headspace and oversized barrel pins

[slowshooter]

My headspace battle with one build continues. This board has helped with several issues faced, and now I need more advice on an AMD65 with headspace problems. I bought the oversized (0.281") pins and high quality J-sized bits. Turns out that the bit fit in the barrel pin hole without any need to drill, so I just pushed in the oversized pin. It was a tight fit going through the trunion, noticeably easier going past the barrel and then a tight fit at the other size of the trunion. I shot it five times, and from shot 1 to shot 5, I saw the headspace change from not being able to fit the no go gauge to being able to close on the field gauge. (At least I'm getting proficient at using headspace gauges.)

What's the largest safe hole that may be made for the barrel pin?

[Random Chaos]

Here's a thread where I had the same problem and a couple of experienced builders worked me through it. I ended up drilling mine to .311 with a .316 pin.

http://www.akfiles.com/forums/showthread.php?t=29979

hth

[7.62x39]

I use 19/64ths which is .297

[slowshooter]

Thanks for the link and the information. So all I need to do is remove the barrel and find the drill-end of the bit that does not fit completely in the barrel groove, replace the barrel in the trunnion, headspace, and drill with the next-size down bit?

It sounds so straightforward.

[Random Chaos]

It did fix the problem. You have guesstimate how large a hole and pin you will need. I bought two sets of drill stock and drill bits, The smaller set did not work, the larger set was a perfect fit. You want to redrill the hole large enough so that the barrel cannot move. My problem was that the barrel channel was elongated on the front to back axis of the barrel. I've never seen anything like it. If you want any of the drill stock, please let me know and I'll send you a pin size length.

Rick

[slowshooter]

The 0.310" bit (5/16") shows a little bit of light between it and the barrel groove when I try to put it in. Just checking: I should drill the hole to 0.310" and chop down an 'O' size bit (0.316") for a barrel pin?

Thanks,

[Random Chaos]

I tried to respond to your pm but I cannot get my pms to work...

explain this to me - "The 0.310" bit (5/16") shows a little bit of light between it and the barrel groove when I try to put it in". When I did mine I was concerned that the barrel pin bottomed out in the barrel channel in order to stabilize any for and aft movement of the barrel. The .316 pin does sound fine, it will be a tight fit, use some anti-sieze and youe'll be fine.

[slowshooter]

Since the barrel is moving forward as I fire, I figured the barrel pin hole would need to be slightly larger than it is now to hold it in place. I tried the chuck end of different bits until I found one that would nearly fit into the barrel groove. The drill bit has a diameter that is slightly larger than the length of the barrel groove, so it will not fit in perfectly. It is hard to tell how closely the bit fits the groove, so I held it up to a bright light and saw a bit of light between bit and the bottom of the groove. There also is some light on the sides of the bit.

The thinking is that once the new barrel pin hole is drilled and the oversized pin installed, there should be no movement.

Should I be going down a size with the plan to use a barrel pin of 5/16ths? The next smaller drill bit size I found at McMasters was 0.008" smaller.

[Random Chaos]

Ok, I understand what you were saying... I would not go any smaller because if you can see light around the chuck end of the bit you are probably close to the size of the hole. How does the .316 pin look in the channel? Can you still see light in the channel around pin? I'm just curious if the .316 pin fits better. The light could be caused by the hole simply being out of round. I'd go ahead and drill it and press the pin in. Let me know if you need the .316 stock.

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