how to drill gas port on a groove?

[tensnxs]

I have a chambered barrel blank already contoured. I'm ready to install it on my gas gun. How do I index the barrel so I can drill the gas port on a groove? TIA

 

[spclark]

Some experienced 'smiths don't believe that's necessary, and in fact that doing so can lead to increased erosion at the port between the gas port and the muzzle.

 

[rcw3]

After inspecting literally hundreds of AR gas ports,both with new barrels and well shot barrels) I can say without reservation that an 'in the groove' gas port is not the optimum place for a port.

The 'in the groove' ports exhibit more barrel erosion and disruption,on the muzzle end of the port) than just about any other place in the barrel a port can be put. I have no idea why people feel it is the best place for the port to be but I believe that conventional thinking in that regard is dead wrong. When you look at 'in the groove' ports that have been shot a while you can see obvious evidence of bullets having slammed into the forward side of the port just like a car tire hitting a deep sharp edged pothole square away at high speed - WHAM! Not good - bits of bullet jacket ripped off the bullet and embedded in the area toward the muzzle side of the port and barrel steel eroded away - how can that be good for passing bullets?

The best place,in my opinion) for the port is half in a land and half in the groove. Why this turns out to be best is because the jacket of the bullet,since it is conformed to the interior of the barrel) has two 90 degree bends in it,with conventional rifling) in that spot and the bullet tends to want to go past the port without wanting to deform and squeeze into the port. Just like angle iron is more rigid than flat iron, so is a bullet jacket with bends in it from the rifling which makes it want to hold shape as it goes past the port.

The 50/50 ports seem to always show much less barrel erosion and disruption than the 'in the groove' ports, but drilling a port 50/50 is more a matter of chance than anything else.

We drill them where the fall because it's about impossible to do anything else once the barrel is already rifled, and if you make uppers with different makers barrels, different lands and groove configurations, different twists, etc.. If they hit the groove 100%, so be it, but we would prefer they are not there for the reasons stated.

Robert Whitley
www.6mmAR.com

 

[rcw3]

By the way, as I understand it, some have it backwards on 'drilling the port in the groove' in that some barrel makers rifle the barrel after they drill the gas port so that the rifling leaves the port in the groove. I realize this can be a 'which came first the chicken or the egg' thing, but it does sometimes answer the question of 'how did they drill the port in the groove' question - they didn't, the barrel was not made that way.

Robert Whitley

 

[TRECustom]

Robert, you may get some flack on this, but I believe that experience trumps theory every time. I have always heard that 'in the groove' was desirable, but have rarely seen it consistently in gas gun barrels,,mostly M-1A's). I recently got a used Hawkeye borescope, but haven't shot service rifle competitively for years, so I don't get to look in barrels like I used to. It figures; now I could really look for selective erosion, etc.
I don't even remember where the gas port is in my old match gun, but it's sub moa anytime I am.

Thanks for a very informative reply.

Good shooting, Tom

 

[tensnxs]

Krieger is one company that drills the gas port on a groove. I spoke to several barrel makers and they all told me gas port is best in a groove so it doesn't damage the bullet as it passes by. They've seen many barrels came back due to accuracy problems because of this.

Krieger actually marks where the gas port needs to be and then cut the rifling groove on it. The port is drilled after it's been rifled, not before. for it causes the cutter box chatter,whatever that means) during rifling process.

I guess there is no way to do this?

 

[Robert]

Krieger is one company that drills the gas port on a groove. I spoke to several barrel makers and they all told me gas port is best in a groove so it doesn't damage the bullet as it passes by. They've seen many barrels came back due to accuracy problems because of this.

Krieger actually marks where the gas port needs to be and then cut the rifling groove on it. The port is drilled after it's been rifled, not before. for it causes the cutter box chatter,whatever that means) during rifling process.

I guess there is no way to do this?

Hi, all,

Not being a specialist in this, I however believe drilling by mechanical means a gas hole in a barrel after it being finished might leave burrs inside difficult to remove and damaging jackets.

Barrel makers seem to say they see problems in rifling after the hole has been drilled..Maybe so, but I am not too convainced of this.

Is EDM erosion-drilling of the hole not a solution? this would prevent any burrs or sharp edges at the hole/bore junction?
F.W.I.W
R.G.C

 

[rcw3]

Krieger is one company that drills the gas port on a groove. I spoke to several barrel makers and they all told me gas port is best in a groove so it doesn't damage the bullet as it passes by. They've seen many barrels came back due to accuracy problems because of this.

I am sorry,and I am going to stick my neck out here) but I believe some barrel makers don't know of what they speak in this regard or they just repeat conventional wisdom,if it really is 'wisdom') without having actual factual data to back up what they say.

Just remember, barrel makers are just that, barrel makers. They typically make barrels, not AR's. I don't know any of them that regularly make and assemble highly accurate AR-15 upper assemblies and do inspections on them and accuracy test firing work,like we do regularly). I inspect the gas port on every upper that goes out the door,before and after test firing) as well as just about all the shot out barrels that come back for re-barreling and every upper that comes back for servicing. Plus I get field reports back from customers on the accuracy of their uppers, many of which I have records on the location of the gas ports in their barrels. There is no way a barrel maker has access to that kind of data. What they get back is a barrel the customer claims does not shoot, they presumably inspect it and make an educated guess in many cases as to the nature of the problem,keeping in mind that attributing accuracy issues to a gas port gets a barrel maker off the hook for a warranty claim on barrel accuracy problems unless the barrel maker put the port in there).

Frankly, Krieger makes a great barrel and I consider them authoritative on the subject of barrels, but I don't consider them necessarily authoritative on the subject of gas ports in AR barrels.

Let me also pose this: Why is 100% 'in the groove' better than 100% through a land? From the perspective of the bullet it's the reverse of what you see when you look down the bore, and if the port is 100% in a land of the barrel, the part of the bullet that passes the port is then the part of the bullet jacket that's inside the groove in the bullet. Why isn't that the best place? There is no logic,or even a debate out there) why 100% 'in the groove' is better than 100% in the land.

The reality is, a bullet going down the bore is a lot more plastic than most people realize and bullets tend to want to squeeze into the port as they go by it and if the port is in a location in the barrel where there are angled bends in the jacket of the bullet, this is a place where the bullet tends to be more rigid and less likely to deform and squeeze into the port,just like angle iron is more rigid than flat iron).

As far as drilling ports, there is no way to totally eliminate a burr inside the barrel when you drill a port, but it sure can be minimized very significantly. The burr inside a barrel is and should be a concern if you are working to build accuracy uppers. Do what we did, experiment, try drilling ports in a junk barrel to get the rpm and feed rate just right and by all means use new and sharp high speed steel bits and replace them often and come up on the final diameter incrementally.

Robert Whitley
www.6mmAR.com

 

[jlmurphy]

I used a borescope to locate the groove, it could be done with a lead slug attached to a cleaning rod. I drilled the gas port hole with a lead slug in the bore to leave a clean hole, I also started with a smaller pilot hole. The hole is definitely a disturbance in the bore, I don't know which is better.

 

[spclark]

I was gonna suggest that idea: lead slug 'indexed' as a tell-tale to a rod of some sort; measure back to a point on the rod so as to reference the groove position on the slug when it's at the proper point inside the bore, then mark the barrel for drilling.

Hesitated as this is all theoretical to me: I do far better working wood than I ever have working metal, in any capacity.

 

[msalm]

I've chambered and installed quite a few AR barrels in the last 5 years, and if I really want the port in the groove, I'll measure from the breech end to the port location and using the borescope mark the breech end of the barrel with a sharpie w/ the locations of the grooves. When the tenon is turned and threaded, I can face off the shoulder a whisker to index the barrel extension properly so the port does end up in the groove. Honestly I don't think it makes a difference, but some customers want to be assured it's done that way. Usually I just chamber and thread, and drill the port w/ a jig of sorts that uses drill bushings. I start with a SHARP drill bit at least .010' under target diameter for the initial hole. If it's a barrel length and caliber I'm not as accustomed to, I'll test fire as I incrementally increase the dia. of the port to assure proper function without over doing things.

 

[SamLS]

I have also seen evidence of peening of the gas port where ever the port is placed. I can't comment on the 50/50. Chamfering the port on the exit side does help. You can also lap the barrel after a few hundred rounds to help with fouling. Lothar Walther blanks seem to be less of a problem than others I have seen.