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Discussion Starter #1 (Edited)
I finally amassed all the parts to complete my cohaire painted barreled upper k gun. after a little bit of fitting, thorough cleaning, and lubing, I took it to the range.

to my surprise, it ran 100% from the first round, at this point I have 5-600 rounds thru it, WWB, maroon box federal, S&B, PMC, and one mag of hollow points just to see, Ive also used HK mags, SW mags, POF mags, and some really crappy knock off mag, also without issue. recoil is less than my fullsize, although the recoil impulse is significantly shorter than the fullsize, and accuracy is so much better than I could hope for. like 6 rounds in one raggedy hole from 20 yards standing. I was almost as impressed with myself as much as the gun.

when I returned from the range last friday, I stripped it down for its maiden after firing clean. all the internals look great, no weird scratches or wear marks. all is well in kgun land.

well until I looked at the bolt gap, it is a fair bit bigger than any of my other roller locked guns. dont know where my feeler gauge is, so no hard numbers, but it looks too big, I'm guessing at .022-.025. thinking I'm a know it all, I get on HKparts and order myself a set of -2 rollers. they come in monday and I just got them installed tonight. I had a random aluminum shim that I got a feel for the original gap with, and the new rollers didnt seem to change it at all.

at this point, Im guessing that the barrel will most likely need repressed to get the gap into spec. I know that will entail at minimum a refinish, and a couple hundred of my dollars. I'm not really interested in throwing any more time or money at a gun that so far has worked better than ANY clone I have ever owned.

so I ask the group, what damage will be done with a bolt gap thats too big? its not gigantic, but definitely bigger than it should be, and showing ZERO functionality issues.
 

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Measure it with real feeler gauges. Slight too large shouldn't be a problem. Light primer strikes would occur if the gap was too big. Also, extra recoil impact.
 

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fullman,
First, congrats on your successful project. After spending all that money for your build......please get a set of feeler guages. When in battery, make sure the carrier is not "bottoming out" on the trunion or cocking tube support. That could give you a "false" gap reading. But first, please get feeler guages before you start swap rollers around.
Dj
 

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Discussion Starter #5 (Edited)
Ok, found the feeler gauges, looks like the bolt gap with the -2 rollers in it is about .024". So on the high side.

The standard rollers sat right about .027. So my gap is shrinking with smaller rollers, but it looks like the rollers aren't sticking out into the trunion quite as far as any of my other roller locked guns.

I also swapped in my HK bolt head from my fullsize, as well as the clone bolt head from the bobcat, both showed the same gap (.027). So the RCM bolt isn't the issue(doubted it anyway).

Any chance it could be the LP? Again RCM, so I doubt it, but I don't have another k gun LP to try.

So what do you all think? Shoot it til it fails, or am I desperately in need of a barrel repress?

ETA:
Any estimate on a repress? And can it be done without a refinish?

Oh, pic of the roller protrusion...
 

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We need chopstix or ghillie to chime in
 

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I would go with -4 or -6 to get you down to .020. Your rollers won't pertrude as much with -rollers that is what shrinks bolt gap. Like Dog said, heavy recoil and buffer/ stock wear with too large of gap. I would find a vey well used locking piece and some -4 rollers before repressing the barrel. You can still achieve propper gap without repressing.
 

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Can you put up a photo of the ejection port view with the carrier in battery?
 

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I would go with -4 or -6 to get you down to .020. .
I got one of those unpainted uppers about a year ago and the BG was excessive > 0.50mm. With an RCM bolt, LP and carrier. These all seemed to have the barrels pressed in a little far from all reports. I used -6 rollers and actually got a smaller BG with and MKE LP as opposed to the RCM LP. The BG now is @ 0.43mm.
 

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Stick your finger up into the mag well and try to wiggle the bolt head. If it's tight, then there shouldn't be a problem. If it easily wiggles side-to-side or front-to-back, well then your bolt carrier is running into the trunion or charging tube. Then you'll need to do something to it.
 

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Discussion Starter #12
I pulled the BCG and removed the head, the carrier is not binding on anything but when it actually bottoms out on the trunion. this is about a millimeter farther forward than it travels with the bolt head attached. when the assembly is all together, there is zero movement of the bolt head when closed. so it is in fact the bolt head bottoming out and forcing the rollers out like it should be.

when I compare it to the fullsize, I can definitely see more of the bolt head in the kgun.

 

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Well just to be clear here, your title is a little misleading, or at least a little inaccurate. What you have is not what I would call "excessive". It might be out of spec on the high end, but excessive is a strong word. If it's operating correctly, nothing is loose, and you are getting good deep primer strikes... you're probably ok. If you really care about it, a pair of -4 rollers would probably do the job. But that's just my opinion and I'm no expert.
 

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Stick your finger up into the mag well and try to wiggle the bolt head. If it's tight, then there shouldn't be a problem. If it easily wiggles side-to-side or front-to-back, well then your bolt carrier is running into the trunion or charging tube. Then you'll need to do something to it.
This is where I was going. In addition to what Retro has pointed out, when this is occuring you can often times see a smal gap between the case deflector and the back edge of the carrier. This would indicate a barrel what was pressed forward too far creating a false larger bolt-gap reading.

I pulled the BCG and removed the head, the carrier is not binding on anything but when it actually bottoms out on the trunion. this is about a millimeter farther forward than it travels with the bolt head attached. when the assembly is all together, there is zero movement of the bolt head when closed. so it is in fact the bolt head bottoming out and forcing the rollers out like it should be.

when I compare it to the fullsize, I can definitely see more of the bolt head in the kgun.

Nope, you are not experiencing what Retro was getting at. It hoestly looks like you just have a barrel that was not pressed far enough forward. And after re-reading the thread it sounds like that it is not an ucommon occurance with these upper. As suggested earlier, find a well used LP and some (-) roller to get your the gap down.

Also, be certain the trunion is clean in the roller recessses and around the chamber-face. Also make certain your bolt-fact is clean. Excessive debris can also give you a false-larger reading.
 

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Discussion Starter #15
Changed the title to reflect the measurement.

The bolt face and barrel face is clean and free of all debris and carbon. When I compare the barrel position to my others, this one does look like it may not have been pressed in as far as it could have been.

I'm thinking I'll start with a used LP, and do my best to run as much ammo thru this thing and wear in the gap.

I mostly wanted to make sure it wasnt going to blow up in my face, and that the rollers sticking out so little weren't going to turn this thing into blowback operation.

Thanks all for the help.
 

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Discussion Starter #17
Your barrel is pressed into the trunnion too far. From muzzle to rear. Many are pressed in from the rear but the reference is barrel chamber within the trunnion. Not pressed in enough yeilds a decreasing gap.
ah, I thought it was pressed in from the rear of the receiver, rather than the front of the trunnion.

so in that case, I guess it has been pressed in too far (too much barrel sticking into the receiver)

thank you for the clarification.
 

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Discussion Starter #18
Bumping an old thread here......

I've managed to get some more rounds through this thing, and the gap is right where it always has been. So that's good news. No weird wear issues.

I'm having issue with it running with a registered pack. Semi pack seems fine, but auto seems like it's not getting the trip lever pressed.

I've tried -2, -4, and now -6 rollers, the bolt gap is unchanged between all of them.

Do I get to source a barrel pull and repress?
 

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I'm having issue with it running with a registered pack. Semi pack seems fine, but auto seems like it's not getting the trip lever pressed.

I've tried -2, -4, and now -6 rollers, the bolt gap is unchanged between all of them.

Do I get to source a barrel pull and repress?
So I'm clear, when you say "issue with it running with a registered pack" and it "seems like it's not getting the trip lever pressed", there is a live round in the chamber, with the hammer back, and the trigger pulled back? If so, it would think the best thing to do would be to repress the barrel. If there is no change in bolt gap from -2 to -6, there is a definite problem.

There is a bunch of smiths listed in the Supporting Vendors Forum. Jeff has built a pile of hosts for me at very reasonable prices. While you have it at the smith, I'd also recommend welding an optic rail on top. Good luck with your "K" problem.

Scott
 

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Discussion Starter #20
When using the registered pack, it acts like there is a semi carrier in it, though sometimes I may get a round or two off.

Already talked to Jeff, he no longer works on cohaire guns.

I agree that it needs a barrel repress, and possibly replacement, as I have read some have been tried once or twice already from the factory.

Seems it's hit or miss on smiths that will have at a CA gun, and even more hit or miss with smiths that even understand the roller lock system.

So I'm open to suggestions as to who to send it to.
 
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