Using an Annular Cutter to Trim an Armalite AR-180B Rifle Muzzle for Threading 

in the description

It's a bit faster than the manufacturer reccomends, but the bit is also not taking a full bite across the width of the cutter tooth. The teeth of the cutter are more than 1/8" wide, and, for this project, I'd be shocked if I removed more than 1/16" of steel. If I were cutting down a bull barrel to a 1/2" muzzle thread, the speed would be much more of a concern for me. It's definitely not a best practice, but it is a trick I've gotten away with. Excellent question!

@@DrThunder88 thank you for the reply. New to this and trying to learn more about it.

armyidaho Thank you!

It's a tricky situation to be sure. My only can is a 30 caliber, so I wouldn't have any problem putting it on a 22 caliber rifle that had been threaded this way. If it were a 22 caliber can, though, I might think twice! That said, I would probably take the risk on my own guns and with my own suppressor, but whether you're willing to take the risk yourself is entirely up to you. Good luck either way you go!

The floating bore mandrel does a good job of isolating the rotation of the drill from the rifling, and it's machined tightly enough that I am not worried about it knocking around side to side. Even if it has done damage, I haven't noticed any ill effects to the accuracy of the gun. The rifles I've done this to started out as 2-3MOA guns at best, so there's a huge margin of error to hide any changes. I don't think I mentioned in the video that I wouldn't do this on a precision rifle, but that's mainly for reasons of concentricity rather than damage to the bore, but that would be the best case for demonstrating that this process will adversely affect accuracy.

That's an excellent question! I immediately thought "Tapco", but after some consideration I don't think Tapco ever made a flash hider of this type. As near as I can tell, it's a pretty run-of-the-mill, three-prong M16 flash hider. I see Luth-AR and Brownells have very similar clones of the same style device, but whether they were the original source for mine, I just don't know.

Sorry to keep you on the edge of your seat! You can rest your weary heart, though, as it wasn't truly freehand. The pilot keeps the cutter concentric and parallel to the bore. The only aspect that could be said to be really "free" is the feed pressure, which would also be true if it were in a magnetic drill.

@@DrThunder88 ok, I didn't see your pilot pin, I usually drill with a mag drill and the pilot is sprung and activates the cutting fluid, with the weldon type chuck. I was cringing expecting the bit to grab and spin the rifle lol. Cheers , sorry for reviving such an old thread!

It is 7/8" outer diameter. The specific part is a Hougen Rotabroach 12128.

Thank you

Excellent catch! The rifling actually ends 1/8" behind the new end of the barrel, meaning those critical surfaces were never touched by my cutting tools. There's no way to know this from the video, which is my fault. It also was not intentional. I did not realize where the "barrel" ended and the "brake" began until I cut into it.

A note in the vid will solve that. Good to know.

CNC Warrior I know sells all the tools I used for both cutting the muzzle diameter and threading it. The only tool I used that they don't sell was the bandsaw! My tools, however, came from a number of sources, but the annular cutter was CNC Warrior's. And yes, it was threaded to 1/2-28.

It is a Hougen 12128.

Are we talking about the contour being tapered or the bore? A tapered contour would be no problem. The only concern I might have is making sure there is enough of a shoulder behind the threads to support whatever muzzle device you're thinking of using. Generally, I'd say the diameter of the barrel directly behind the threaded area should be at least 0.1 inches larger than the nominal diameter of the threads, e.g. if you're threading 1/2-28, make sure the barrel is at least 0.6 inches in diameter at 0.5 inches behind the crown. Now if we're talking about the mythical tapered bores, that might cause an issue if the taper is abrupt enough to allow the annular cutter pilot and thread alignment tool to wobble between the lands.

It's difficult to say, as I don't know if there is a standard width of the cutter teeth. Otherwise it'd be easy to say "take the cutter diameter and subtract two times the tooth width". Looking at CNC Warrior's site, they offer an annular cutter with a 0.57 inch slug diameter, and the page says that barrels should not have an OD larger than 0.875 inch. Since 0.875 inch is 7/8 inch, my guess would be that a 7/8 inch cutter is what you're looking for, but I do not know that for sure.

I got my annular cutter as part of a kit from CNC Warrior. It ended up being a 7/8 inch cutter to get the correct muzzle diameter for 1/2 inch threads. I believe--but am by no means sure--that a 15/16 inch annular cutter will leave the correct muzzle diameter for 14mm threads.

@@DrThunder88 1/2 × 28 is the thread mostly i would use . My SKS is the Norico hunting version. Thank u for your response.

@@charlesc.plumley9755 You're welcome! I lucked out on my Norinco Hunter project, since the muzzle on mine was just about right for threading at 1/2 inch. You can see that project here: ru-vid.com/group/PL00374856840BC330

I certainly wouldn't use this tool on a precision rifle for fear of messing up the crown, but on a utility rifle like the AR-180B I had no qualms. Importantly, it did not cause keyholing. Given the rifle's accuracy out of the box, I don't think I would have noticed a change in group size unless it was more than an inch.

@HeadingEast33 The venerable but no longer online Steve Wagner would use the top of a carriage bolt and 400 grit Clover compound, but that's exactly how to do it! Though, on a rifle like this one, having a pristine muzzle isn't going to affect accuracy as much as it would on a precision rig.

The cutting bit itself is a Hougen Rotabroach 12128, however, the bit is designed for magnetic drills, not hand drills or drill presses. The bit holder made by CNC Warrior is what allows the cutting bit to be used in a standard drill and accept a bore diameter-specific pilot, and it is not as readily available as the cutting bits. Dinzag Arms also sells an extremely similar setup to the one sold by CNC Warrior. Dinzag sells the parts individually, so you can order the holder and pilot from him and find the appropriate size cutting bit from another vendor if you find a better deal locally. The terms and conditions page also seems to indicate that Dinzag will ship tools internationally, but you'd need to call him.

@@DrThunder88 so it's actually a Hougen 7/8" x 1" Rotabroach Annular Cutter? That's what I located with the part number 12128.

@@CaFFeineSouLja Correct. The inner diameter ends up being the right size for threading.

I didn't cut the muzzle brake back far enough to get to the factory crown. It might be something I pursue in the future. Thanks for watching!

According to info from CNC Warrior, 1/2" threads should be between 0.4924" and 0.4989" with an absolute maximum of 0.510". I think the annular cutter left the muzzle diameter just a hair under 0.5"

@@DrThunder88 I'm now at 0.5005, I'll take off a few and try to thread it. Thank you.

@@ARC928 Good luck on your project! Let me know how it turns out.

So I have this adjustable 1/2-28 die and not sure what to adjust it to. Any idea?

@@ARC928 Generally speaking adjustable dies have a set screw that runs across the gap. Tightening that screw will expand the die ever so slightly (almost imperceptibly) and create a slightly thicker male thread. Similarly, backing out the screw and clamping down on opposite sides in a particular type of die stock will close the die slightly and result in a slightly thinner male thread. I've not found it too important to play with the thread specs in this way for this type of threading. To get the most precise threads possible, it's best to remove the barrel and have them single-point cut on a lathe.

Not that I've found, though there is that nagging feeling I get whenever something other than a bullet goes down the bore. The mandrel is machined to be a close fit between the lands, so it doesn't knock around inside the barrel. It's also smoothly finished. As long as it doesn't get abrasive grit on it, there shouldn't be much unusual wear. It's also worth pointing out I did this on an AR-180B, not a target rifle. If I were having one of my long range guns threaded, I'd almost certainly have it done on a lathe for the utmost in concentricity and to eliminate even the possibility of damaging the bore. The other problem is AR-180Bs aren't common guns for gunsmiths to work on. I don't know of many who'd take on such a gun.

DrThunder88 right on thanks for the info I appreciate it

+SOJU Mik The annular cutter kit from CNC Warrior I used currently costs $77.63.

Información

The precision was sufficient for the task at hand. You might as well have said "THERE COULD HAVE BEEN MORE ELEPHANTS INVOLVED" True, but to what end? More to the point, neither you nor I nor anyone knows how accurate the threads actually are. All I know for sure is the rifle is just as accurate as it was before (which is to say "okay, but not great") and it can host my 30 cal suppressor with no problem. "Butcher," by the way is pretty apt. Breaking down carcasses into more useful, desirable parts is pretty much how I go about my gun projects. If you go to a surgeon to cut you a porterhouse, you are overpaying!

@@DrThunder88 🤣 what a great response

Thanks! You're right that the rifle does work better than before, and having muzzle threads actually makes it more marketable upon resale. You're using the word "destroy" in a way I'm not familiar with, but I don't think that matters as long as we're on the same page.

You did a great job. I’m using this as a guide on how to do mine the same way.

None of the barrels I used the annular cutters on were nitrided, so I can't speak to that from firsthand experience. On the one hand, it makes sense: increased hardness and wear resistance. On the other hand, I know I have dimpled a few nitrided barrels with regular twist bits. I wish I could provide you with some more info. Best of luck!