View Full Version : A2 to A4 AR upper

December 31, 2010, 11:05 AM
I have a 10.5 inch upper on my SBR AR-15 that I honestly just bought as a play-toy after watching the bank shootout on Heat :rolleyes: (damn you Robert De Niro!). The upper runs great and after several 100 rounds of Wolf & other cheapo brands of ammo still has yet to Jam even though I've never bothered to clean it (if you've played with shorty ARs you know how finicky they can be, so I feel pretty lucky).

Though I love the looks & simplicity of the upper, I'd like to mount an EoTech I have on it. But mounting an EoTech on an A2 upper looks retarded IMHO. So I'd like to replace the A2 upper receiver with a new A4 upper receiver (just the upper receiver not a complete upper).

Would putting a new upper on it change the reliability of the weapon, or would the gas system pretty much stay the same?

Does this make sense or should I just buy a whole new upper assembly?

December 31, 2010, 12:17 PM
The new upper won't change reliability, or at least it shouldn't; the barrel/chamber and bolt carrier group tend to be the determining factors there.

The only possible catch might be the zero of your iron sights; you might need to adjust the elevation of the front sight post since a flat top and iron sight tends to result in a slightly different aim point (especially with shorter barrels) than the traditional A2 upper.

December 31, 2010, 12:38 PM
If your current upper has M4 feedramps on the barrel extension, you should be fine. Take it to someone who knows what he's doing if you decide to swap.

I'd just buy another complete upper. :D

December 31, 2010, 01:57 PM
Good points above!

I'll just touch on the legalities... Since we're talking a non-serialized upper there's probably no issue but definitely worth double checking the law in your state, etc. Don't know that the paperwork tracks whether the upper is A2 or A4, seems unlikely.

December 31, 2010, 02:49 PM
The lower receiver is the SBR, so putting a different upper on it won't effect that part of the equation at all.

There could be reliability issues, but you should be fine as long as you buy from a reputable manufacturer.

December 31, 2010, 02:51 PM
Paperwork doesn't look at the upper receiver.

If you have the proper tools, it is a snap.

Remove handguards
Remove gas tube roll pin and gas tube
Use a barrel wrench to remove the large nut that holds the barrel to the receiver
Pull barrel off upper

Reverse above steps with new receiver

December 31, 2010, 02:52 PM
What about torque spec and moly grease?

December 31, 2010, 03:03 PM
Fine, full length instructions with pictures. From AR15.com

Changing your own AR15 barrel for dummies

This web page will demonstrate just how easy it is to re-barrel your AR-15 upper. This whole process, start to finish, on my very first build, took less than 30 minutes…. And that was taking notes and pictures! If you can tighten a single nut, then you can change your barrel, and build your own uppers, in minutes. I cannot get over how simple this is.

Ok, lets cover the necessary tools. What you need:

1. A quality upper receiver action block. I went with the Bushmaster model (~$40):



2. A quality armorer’s wrench. Based on what I read about all the choices, I chose the DPMS model (~$35):


Bushmaster also sells one:



3. A small punch to knock out the gas tube roll pin. I used a 1/16” drill bit because I did not have a punch that small.

4. A set of snap ring pliers. I had a set laying around, but you can get these at any hardware store.

5. Molybdenum-Disulfide grease (per the TM) This is for the upper receiver threads. This is found at any auto parts store or hardware store for cheap. At the auto parts store - just buy generic WHEEL BEARING grease.

6. A vise. Pretty much any size will do, but must be firmly mounted to a bench of some sort.

That’s it for tools! Now here are the step-by-step instructions:

1. First thing is first. Remove the upper from the rifle. Remove the bolt/carrier/charging handle from the upper.

2. Remove the hand-guards from the upper. (This assumes you have typical carbine/rifle guards) You do this by pulling on the delta ring toward the receiver, while using your other hand to slip off the upper and lower guard. It’s a little tricky at first, but you will get the hang of it with practice.


3. Next, we need to knock out the roll pin that holds the gas tube into the front sight assembly. The correct size if 5/64". Try finding a 5/64" punch at your locl hardware store!. Use a 1/16” punch. I didn't have one that small, so I just used a really small drill bit to knock it out. It can be tapped out from either direction. Here is a close-up of the roll pin to knock out:


4. Once that is knocked out, you can remove the gas tube. Pull the tube from the front sight base into the upper receiver, until the tube can clear the front sight, then pull it forward to remove it. Sometimes they can be stubborn, and you might have to LIGHTLY grab the tube with a pair of pliers and tap the pliers rearward with a light hammer. Don’t crush the gas tube. If for any reason you damage it, they are cheap. Replace with a new one. Note the gas port on the tube faces down, and note the hole for the roll pin:


5. Now, we are ready to slap on the action block. It only goes on one way, so you cannot screw it up. Clamp the upper/block into the vise as shown:


December 31, 2010, 03:04 PM
Continued from above

6. Take your armorer’s wrench out, push it onto the barrel nut, and unscrew the nut. There is spring pressure here, so you must push in towards the receiver whille you turn. It uses standard threads. Righty-tighty, Lefty-loosey. Once it breaks free, it should unscrew fairly easily. Take off the delta ring assembly as shown:


7. Now, your barrel will just pull straight from the receiver. Remove the barrel.

8. At this point, we need to remove the delta ring, spring, and snap-ring that holds it all together. Grab your snap ring pliers, and slide off the snap ring, spring, and delta ring as shown:



9. Halfway there! Now, we need to clean the surfaces of the new barrel, and the threaded area of the upper receiver, to make sure they are free of grit and particles that might keep them from going together smoothly. I like to use gunscrubber, or brake parts cleaner for this. Then, per the TM, apply a little moly grease to the threads of the upper receiver:


10. If necessary, assemble the delta ring assembly on the new barrel. Now, slide the new barrel into the receiver. There is a locator stub on the barrel that fits in a notch in the upper receiver. If there is any play here, line up your front sight so that it is the straightest with the rear sights. Sometimes, it *may* necessary to file this notch open in one direction, if your front sight will not line up straight. I dont recommend cutting anything at first; assemble your barrel, and only perform this procedure if your sights require a lot of windage adjustment in one direction to zero it.


11. Thread on the barrel nut. Be careful not to cross-thread it, so just go easy at first. Some people like to take a short piece of gas tube or 3/16" bar and insert into the receiver, to hold the spring/snap ring from spinning as you tighten the barrel nut. Then use your armorers tool to get the barrel nut snug (not fully tight). Inspect the barrel nut lugs, and continue to tighten the nut until the lugs line up with the gas tube hole in the receiver. Once that is done, you need to make sure that the hole in the spring and the snap ring is aligned so you can insert the gas tube. Insert the gas tube through the barrel nut, delta ring, and into the receiver. Then insert the other end into the front sight block:


12. Now, you can remove the upper from the vise. Tap the roll pin back into the front sight block to hold the gas tube in place. Install the handguards.

13. Sight the upper in and go shooting!

December 31, 2010, 03:20 PM
:D Much better!

January 1, 2011, 09:49 AM
Then use your armorers tool to get the barrel nut snug (not fully tight). Inspect the barrel nut lugs, and continue to tighten the nut until the lugs line up with the gas tube hole in the receiver. Once that is done, you need to make sure that the hole in the spring and the snap ring is aligned so you can insert the gas tube. Insert the gas tube through the barrel nut, delta ring, and into the receiver. Then insert the other end into the front sight block:

They didn't mention that you should torque to 30 lb ft three times, then final torque of 30-50 lb ft for gas tube alignment. If you can't align the gas tube by 50 lb ft torque try a different barrel nut.