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Good Books on threading and Chambering AR platform weapons

boomboompow

Plastic
Joined
Dec 17, 2012
Location
Georgia
What are some good reference materials on building AR platform rifles. I am more interested turning, threading, and chambering. Any suggestions will be appreciated.
 
I don't have any books on threading and chambering AR's, I just have some notes and engineering drawings. The specs for the barrel tenon are pretty specific and tight: 0.620" long, threads are 0.812 major diameter, 16 TPI, class 3 fit.

Chambering: I chamber the barrel first, so that if I'm a tad too deep, I can just handle it on the cut for the tenon. So I:

1. Set up the barrel as I would for chambering any other rifle. This means whatever you want - if you chamber through the headstock, then you know what to do. If you chamber in a steady rest, you know what to do.

2. Chamber the barrel.

3. Put on your tenon, 0.620" long, thread per above. The barrel forward of the barrel extension should be 1" in diameter. A go gage should stick out about 0.128, if memory serves.

4. You should think about profiling your barrel now, if you haven't done it yet. If your barrel already has a finished profile, thread the muzzle end if you want such things and get that done.

5. When you put on the barrel extension, you're going to torque that thing on to 150 ft-lbs. Get a good grip on the barrel in a vise with some soft jaws or a clamp setup coated with rosin. Use a fluted driver made for putting on barrel extensions and a 1/2 drive torque wrench.

6. Now you need to drill the hole for the indexing pin - which is 0.125. If you're making a pin, make it a couple tenths too big, put a chamfer on the bottom end. Coat it with red loctite and press it in. The pin should be no higher than the bottom of the threads on the upper receiver when you push the barrel extension into the upper.

7. Now you need to determine where the gas port is going to go. Before I type in a whole bunch of material on this, I'd want to know that your barrel has no gas port hole and that you need this info. You can put the gas port just about anywhere between 5" forward of the chamber and about 2" short of the muzzle, and you adjust the gas into the action by choosing a larger or smaller gas port. You should drill the gas port hole .005" under your desired size, make sure that the drill bit doesn't bottom into the other side of the bore. Put a 3/16's wood dowel into the bore to tell you (by manual feedback) that you've punched through the barrel wall. Then change to a chucking reamer that is your final size (drill size + 0.005") and ream the hole. This gets rid of the burr that happens if you use a twist drill to make the hole the final size.
 
Wyop,

I'd be inerested in more detail info if you don't mind. Are drawings available on the web? How large is the gas port hole? Does it vary by caliber?

I've been looking for a Grendel upper assembly for a few years, or even a barrel. Every time I've looked for one they are unavailable or on back order. Looks like I may have to purchase a .264 barrel blank and purchase/rent a reamer and gauges if I'm going to get one.

Hope this isn't construed as high jacking the OP post. Just looking for more info.

Any help would be appreciated.

thanks,
Ken
 
There might be some drawings out there. I guess I'd start by sniffing around the AR groups on the web. I have drawings on hard copy and I have some Armalite engineering memos, but I gotta be honest: I'm too busy right now to dig that stuff out and scan it in.

There's nothing about the AR platform that's different than any other gas-operated semi-auto. You need to pick off "just enough" pressure for a required duration to cycle the action.

The port hole doesn't vary by caliber so much as it varies by pressure of the cartridge. 5.56 is higher pressure than some larger caliber cartridges. The size of the hole varies according to what the pressure is where you choose to pick off the gas to cycle the action. You need to consider the length of the barrel beyond the port - if your port hole is close to the muzzle, you'll need to make your port larger to get that action cycling faster before you lose all pressure. Conversely, if you're porting the barrel closer to the chamber, you might have a tiny little hole, and you're going to have pressure of a longer duration.

There's rifle length, mid-length, carbine length and pistol length gas tubes. In general, I'd try to have at least 4" of barrel between your port and the muzzle. That's just a generalization to make your life easier. If you really like being a pioneer (definition of a pioneer: Arrows in your back and your face in the mud), then locate the gas port where ever you want and try to make it work.

I can't tell you what size you should be using for a particular cartridge unless it's a .223/5.56. I can tell you where to start according to your gas port location, but it is probably better that you measure it directly yourself. You should assemble your upper (with the barrel, barrel extension, and D-ring or floating handguard nut) torqued down to final spec. Then you should lock the complete bolt into the extension.

Now, put your gas tube into the upper. Bottom it into the gas key. Pull it back out by, oh, 0.050 or so. You do NOT want the gas tube to bottom into the gas key when the bolt goes into battery - it will peen the gas tube shut and you'll have to a) replace it and b) cut down the gas tube to make some clearance.

Mark where your gas tube (whatever the length of your gas system) ends with .050" clearance off bottoming in the gas key. Dykem the barrel, mark where the end of the gas tube is. Don't get itchy with the drill bits just yet.

Now you need to look at the drawings for your gas block and figure out where the hole in the barrel needs to be - it might not be directly under the end of the gas tube. You might need to position your actual hole in the barrel forward of the end of the gas tube in some gas blocks in some adjustable gas blocks.

Put the barrel into a vise with soft jaws (if you have a clamp-on gas block) or a super-spacer (if you need to put dimples on the bottom of the barrel for set screws), and then figure out how to get the gas port hole indexed to be vertical - you should determine what is "straight up" from the alignment pin that goes into the barrel extension (and into the barrel threads). That should be your indication of what the "top" of the barrel is. Run out to where your gas port needs to be and position the hole so that you can make your gas block work with your gas tube position and the hole position in the barrel. I can't tell you how to do this without seeing a drawing for your gas block. I make my own gas blocks from 1.5" aluminum round stock, which allows me to choose more barrel diameters than what are "standard."

OK, so let's assume you know exactly where your port is going and you've figured out the dimensions of your gas block. You do the usual stuff to make sure you're about to sink the hole into the top center of the round stock that is your barrel, center drill, then use a small drill bit to set up the first hole. Then run in a chucking reamer to enlarge the hole by no more than .005. This gets rid of your burr in the bore. Put on your gas system and go test fire your AR. Your empties should eject at about 3 o'Clock to 4 o'Clock, and you should get reliable stripping & feeding of the next round. If you over-gas your action, it is entirely possible that you fail to feed the next round - this is true of every semi-auto that is gas operated and some that are recoil operated (eg, the Browning A-5). If the action cycles too fast (from being over-gassed), the first round extracts/ejects (with quite some force, I might add) and then the bolt passes over the next round from the magazine - the bolt is cycling too fast for the spring in the magazine to feed the next round up into the path of the bolt.

If your AR is short cycling (ie, ejecting at about 2 to 3 o'clock, and not stripping the next round), you need to enlarge your port by .005 at a time until it reliably strips and feeds the next cartridge. Then you're done.

You should always start smaller than what you expect the final size to be - if you over-gas the action, all you can do is put a heavier recoil buffer into place, and that's a pretty feeble solution, IMO.
 








 
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