Drilling Near the Chamber

[MoscowMike]

I understand that you will ruin a rifle by drilling into the chamber while mounting a scope, etc., but would like to know how deeply you can drill near the chamber without significant risk.

Of course the rules are different between a 30-30 and a 300 Weatherby! My specific question relates to a 7.62X39, which is not a high pressure round. I was drilling out a broken tap and the stop slipped on my drill press, so I have a 1/8" hole part way into the front of the chamber that leaves about .18-.20" of chamber wall thickness.

At first I thought I was hosed, and then I got to looking at my Ruger Blackhawk .357. The chamber wall is about .13 at the edge of the cylinder, and in the section the cylinder latch drops into, it's only about .07 thick! I realize we are talking about 35,000 psi vs 45-50,000 psi, and probably different steel but we are also talking about 3 times thicker over a smaller area, the 1/8" drill hole vs the cutout for the cylinder latch.

I also noticed that I have drilled in about as deeply as the cut for the barrel retaining pin which runs across the chamber. That area is supported by the retaining pin, but it is a deep cut.

After a few test shots with the rifle tied to a bench and pulling the trigger with a string, everything works fine, and the AK has a real reputation for durability, so I would hate to go to a lot of work if this is only a minor risk.

On the scale of no big deal to it's a ticking time bomb (1-5), where would you put this?

Thanks

- Mike

 

[brian923]

its up to you, but that "minor" risk could turn into a lost hand, eye, or even KILL you, but at least its only minor....right?!?

 

[brian923]

its up to you, but that "minor" risk could turn into a lost hand, eye, or even KILL you, but at least its only minor....right?!?










is the risk worth a new barrel??? what type of rifle is it?? 7.62x39 doesent give the make and model. you might be okay, but I am NOT telling you that. i would peronally look into a new barrel. barrels are cheap. fingers are not.

 

[moosehunt]

If you measured correctly and indeed you have .180 to .200 of steel remaining, you are plenty safe. Are you certain you measured correctly? Giving that wide of a range makes me wonder. If you had said .187 to .189, I'd feel a little more confident in the measurement. If that's the case, though (i.e. a correct measurement), I wouldn't give it a thought. But I can't help asking, why didn't you just take the barrel out to begin with?

 

[Jim K]

If you want, you can calculate the area of the hole, then divide its area in a fraction of a square inch into the average chamber pressure. I don't think there is anything to worry about.

Hi, Moosehunt, just FWIW, I don't know of any gunsmith who pulls the barrel (a major operation) before drilling and tapping the receiver ring.

Jim

 

[moosehunt]

I don't either, but before I had a mill by which I knew I could control hole depth, I did often. That way you don't make mistakes! That appears to be how this fellow got into trouble--though as you, I don't think he is in trouble

 

[rcmodel]

and the AK has a real reputation for durability

I don't think anyone in his right mind would take the barrel out of an AK-47, which seems to be what he was working on.


rcmodel

 

[MoscowMike]

Moosehunt -

Thanks for making me re-think the measurement. I am much more comfortable with wood, where 1/32" is precision!

As the tapped hole is near the front of a tapering chamber and I don't have the tools for measuring at that point, I used a vernier caliper to measure the mouth of the chamber and the cartridge specs to extrapolate the dimensions. The hole is at the shoulder of the tapered cartridge where begins to be necked down to 30 caliber.

When I remeasured this morning, though, I found I had made a mistake. To be pessimistic, I think I should assume the chamber wall thickness at the very tip of the 1/8" drilled hole is ~.16. At the shoulders it is ~ .2

My drill press is homemade, and the drill stop consists of a hose clamp on a rod. It wasn't tightened securely. Same old story, checking twice slows you down, but can save you a ton of grief!

As far as why I didn't pull the barrel - I had been reading about building a semi-auto AK-47 from the kits various vendors offered. You cut off the remainder of the torched receiver and replace it with a new one. Either press out the barrel and use rivets to attach the front and rear trunnions, or leave the barrel in place, drill them out, tap the holes and use high-strength screws. That sounded like an interesting winter project, and as I didn't want to invest in the press and other riveting tools, or have to deal with headspace issues, the screw build option sounded like a more practical method.

I've learned a lot, and if I were going to build several I would invest in the press and riveting tools, but if you are careful you can build a fun project for ~ $250. I was doing fine until the tap broke! Next time I would use a 5/32 end mill to finish the blind holes before tapping.

 

[Jim K]

Use a bottoming tap.

(Home made drill press! Stop made from a hose clamp! Is your lathe foot treadle powered? And do the Taliban bother you much up there in the Darra pass? )

Jim

 

[moosehunt]

Mr. Keenan--I don't think that we are on different wave lengths!! Home made drill press--more power to the gent--hose clamp drill stop??--the basis of this hole thread!!!

Mr. Mike.--not degrading you, not at all, I'm just picky, probably way too picky. Monkeying around with those "cheap" guns is a good and fun way to learn. You can't loose much in so far as $$$, just be careful you don't loose too much of you!

 

[MoscowMike]

I have no problem with people kidding me about some of my primitive tools. With a limited budget I have learned to make do and find alternate methods rather than not tackling projects. Sometimes I make mistakes but that teaches me, too. I just want to avoid really dangerous mistakes!

It's interesting to look at the sort of things people can do with very basic hand tools, if they have patience and skill. I have heard that gunsmiths in the Khyber Pass can turn out working versions of firearms with little more than files and hammers. I would worry about the quality of the steel, though!

Thanks for the advice.