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Discussion Starter · #1 ·
I am just about to drill my trunnion and barrel for pinning; the trunnion isn't welded to the receiver yet. It seems it would be easier to have the trunnion and barrel assembly out of the receiver so I can clamp it in a drill press vise for proper positioning. This leads to my question: How do I set the alignment of the hole already in place so I do not end up with an off center hole? And, do I drill successively larger holes until I get to 7/32"? I'm not all that worried about pressing in the pin; it will be covered with weld once I am done. I know I should have mentioned this earlier, but I already headspaced the barrel at .016", and set up witness marks on the trunnion and receiver for proper re-alignment during re-assembly.
 

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If you're using a used trunion, you will need to use an end mill and drill a flat spot on your barrel. If you attempt to bypass this route and just use a drill, it will wander while you're drilling and probably screw up the barrel/pin fit. The bolt gap can drop because of a sloppy fit. My suggestion is to use a 3/16" end mill flipped upside down and center the trunion hole. Once it's centered, flip the end mill around and mill the barrel about 1/4" deep. Remove the end mill and use a #9 drill to drill the barrel all the way through.
 

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Discussion Starter · #3 ·
My trunnion is new RCM, and I'll be using a drill press for the job. If the hole I drill is just a bit too large, I figure I can just use oversize pins. Thanks for the info.
 

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New trunion is different. Use a #9 drill, go slow and a lot of cutting oil. No need to use an end mill.
 

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Discussion Starter · #5 ·
Thanks for the clarification. I don't have a #9 drill bit, but figure I can pick one up cheaply enough. I alredy hvae the cutting fluid, and heat control paste as added insurance against sofetning the roller cavity of the trunnion.
 

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Discussion Starter · #6 ·
An update on my progress so far: Got the barrel drilled with a 5mm end mill, pinned no problem. Starting headspace was .022, after taking out the slack between barrel and trunnion, it's now .004". I'm thinking of using oversize rollers to get up to .012. Also got the cocking tube and trunnion welded in with a wire welder; no it's not pretty, but that's what a die grinder and dremel are for. I didn't figure an end mill would drill all the way through, but it did at 3k RPM with plenty of cutting oil. Easily, too. Now to touch up some of the welds, solder on the front sight tower, drill, pin, and finish. I have +2 rollers in right now; would +4 rollers get me close to .010 or more? Thanks. I know it's not kosher to use oversize rollers on a build, but I'm working with what I have.
 

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What made the gap move from .016 to .022 to .004? You could always start over, flip the barrel and drill it again.
 

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Discussion Starter · #8 ·
I pulled the barrel out, re-headspaced. It was .022, then I drilled and pinned. Then I took out the slack between the trunnion and barrel, which brought it to .004. Can't drill again, as I messed up the first headspace, so I had already flipped it over.
 

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Sounds weird, that shouldn't have happened. What if you removed the pin, nudged the barrel to the correct headspace and reamed the hole for an oversized pin?
 

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Discussion Starter · #10 ·
Already using an oversize pin. I think I may have taken out too much slack when pressing the barrel again after pinning to remove slack. How much headspace would I gain by using +6 rollers, or changing out the locking piece. I'm currently using a new standard piece from HKParts, though the bolt is used. Will a new bolt help as well?
 

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An update on my progress so far: Got the barrel drilled with a 5mm end mill, pinned no problem. Starting headspace was .022, after taking out the slack between barrel and trunnion, it's now .004". I'm thinking of using oversize rollers to get up to .012. Also got the cocking tube and trunnion welded in with a wire welder; no it's not pretty, but that's what a die grinder and dremel are for. I didn't figure an end mill would drill all the way through, but it did at 3k RPM with plenty of cutting oil. Easily, too. Now to touch up some of the welds, solder on the front sight tower, drill, pin, and finish. I have +2 rollers in right now; would +4 rollers get me close to .010 or more? Thanks. I know it's not kosher to use oversize rollers on a build, but I'm working with what I have.
You did not take the advise that was given to you. Member JFK is highy knowledgeble and any advise he gives can and should be taken as near gospel. He suggested a #9 drill bit (carbide), not a 5mm end mill. The barrel pin is 5mm so you essentially have no press-fit. The barrel pin should have a 1-2 ton press fit which will prevent the barrel from moving and elimiate the need for oversizing the gap. Also, endmills will side-cut so in actuallity, your hole is larger than 5mm and probably slightly oblonged.

My suggestion now would be to remove the pin, tweak the gap back to your intial 0.016" and see if a #8 (0.199") carbide drill bit (reamer preferred) will remove material and clean-up the hole. If so, use a standard +1 oversized barrel pin (0.201") which should press in between 1-2 tons.

If 0.199" is too small for your hole, try a #7 and go to a +2 oversized barrel pin. If that is even too small then you've really tore up the hole and will need to make a custom barerl pin, however I would suggest a new trunion and rotate the barrel index 1/3 and start over with the recommended #9.

THERE IS NO NEED TO OVERSIZE YOUR BOLT-GAP OR "TAKEUP SLACK" WHEN DONE PROPERLY. For beginners, starting between .018"-.020" isn't a bad idea but if you setup accurately and your drill press/mill can give the precison, there is no need to.

Don't feel bad, this story replays itself once every few months.


EDIT: just noticed that you rotated 1/2 already and have already tried an oversized pin. A barrel pin .001" to .002" larger than the hole should give you the proper press fit assuming that the hole is true and the pin is of adequate hardness. For reference, if you need to rotate a barrel, rotate it 1/3 that way you still have one more rotation index if needed.
 
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