Using 223 upper receiver for 9mm

[andy t]

I am wondering if it is possible to use .223 upper receiver for a 9mm rifle (i.e. attach 9mm barrell, etc) instead of a 9mm upper receiver, since I have a Colt A1 upper that I would like to convert to a flat top.

[khc3]

Yes, it works just fine.

[LonghunterCO]

There are two difference between the .223 and the 9mm upper. The .223 has a hole in the front to allow the gas tube to enter the upper (not needed on a 9mm build), and there is also not bolt cam recess in the 9mm. So building a 9mm on a .223 upper is ok but would not work the other way around.

[CarlosDJackal]

There are two difference between the .223 and the 9mm upper. The .223 has a hole in the front to allow the gas tube to enter the upper (not needed on a 9mm build), and there is also not bolt cam recess in the 9mm. So building a 9mm on a .223 upper is ok but would not work the other way around.

Don't forget that the .223 upper has a much larger ejection port as well. My question is would the 9mm bolt work with the .223 Dust Cover?

[LonghunterCO]

Don't forget that the .223 upper has a much larger ejection port as well. My question is would the 9mm bolt work with the .223 Dust Cover?

The ejection port is on the upper is actually the same dimention the gas block that is setting in the back of the port makes is shorter. The Dust cover is only 1/4 shorter the the standard one. Replace the port door with the shorter 9mm and the gas deflector and its spring. Reuse the ejection port rod, spring, and "C" clip retainer from the .233. Some run the regular the ejection port cover on their 9mm builds also. Anyway the 9mm bolt will work with the standard .223 cover, the 9mm bolt has the notch in it to clear the ejection port cover when the cover is close on a closed bolt, and will open the port door upon cycling.