Drill barrel blank for gas ports

gamma

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Ive been asked bout fitting a barrel to a semi auto rifle, its a Sig 551, I looked at the barrel and it looks to be a simple enough job but I could be wrong. Can I simply use any custom barrel and drill it for the gas block or is there more to it than that. Has anyone here ever done anything like this.
 

pagana

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Re: Drill barrel blank for gas ports

Not sure about a Sig, but have done it for an AR. With that rifle there is a need to contour the blank so it fits under the barrel nut, and, unless you make your own gas block, turn a section at the appropriate location for a gas block. Then, in reference to this shoulder for the gas block drill your port hole. All this should be done after chambering and timing the extension/bolt/barrel assembly for proper headspace and bolt clearance. The reason is that it is much easier to ensure the port location is at TDC and the gas block section and port hole are drilled the proper distance. Not really as complicated as it sounds, but not for the average individual. Derrick Martin's book outlines the above in detail and even gives dimensions. It could be used as a guide for your Sig even though it is a piston system vs. DGI.
 

gamma

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Re: Drill barrel blank for gas ports

Thanks, thats the kind of reply I need. Im well on top of the other stuff, timing etc. Im just wondering is there anything to worry about in relation to where the holes or hole is placed e.g should the hole be placed on the lands or below the lands on the barrel or does it matter. I see your name is toolmakr thats what I am, an injection mouldmaker.
 

pagana

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Re: Drill barrel blank for gas ports

Much has been written about the best location for the port, i.e. in a groove, does not matter, etc... (all opinions). Here is my opinion based on those articles. Ideally the hole should be half in a land and half in a groove because it should minimize erosion of the hole, but since it is virtually impossible to time it exactly, and since the hole diameter required will likely be larger than the width of a groove anyway, don't worry about where in relation to land and groove the hole winds up - just drill it. And that too is a topic of discussion - drill into a lead slug - EDM to prevent burr creation in the 1st place. According to the book reference in my last post, he just drills it and shoots the burr off and that is what I did as well with no ill effect.
 

cal50

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Re: Drill barrel blank for gas ports

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mikki</div><div class="ubbcode-body">Ive been asked bout fitting a barrel to a semi auto rifle, its a Sig 551, I looked at the barrel and it looks to be a simple enough job but I could be wrong. Can I simply use any custom barrel and drill it for the gas block or is there more to it than that. Has anyone here ever done anything like this. </div></div>


I have done a Galil and it's not hard but you have to do it in the proper order. I am not familiar with the SIG but the Galil needed the barrel headspaced and torqued in place then the gas port hole drilled. The gas port also passes through the front sight base / gas block. There is not a lot of windage adjustment so the base MUST be centered well before drilling and pinning it.

I barreled my Garand with a Krieger and it was short chambered. Once it was tightened down the gas cylinder and front sight lined up properly. I used a pull through reamer and the bolt to finish the chamber and headspace properly.

The Sig looks similar that you want the headspace correct and barrel in place before drilling the gas port and gas block.
 

lfagan

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Re: Drill barrel blank for gas ports

I hope that this reply isn't off topic.

First, Let me say that I agree with Toolmakr regarding the placement of the gas port into the bore. Enough said, moving on...

A large portion of the work that I do is centered around the AR platform. I drill all of my gas ports using a milling machine and I don't get a burr in the bore. Here is the way that I do it....

I will use a .096" diameter gas port as an example.

-the first thing that I do is locate the gas port position.
-Then I spot the location with a good #2 center drill.
-I use a #43(.089")drill bit to make my first pass through the barrel wall.
Note: I will look down the bore when I start to "feel" the bit breaking through the barrel wall and I slowly continue to lower the quill while watching the drill tip protrude. When it protrudes I stop drilling and retract the bit. I don't use a lead plug and I don't crash into the lands/grooves on the other side.
-Next I will make another pass using a #42(.0935)drill bit.
-Then I make a final pass using a #41(.096")drill bit.
After drilling I swab the bore with a new bronze brush, then follow with a patch and I'm done. No burr.

Stepping up drill sizes as I have outlined will generally leave me no burr in the bore.

Don't skimp when purchasing your drill bits. Use high quality bits, like Guhring and make sure to use some Tap Magic when you are drilling.

I hope that this makes sense.
-Len
 

gamma

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Re: Drill barrel blank for gas ports

Guys thanks a lot for your help here, this is great, I think Im ready to go ahead with this project, is there anything else
you think I should be aware of, headpace differences etc.
 

biffj

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Re: Drill barrel blank for gas ports

I've made quite a number of barrels for the SIG 55X series and its not too terribly difficult. You do need to have the barrel threaded into the receiver to whatever torque you are using, have the chamber done and headspaced and then you can locate and drill the gasport. You don't really have any leeway in locating it as the gasblock will define its position. What this means is that you may be in a land, a groove or both so no choice there. You can EDM the hole for a no burr edge but its important not to go too far and dink the opposite side of the barrel Thats also true for drilling the port. Shooting off the burr isn't really a good method as it will take many rounds to do so and fill your port with copper from the jackets. If you do drill the port you can deburr with a ball-end carbide burr. It needs to be done very carefully to insure an even, consistant bevel.
good luck with your project, what cal are you building?

Frank
 

cal50

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Re: Drill barrel blank for gas ports

Also if you are using a vertical mill with a crank up table / knee I always drill the hole shallow just to the point of breaking through to the bore (set quill stop). Retract to clean chips and add lube. Advance quill to the stop you set then crank up the table or knee to break through to the bore diameter. The drill wants to grab and cranking the table up prevents this. You don't want it to grab and nick the opposite side of the bore.

I don't know what the angle is for a Sig but the Galil is at a really steep angle that wants to grab really bad.

Stay away from carbide drills for interrupted cuts or breaking into other holes. Cobalt is great and a reduced chance of breakage.
 

gamma

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Re: Drill barrel blank for gas ports

Thanks a lot guys great info here. Yes its actually a .223 Im planning on working on. Cal50 you talk about a very steep angle, what angle are you talking about here, is it a very steep angle into the port, I havent looked at it at all yet, I thought the hole was actually straight down at 90 degrees.
 

cal50

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Re: Drill barrel blank for gas ports

I was referring to the Galil gas block. Drills want to grab when breaking through. It's a little more touchy when you have a steep angle (Galil) and breaking through. By cranking the table or knee up you are pretty much assured the drill will not grab on you.

If your drilling a straight through (90 Deg.) hole its simple but I would still go easy when breaking through.

Sorry for the confusion.
 

gamma

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Re: Drill barrel blank for gas ports

Thnks cal50 thats a great help to me, Im getting ready now to go for it. Ill let you know how it went, if you dont hear from me your invited to my funeral anyway. Thanks again.