he'd get lucky and one would align, but he spent more time trying to get them aligned than just reinstalling the front sight from the beginning.

    Shattered Mind has already stated he has some that don't line up. And I just looked at 3 93 barrels and 2 HK53 barrels. The rifling orientation varies from barrel to barrel with respect to the groove that was drilled by HK. But if you did want to get it closer to the conversion kit rifling, you could do it fairly easily. It doesn't HAVE to be rotated 180 degrees. There are 6 grooves, or one every 60 degrees. At some orientation between 150 degrees rotation and 210 degrees rotation from the original slot, the barrel grooves will line up with the adapter grooves. It's completely unnecesary, but you could do it.
    Last edited by AviatorDave; 06-06-2009 at 01:30 AM.

  7. #7
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    Guys,

    I have to disagree. HK does can and does indeed press the barrels in without turning them.

    Case in point is is MP5 10/40 barrels. They have a relief cut in the chamber face for extractor. This relief cut is similiar to the one on the 21e and is done to for clearance for extractor. The cut is made on barrel when the barrel is being produced, not after the barrel is pressed into the trunion. The orientation of the barrel with relief cut must be a specific position relative to the extractor. Look at any factory MP5 10/40 gun and you will see what I mean. If HK does not press the barrel in without spinning it, they would never get this orientation correct. Another case in point, is HK new barrels come painted with the front sight on. I have had and sold a number of MP5K PDW German barrels and they came brand new in butcher wrap painted with the front sight one, and a small starter cut where the barrel pin goes. Now before you say these were pulled barrels, the cut was no where deep enough for any barrel pin. Anyone care to call Deb at HK USA to see how she presses in the barrel?

    That being said, the press and non-factory jigs that American gunsmiths use make it easier to just remove the front sight and press the barrel from the rear. I have often wondered how they plan to manage replacement of MP5 10/40 barrel with a used barrel. You can't take a pull out barrel, remove the front sight rotate it 90 degrees and press it in and expect the extractor cut out to be in the right place!

  8. #8
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    Default Installing a barrel to a trunion

    If rotating the trunion once installed is an option what do you recommend for holding the barrel without damaging it?

  9. #9
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    Quote Originally Posted by HK23E View Post
    Guys,

    I have to disagree. HK does can and does indeed press the barrels in without turning them.

    Case in point is is MP5 10/40 barrels. They have a relief cut in the chamber face for extractor. This relief cut is similiar to the one on the 21e and is done to for clearance for extractor. The cut is made on barrel when the barrel is being produced, not after the barrel is pressed into the trunion. The orientation of the barrel with relief cut must be a specific position relative to the extractor. Look at any factory MP5 10/40 gun and you will see what I mean. If HK does not press the barrel in without spinning it, they would never get this orientation correct. Another case in point, is HK new barrels come painted with the front sight on. I have had and sold a number of MP5K PDW German barrels and they came brand new in butcher wrap painted with the front sight one, and a small starter cut where the barrel pin goes. Now before you say these were pulled barrels, the cut was no where deep enough for any barrel pin. Anyone care to call Deb at HK USA to see how she presses in the barrel?

    That being said, the press and non-factory jigs that American gunsmiths use make it easier to just remove the front sight and press the barrel from the rear. I have often wondered how they plan to manage replacement of MP5 10/40 barrel with a used barrel. You can't take a pull out barrel, remove the front sight rotate it 90 degrees and press it in and expect the extractor cut out to be in the right place!
    Yes, but it is the amount of turning that is the key. When a barrel is installed, the amount it may turn would be measured in seconds. Maybe 30 seconds + or -. There's more than enough clearance in the parts you mention so that this amount wouldn't be a problem. But it is when you're trying to align an interference fit dowel pin through a hole that passes through 2 parts in 3 places.

    But I have wondered the same thing with the MP5 10/40 barrels. I've wondered if they don't either weld the pin slot up and redrill that, or if they weld a plug in the barrel face where the old extractor was, and just recut the extractor channel in the new orientation.

    But there's one other possibility - with the cut for the extractor, you now have a good solid feature you could key off of that could prevent the barrel from rotating, and you likely COULD force perfect alignment when installing it. I've never built one, so I'm not sure.

    I still think I might be able to build a fixture that would let me reuse the existing slot. It's already designed, I just haven't bought the $500 worth of linear bearings I designed the thing around.

    But all of this is a total side discussion - the original posters question was "how to align the slot with the trunion hole." The answer is that "you don't, you drill a new hole."

    Can we at least agree that that's the right answer for a home builder, and worry about how HK does it from somebody that knows, like G3Kurz perhaps?


    But Krugger - when other posts were talking about rotating it or not rotating it, they did not mean that HK intentionally rotates the barrel after it's pressed in to the trunion. They were commenting that they think HK can get it in the right orientation before it's pressed in, and keep it in that orientation as it's pressed. Once it's pressed in, you're not going to be able to turn it. Drilling a new hole is your only option.

    Use the advanced search for some of JFKs posts, use SP89 and HK53 in the search string, he's got a few posts with many pictures showing how he builds them.
    Last edited by AviatorDave; 06-06-2009 at 06:03 AM.

  10. #10
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    Default installing a barrel to a trunion

    Thanks for the info

    Krugger

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