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I picked up a used Bushmaster AR15 recently, but it's the type with the permanently attached carry handle (A2?). It came with a low quality scope on a low quality carry handle mount, which I wish to replace. After looking at scope mounting options, it's become painfully obvious that a "flat top" upper reciever is a much better solution for optics mounting.

So... I'm thinking about replacing the upper with another flat-top receiver. I'd like to stay with a recognized upper manufacturer, such as CMMG.

My understanding is that either of these would basically be quick replacement onto my existing AR-15's upper?

The "16" M4 upper" from http://www.fireforeffect.biz/shop/index ... t=3&shop=1&

or

http://www.aimsurplus.com/acatalog/CMMG ... _Half.html

While this is seems like an expensive solution, I think I'll be happer with this setup than trying to get a decent mount on my carry handle, and I'll be halfway (just short a completed lower receiver!) to another AR.

Any comments or suggestions? Is the 1/7 vs 1/9 twist and 4140 vs 4150 steel used in the above two receievers worth the $50 difference in price?

Another option is to just swap the upper receiver, not the entire upper half. I'm thinking about it too.

135boomer wrote:Another option is to just swap the upper receiver, not the entire upper half. I'm thinking about it too.

[slapping forehead]

Aha - obviously I hadn't done enough research here! I assume something like the top link at

http://cmmginc.secure-mall.com/shop/ind ... 353&cat=30&

would be the right approach? $140 certainly sounds more appealing than $500! :-)

Velocity wrote:

135boomer wrote:Another option is to just swap the upper receiver, not the entire upper half. I'm thinking about it too.

[slapping forehead]

Aha - obviously I hadn't done enough research here! I assume something like the top link at

http://cmmginc.secure-mall.com/shop/ind ... 353&cat=30&

would be the right approach? $140 certainly sounds more appealing than $500! :-)

The upper receiver is much of the accuracy in an AR 15. You may want to have a gunsmith match up all the parts etc. if you get just an upper receiver.

Just sale the one you have and buy a flat. There are many manufactures of quality uppers to choose from

MGI Military make a very good advanced upper design. It has a quick change barrel system. They are fast easy and accurate.

GC

KaiserB wrote: The upper receiver is much of the accuracy in an AR 15. You may want to have a gunsmith match up all the parts etc. if you get just an upper receiver.

Just out of curiousity, what's involved in "matching up the parts" that would require a gunsmith? Not being sarcastic at all, I just figured as long as the reassembly was done correctly and the build is solid, the rezero'ing process of whatever sights/optics you have back on the rifle would get it accurate again.

Now if the parts were loose or something, I could see that being a problem...

On the same topic, should I need one, anyone have recommendations for a gunsmith in the Houston area? Is this something a "gander mountain" could handle, or is it more specialized?

I for one don't agree that you need to match the parts up. A good quality upper and bolt & carrier will give you an accurate rifle.

If you are building a super accurate target rifle then I would say have a custom bbl made with a match chamber and have the bolt fitted for that barrel.


GC

As mentioned above the barrel and bolt have more to do with accuracy than the upper receiver itself. The barrel slides into the upper receiver the barrel nut screws on the the receiver securing the barrel between the barrel nut and the receiver. The chamber itself is formed when the barrel extension is fitted to the barrel which happens before the barrel has a finished applied to it. Simply swapping from an A2 upper to a flat top goes something like this:

**Disassembly**
Remove handguards.
Remove upper from lower (if you have not done this already).
Drive out gas tube roll pin.
Remove gas tube.
Place upper into action block and secure in a vice.
Using the armorer's wrench break loose the barrel nut and delta ring assembly.
spin the barrel nut and delta ring assembly until they are no longer connected to the upper receiver.
slide barrel out of upper receiver.
Remove upper receiver from action block and vice

**These do not apply if you are using a complete upper or an upper without provisions for the forward assist and dust cover**
Remove dust cover from A2 receiver
Remove forward assist from A2 receiver (drive out 1 roll pin)
Install forward assist into new receiver
Install dust cover into new receiver

**Reassembly of upper**
Place new upper receiver into action block and vice.
Slide barrel into upper receiver.
Spin barrel nut onto upper receiver.
Using the armorer's wrench, tighten barrel nut to spec (30-80 lb/ft) ensuring gas hole in barrel nut aligns with gas port in upper.
Reinsert gas tube
Reinstall gas tube roll pin
Inspect all parts to ensure proper installation.
Install upper onto lower.
Install handguards.
Install sighting device (iron sight, scope, ect).
zero rifle at the range.
pat yourself on the back for a job well done .

The rifle will be just as accurate afterwards as it was before, unless you installed a free float while you had it apart, then it could easily be more accurate than before.