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View Full Version : Changing A2 upper to an A4 Flattop


Gunhacker
09-22-2007, 4:28 PM
I have a Colt 7.62x39 A2 upper and I would like to change it into an A4 flattop using all of the existing parts.

Is there any difference between the upper receivers used on the .223 and 7.63x39 guns as far as being reinforced for the larger caliber?

Or is switching a matter of buying an A4 .223 flattop receiver and swapping parts?

Thanks in advance for any help that can be provided.

Technical Ted
09-22-2007, 5:29 PM
I have a Colt 7.62x39 A2 upper and I would like to change it into an A4 flattop using all of the existing parts.

Is there any difference between the upper receivers used on the .223 and 7.63x39 guns as far as being reinforced for the larger caliber?
No difference in receivers construction between calibers.

Or is switching a matter of buying an A4 .223 flattop receiver and swapping parts?
Not quite that simple.

More than likely your barrel has an A2 height Front Sight Base which is 0.04" lower than the "F" stamped Front Sight Base used with flat top upper receivers. Using a detachable carry handle, or a properly designed aftermarket rear Back Up Iron Sight (BUIS), may require the front sight post to be raised high enough to expose the threads underneath the flange

Swapping out FSB's is not your typical simple garage work bench chore. You can get by with a taller front sight post, but it will protrude above the FSB's protective ears.

You also want to avoid a flat top upper with extended feed ramps (M4 style). You're barrel more than likely has standard feed ramps. For clarification see this thread about M4 Feed Ramps:
http://www.calguns.net/calgunforum/showthread.php?t=69729

Otherwise, you should be able to swap out the other parts without issues.

Gunhacker
09-22-2007, 6:38 PM
Thanks much for the info... the front sight issue won't come into play with me as I'm doing the swap to be able to go to a clamp on open optic sight. My aging eyes are finding it more and more difficult to use the "iron sights".

But I will still try to do this the right way for the next owner of the gun, who may wish to have a carry handle or a BUIS on it. I'm a bit more advanced than "garage work bench" level, so I'll check out by reference books for what swaping out the FSB entails.

ar15barrels
09-22-2007, 9:05 PM
I'm a bit more advanced than "garage work bench" level, so I'll check out by reference books for what swaping out the FSB entails.

You won't find info about swapping front sight bases because it's just not done.
The barrel and FSB get drilled as a mated pair.

Trust me on this.

Go with the extended post if required.

Santa Cruz Armory
09-22-2007, 9:34 PM
You could just sell the 7.62 upper and buy another .223... just a thought? :confused:

How much for the 7.62 upper? :D

Technical Ted
09-22-2007, 9:36 PM
You could just sell the 7.62 upper and buy another .223... just a thought? :confused:

How much for the 7.62 upper? :D
I believe he wants to keep the 7.62X39 barrel and just change it from a fixed carry handle A2 receiver to a flat top, railed receiver.

TonyM
09-22-2007, 10:24 PM
If you need to, you could replace the FSB with a gasblock that has a rail, and mount a front sight on it. Or you could use a PRI type flip up/gasblock. If the one you have won't work for the new config.

Gunhacker
09-23-2007, 3:40 AM
You won't find info about swapping front sight bases because it's just not done.
The barrel and FSB get drilled as a mated pair.

Trust me on this.

Go with the extended post if required.

Thanks much for that info, I appreciate it greatly!!

Gunhacker
09-23-2007, 3:54 AM
I believe he wants to keep the 7.62X39 barrel and just change it from a fixed carry handle A2 receiver to a flat top, railed receiver.

Yep... that's exactly what I was planning to do in order to go with an open "red dot" style optic sight, as I'm having problems using the stock apeture sights with my aging eyes.

But my wife has come up with an alternative and says leave the rifle along and go have my eyes altered... Lasik.

Damn :cuss:, she may have something there, it would solve a multitude of sins considering all of the different guns I go shooting with.

supersonic
09-28-2007, 10:30 AM
You won't find info about swapping front sight bases because it's just not done.
The barrel and FSB get drilled as a mated pair.

Trust me on this.

Go with the extended post if required.

What about REMOVING A2 FSB to get off Delta ring/handguard cap/BBL nut/etc., then REINSTALLING FSB (needs to be done if you want to install BBL assy to MGI upper?
SS:43:

ar15barrels
09-28-2007, 1:20 PM
What about REMOVING A2 FSB to get off Delta ring/handguard cap/BBL nut/etc., then REINSTALLING FSB (needs to be done if you want to install BBL assy to MGI upper?

No problem.
You can remove the taper pins, slide off the FSB, handguard cap and delta ring and then re-install the FSB and taper pins.

You just can't swap out the FSB from one barrel to another, or easily install a new FSB on a barrel that's been previously drilled.

supersonic
09-29-2007, 8:50 AM
No problem.
You can remove the taper pins, slide off the FSB, handguard cap and delta ring and then re-install the FSB and taper pins.

You just can't swap out the FSB from one barrel to another, or easily install a new FSB on a barrel that's been previously drilled.

Aaaahhhh.....okay. Thanks for the clarification, as I have a "clamp-on" FSB coming from Armalite to replace my 4-rail GB. Maybe the two aren't even related, I don't know. :confused:
SS:43:

ar15barrels
09-29-2007, 2:14 PM
I have a "clamp-on" FSB coming from Armalite

A clamp-on FSB can go on ANY barrel of the proper diameter.
It does not matter if there are taper pin holes or not.