More armorer bench stuff, installed a 14.5 barrel this AM. I really like the fact that armorer wrenching is part of my job, I enjoy it and it's good to have a really decent work space and a full set of armorers tools that I can't afford to buy. Hell, the full starter/roll pin punch sets are a couple hundred dollars alone. Anyway here we go.
First get the A1 upper all padded up in the vice block and secure it in the vice. The vice block is the standard Delrin Universal A1/A2/A3 type. I pad the block with Styrofoam sheet because if you don't there will be faint markings, kind of slight shading, left on the upper where the block makes contact. No big deal but it is what it is. Then grease the upper receiver threads and barrel extension with Aeroshell 33MS, insert barrel into upper, and snug the barrel nut up ........
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Next I check the barrel extension pin and receiver pin notch fitment by wiggling the Front Sight Base back and forth.
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Most of the time there will be play, sometimes the fit is tight. I prefer play and here's why. The next thing I do is make adjustments to ensure the front sight /FSB will be very close to top dead center (TDC) when the barrel nut is torqued down. The barrel/FSB will want to rotate clockwise (as you look from the muzzle end back toward the upper receiver) as you torque the barrel down. I snug the barrel nut then I use hand pressure to torque the front sight base clockwise then eyeball the the rear sight/front sight post alignment to make sure it close to TDC. If it's over rotated (front sight base appears canted left, which is the case about 90% of the time), I slightly peen the right side of the barrel extension pin (looking from muzzle) using a standard punch and small ballpeen hammer. A little peen/check and make additional peening as necessary until hand torqueing the front sight base clockwise gives the TDC result I want. If after hand torqueing the FSB ends up slightly canted right...then its time to take a jewelers file to the upper receiver barrel pin notch (right side looking from muzzle) and open up the notch. A little filing, check, make small adjustments as necessary until TDC is reached. This sounds like a lot of work, but it's not really and it will insure when the barrel is torqued properly you won't have your rear sight run out of left windage before you reach zero because the FSB is canted left. By the way this is not how it's done at Hartford. The reason all the millions of Colt's don't have the canted FSB issues that some other manufactures suffer from is you will never see a barrel with the FSB installed before the barrel is installed in the upper receiver. Colt (and probably the other high end manufactures like KAC and DD) install barrels first, then measure TDC at the gas port location, mark it, drill it, then install the FSB = TDC every time.
Next, time to torque the barrel nut down. The torque standard is 30 to 80 foot pounds....yeah I know. First, I use the barrel wrench alone and and tighten then loosen the barrel nut three times to set the threads. Using the wrench alone will get the nut somewhere between 30-40 foot pounds of torque, no worries. Next I set the torque wrench to 45 foot pounds and mate up with the wrench and torque the barrel nut down. Now time to align the nut with the upper receiver gas tube hole. I set the wrench to 58 foot pounds and tighten the nut till it looks aligned, I rarely have to exceed the 55 lbs to get it done. I check the alignment with a short section of old cut down gas tube and adjust as necessary.
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You can see above the nut is slightly over torqued. Not a problem, make the small adjustment to line it up. Now time to check from the business end using the bolt carrier only (the pic shows the bolt/cam pin/firing pin installed, but all that needs to be removed). There is a gas tube alignment tool that fits in the carrier key for this very purpose. I have no idea what it costs, but I know it ain't cheap. I have one, but you can make one from an old gas tube, like shown here.
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Make sure to insert the gas port end of the tube into the carrier key (for a tight fit) and insert the carrier only into the upper. The tube should fit through the receiver, snap ring/spring/hand guard ring, and barrel nut with no interference. If it doesn't make VERY small adjustments to the barrel nut until it does.
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Once it all aligns, now it's time to test fit the gas tube, recheck the alignment and verify the torque is good to go. First I install the gas and use the proper punch passed thru the roll pin hole to make sure the tube depth is good to go. I find the gas tube install/disassembly tool is a easy way to do this, but it's not necessary. A good grip and a proper punch will get it done.
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Now eyeball the gas tube inside the receiver, it should look centered.
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Now drop the stripped bolt carrier into the upper. It should slid forward to the full battery position with zero interference between the carrier key and gas tube. If you find it does rub, then almost always it due to the barrel nut being slightly misaligned. Two ways to fix that. Remove the gas tube and adjust the barrel nut (what I would do), or break out the the BIG standard screwdriver and bend the tub inside the receiver in the appropriate direction to relieve the interference. I don't do that, but that's just me. Before I install the gas tube roll pin I remove the tube. I reset the torque wrench to 30 foot pounds and attempt o loosen the the barrel nut. It will snap over before the nut loosens. Now you know the torque is somewhere between 30 - 58 foot pounds...probably closer to 50-55. Now install the gas tube.
The last thing I do is bore sight the upper. Not a lot of folks bother with this. I set a target at 10 meters, use a level to draw a straight vertical line, and set the stripped upper assembly on a flat surface. I use the small aperture and adjust the upper until the front sight post splits the vertical line. I then look through the bore and confirm the line splits (or is very close) the center of the bore. Done!
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Zeroed pretty close to center windage which is a little more critical than normal when running an A2 rear aperture in an A1 upper. The A2 rear sight is wider than the A1 and there is less windage adjustment as a result.
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50 yards final zero group, prone unsupported, M193
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