Colt A2 to Rock River Arms M4 Upper Reciever Swap by Kelly B. New 01-05-08
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A lot of departments started moving towards issuing AR-15s to their officers or, in some cases, such as the Denver Police Department, allowing officers to purchase their own rifles. In Denver's case, only two AR-15's were initally approved, the Colt 6530 and 6520. Both of these rifles use the older A2 style upper with the 'carry handle' built in. At first, this wasnt an issue, since Denver hadn't approved optics for use.
Now as time marches forward, departments of all sorts are upgrading and allowing officers to customize to an extent. There are several ways you can upgrade your rifle to the newest 'M4' sytle flattop. You can buy a whole new rifle (OUCH), and whole new upper reciever (ouch) or if you're handy, for less than 200.00 in parts and tools, swap out just the carry handle upper reciever with a flattop upper reciever. Come on and I'll show you how.
First, you'll need some tools. Fortunatly, I have a LOT of tools. But you won't need nearly as many as this.
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You'll need, at least the following tools:
° A good field giude if you haven't done this before. I highly recommend Fulton-Armory's "The AR-15 Complete Assembly Guide" by Walt Kuleck and Clink McKee, 20.00 from http://www.fulton-armory.com .
° A good quality reciever block set. I used the Brownell's Upper and Lower Action Block set, PN 702-000-001AA, 75.00 plus shipping.
° A pair of hammers, I recommend a brass drift hammer and a 16oz dead blow hammer. This will prevent damage to your punches and if you should slip, minimize damage to your rifle.
° A small pair of vice grips will be handy later.
° A good armorers wrench, I've got a DPMS one that I've had for years that I picked up at a gun show for about 30.00 or so.
° A good torque wrench, I use a Snap-On QD3R250. You can rent a torque wrench at your local auto parts supply house, but since these never get recalibrated and are usually of Chinese origin, I recommend that you beg, borrow or buy a NICE one.
° A small 5/64ths or 1/8th punch. I HIGHLY recommend the Snap-On 1/8th inch roll pin punch, part number PPR2, it has a ball shoulder milled into the punch face to keep the punch centered on the pin.
° A high quality anti-seize lubricant. Some people use molybdenum grease, but I prefer a dedicated anti-sieze such as Permatex 133A (available at any auto parts store)
° A vice <i>(smoking or drinking doesn't count *grin*)</i>, the bigger the better. This will play in later when you're trying to remove the barrel nut thats been torqued to 100+ft lbs.
° Snap ring pliers are an optional item if you intend to replace the delta ring and spring assembly
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BEFORE YOU START, IF YOU ARE UNSURE OF YOUR ABILITIES WORKING ON A RIFLE, SEND IT TO A GUNSMITH. IF THIS IS A WEAPON THAT YOU ENTUST YOUR LIFE WITH (MIL OR LE) REFER TO YOUR ARMORER BEFORE PROCEEDING.
I'm not going to spend a lot of time explaining dummy stuff. I assume that you know how to safely clear the weapon, get the upper reciever off of the lower and remove the hand guards. Once you've done that, the first step is to place the front sight assembly onto a safe, clean edge and drive out the gas tube retainer pin. I tried it initially with a cheapee 1/16ths punch and got a broken punch for my trouble. I switched to the Snap-On PPR2 and it came right out.
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Once you've gotten the pin out, place the entire upper in your action block and secure it in the vice. Start by spraying in a bit of WD-40 or other lube to help loosen the tube up. Then Take your pair of vice grips and gently work the tube back and forth about 10° at both ends to free up any carbon before sliding the tube backwards about 3/4 of an inch to the rear, then rocking it slightly to one side and pulling it GENTLY away.
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After removing the gas tube, take your armorers wrench and loosen the barrel nut. On this upper, the barrel nut was torqued to at LEAST 110ft lbs, so I was really glad to have a huge vice. And remember you have to push in and turn at the same time because of the delta ring. The threads are normal right handed threading so remember, clockwise to tighten, counter clockwise to remove.
Once you've pulled the barrel free, take the old reciever out of the action block, put the new reciever in and chuck it back up in the vice. Once you've completed that, place the old barrel into the new upper reciever, making sure that the barrel seats properly with the alignment pin in the slot on the upper reciever. Apply a generous amount of anti-seize to your reciever threads as this will prevent the steel barrel nut from galling to the aluminum reciever due to galvanic corrosion, often misnamed "electrolysis". As you begin to tighten the barrel nut, remember to keep an eye on your delta ring spring and snap ring assembly. Stop occasionally and check alignment, as you'll have to have the barrel nut, delta snap ring and delta ring spring aligned correctly to reinstall the gas tube.
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Once you've torqued the barrel to 80ft lbs, you can then revese the disassembly process. Once you've aligned the barrel nut, and delta ring assembly, take a bit of scotchbrite and polish off any carbon on the gas tube. Then carefully reinsert the gas tub into the reciever, and then carefully forward into the front sight block. Once you have the gas tube inserted, take your 1/16th punch and line the roll pin hole back up and the carefully drive your roll pin back in.
And there you have it. Replace your handguards, or rails, mount up a BUIS and maybe an optic and go sight it in. In SOME RARE cases, the barrel alignment pin or alignment pin slot in the upper reciever may be out of spec, causing the barrel to be slightly tilted or extreme zero'ing issues. Again these are correctable issues, but often require lapping or grinding to fix. Don't be part of the dremel brigade unless you're 110% sure of what you're doing! As far as headspacing goes, unless you've done something silly, like lapped/stoned the bolt or barrel, you're ok. And since we reused the original bolt and barrel, both of which are Colt, we're good to go. And just in case you're still worried, remember that that the Army, which fires a zillion rounds a year has one (1) gage. Gage, headspace, field. That's it. So sleep well.
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Mount it up and go have fun. Remember the four cardinal rules and that attitude and training should always be high on your list of personal. Many thanks to Brownells and Rock River Arms for their help with this article.
