[a1fabweld]

I know the smart thing to do is send it out & have it done by a pro. I'm a stubborn SOB & I want to do it myself. If this was a real HK91 I would send it out to be professionally rebarreled. But it's not so I thought I'd learn on this one. I have a full fabrication shop (TIG, MIG, Bridgeport mill, 14x40 lathe, precision measuring equipment, etc...). I've searched the net regarding this procedure a few times & I see conflicting answers regarding pressing out/in the barrel.

Does the existing barrel get pressed out the front of the trunion (toward the muzzle)? Then when I reinstall the new barrel do I pass it through the receiver in the same direction I pressed the old barrel out? I heard that the triple frame is soldered to the barrel. Some say to heat it up to loosen the solder while some say just press it out as the solder is soft. I know the eyelet pin needs to be milled on the flared/welded end. Does the triple frame & trunion need to be supported simultaneously as the barrel is being pressed out/in?

I have some ideas for getting the triple frame aligned but I'd like to know of a proven method that's used to accomplish this.

I really feel that I can pull this off with some patience and a few tricks from some experienced HK pros. If anyone can list in order the steps to do a barrel swap I would greatly appreciate it. Thanks.

 

[JeepGuy79]

press out the barrel pin. press the barrel out from the inside. remove the pin in barrel from inside receiver and triple frame. Press in new barrel. check bolt gap. repress barrel check gap. press barrel some more and check bolt gap. Then mill the barrel where the pin goes and repin it with your press. Put on the triple frame. check for accuracy. put new pin in triple frame and press it. then refinish. done. ez.

 

[Xenophobiac]

Quick and easy. An addition to that; use heat control paste during drilling and welding operations so you do not remove the temper inadvertently. Also, for the triple frame, you'll wnat to silver solder after checking alignment with the rear sight, then pin. Press out the barrel from the rear, press in from the rear, and use anti-seize grease to prevent galling on both the inside of the trunnion and the portion of the barrel that contacts the trunnion

 

[Holescreek]

An addition to that; use heat control paste during drilling and welding operations so you do not remove the temper inadvertently.

Remove the temper out of what?

 

[Grumpy]

Remove the temper out of what?

The temper of the steel. I.E. if you heat it too much and let it cool slowly, you can anneal the steel, making it very soft.

Heat treating and tempering are some of the touchy subjects regarding clone guns to begin with, no need to make it worse.

 

[Holescreek]

Not a lot of welding going on in a barrel repress and I expect that "a1fabweld" probably has a good understanding of how to apply heat to soften silver solder (maybe presumtious on my part).:2300000:

 

[hkshooterusp]

Glad I'm not the only one that wondered where the welding was involved during a barrel install.

 

[6500rpm2]

Just because the OP asked the question and I'm always looking to learn, does anyone have pics or links of the press jig/fixture used to press the barrel in and out. Another thing I'll add that always makes AK barrel pressing easier is that I like to throw the barrel in the freezer for a minimum of a few hours just prior to pressing in and I'm a firm believer of reaming the barrel pin hole to size and using a .002" over hole diameter pin.

 

[Holescreek]

A few of us just finished up doing a pretty intensive (although smallish sample size) trunnion metalurgical study to investigate the cracking issues, based on the outcomes, I personally would never intentionally use a pin +.002" over the hole size. But whatever works for you, keep doing it. I have a couple of rifles in need of a repress sitting in the shop to do in the next week or two. I just pressed in my first RCM melonited barrel in last week, if nothing else, the melonite coating made it the smoothest barrel press yet with zero evidence of galling.

 

[hkshooterusp]

I read through the study that Holescreek speaks of and it's very interesting. I'd never try to press in a pin .002 over hole size anyway but that little study firmed up my beliefs. I've always looked for .0005-.001 over hole ID.

 

[JeepGuy79]

I could be wrong but the oversized pin is cause when you try to use a drill on a trunnion that is already used you have a straight hole going into a round barrel. Things get oval and larger. If you flatten the barrel with a endmill then plunge a drill through it you can go back in with a standard pin and be all good. Just how I would do it. I am a total amateur tho.

 

[6500rpm2]

My input was strictly AK bias as I'm learning about this platform at the time and have little practical experience at this point. AK barrel pins are also much larger in diameter than what I've seen on HK's. I always finish ream to size, so it was a true press fit. Based on HC, .0005-.001 is the industry standard for HK if I read correctly and although he didn't come out and say it, it sounds like the trunions in question are extremely hard (read brittle and prone to cracking if the press fit is too tight?). Correct?

 

[hkshooterusp]

Trunnion Survey - Further Analysis

 

[Xenophobiac]

I could be wrong but the oversized pin is cause when you try to use a drill on a trunnion that is already used you have a straight hole going into a round barrel. Things get oval and larger. If you flatten the barrel with a endmill then plunge a drill through it you can go back in with a standard pin and be all good. Just how I would do it. I am a total amateur tho.

That's what I'm doing with mine; using a 5mm end mill, then using a drill made for stainless steel, and finishing up with a 5mm reamer. Finally, I'm pinning with an oversize pin. I'm going oversize because I'm using a used trunion for my build.

 

[Holescreek]

My input was strictly AK bias as I'm learning about this platform at the time and have little practical experience at this point. AK barrel pins are also much larger in diameter than what I've seen on HK's. I always finish ream to size, so it was a true press fit. Based on HC, .0005-.001 is the industry standard for HK if I read correctly and although he didn't come out and say it, it sounds like the trunions in question are extremely hard (read brittle and prone to cracking if the press fit is too tight?). Correct?

I went into the trunnion study thinking I knew something about them only to learn that I did not. The net if full of misinformation just because speculation becomes fact over time. During the study I held a torch on a couple of trunnions trying to modify the hardness on purpose (to achieve full anneal of the end) and was disappointed. I would not worry about a little welding after what I did. In a nutshell, some trunnions are physically better than others but they are all serviceable if you follow some common sense rules. Make a clean-straight pin hole by whatever means, and don't get crazy with oversized pins. The pin serves it's purpose. Adding internal stresses to the trunnion by cramming in too large of a pin doesn't make it work better, just complicates things. I don't know what HK's specs are for pin fit and don't feel compelled to make something up. On my current build (HK33) I went so far oversized to get a clean hole that I ended up using a cut-off drill bit shank for the pin. I might have hesitated to do that before the study but since learning about the core hardness of the trunnions I didn't hesitate for a second. OTOH, I would never advise anyone else to do what I did.

 

[hkshooterusp]

Why is a bigger pin hole any different,besides the obvious reasons?

Besides the obvious reasons, what does it matter?

I'm really confused by your question.

 

[Holescreek]

Was the reluctance due to the size of the pin or because of the chosen "host"? Try some other builders and see what you can find out.

 

[a1fabweld]

What a wealth of info here! Just to confirm a few things on a rookie level:
1: Does the solder in the triple frame need to be heated up prior to pressing it off the barrel? I've heard of guys doing it cold because it's soft in comparison to the other parts. Will it damage anything doing it this way?

2: The direction of travel for pressing out the old barrel & pressing in the new one is the same direction as the bullet travels (rear to front)?

3: What's the most efficient method of getting the headspace correct when pressing in the new barrel? Do you press it in until it looks visually close, drop your bolt in it, measure headspace, then press some more using indicators to creep up on it until it's correct?

4: Is the best method for lining up the triple frame to use a laser boresighter?

 

[Holescreek]

What a wealth of info here! Just to confirm a few things on a rookie level:
1: Does the solder in the triple frame need to be heated up prior to pressing it off the barrel? I've heard of guys doing it cold because it's soft in comparison to the other parts. Will it damage anything doing it this way?

2: The direction of travel for pressing out the old barrel & pressing in the new one is the same direction as the bullet travels (rear to front)?

3: What's the most efficient method of getting the headspace correct when pressing in the new barrel? Do you press it in until it looks visually close, drop your bolt in it, measure headspace, then press some more using indicators to creep up on it until it's correct?

4: Is the best method for lining up the triple frame to use a laser boresighter?

Just my 2cents,
1) heat first, no need to damage the triple trying to beat the solder loose. MAPP gas works quickly.
2) Yes
3)There are several good tricks to use, too much typing for me though. Google is your friend. I can steer you to a good tutorial I know of but its on another board and there might be one here too that I don't know of, PM me if interested or none of the regulars here points you to a thread.
4) I've done it both ways, laser and string. Just used string this week.

 

[a1fabweld]

Holescreek, PM sent. Thanks.