Colt M4 Upper with FN 20 inch barrel...will it work?

AR/M4 AR Technical Discussion
1

Making an M16A4 clone rifle (semi of course). Through the equipment exchange on the other site, I managed to buy a new Colt M4 Upper with M4 feedramp cuts, and a new in the bag FN 20 inch M16A2 barrel. Is this is an ok combo regarding the feedramp cuts on the upper and non F marked FSB? I cannot seem to locate any guidance on the subject within the TO’s. For now, the rifle is only getting a detachable carry handle put on, nothing fancy.

Thanks

2

if the barrel doesn’t have M4 cuts, you’ll have to get them put in. a non-M4 cut barrel extension will overhang the M4 cuts on the receiver, and cause failures to feed.

3

You need to get an upper without M4 ramps or a barrel with them.

4

Thanks, I kind of thought the feedramps would be an issue.

5

And please don’t do something stupid like modify the factory FN barrel with extended feedramps. Find an upper without them.

6

I would never bubba up a nice FN barrel. I’ll just get a DPMS upper to install the barrel on.

7

Curious why you do not advocate this. If it has to do with the value of the barrel, I would intend to shoot it out so it would not be worth anything when I was done with it.

Gunsmiths for years cut and contoured (throated) the feed ramp in government model barrels, before the factories started doing it, and the same principal applies.

I am probably missing something, as I often do, so I would like to hear your reasoning.

8

No explanation as to why it is “stupid” to modify an FN barrel extension to M-4 configuration?

9

I have a feed ramp question too.

I have an Colt A1 slick side upper on the way and I need a bbl for it. I would like a 10.5" or 11.5" and every thing I have been able to locate has feed ramps. There is a sticky her on m4c I read about feed ramps and it said this combination would work.

The other option is adding feed ramps or locating a bbl w/o feed ramps. Any sources for a bbl?

Thanks Guys!

10

Having a barrel with feed ramps and an upper without is not a problem.

11

5 Pins

Thank you. Excellent examples.

12

Dremeling the barrel extension will go through the surface hardening (bad). Dremeling the receiver will go through the anodizing, which has a similar effect (bad).

This is an upper I had with dremeled feed ramps after about 7000 rounds.

13

how hard would it be ? just to change out the barrel extension ?

peabody

14

you would need the right tools, you’d also have to re-headspace the barrel, plug the existing gas port, and redrill a new one. if you plan on using the existing FSB, you’d have to redrill the barrel for the taper pins…

15

You can just buy a flat top upper without M4 cuts: http://www.bravocompanyusa.com/Upper-Receiver-Assembly-Flat-Top-p/flat%20top%20upper1.htm

Theres no good reason to modify the existing feed ramps on either the upper or barrel extension. Anodizing is actually what makes the AL hard on the surface. Cutting through that removes that protective hard surface layer. As seen above it can cause that area to get banged up with long term use because the metal has nothing protecting it.

Just buy the proper parts, and sell what you have.

16

Barrel extensions aren’t aluminum or anodized. (obviously since it is not aluminum)

Again, what is the thought behind not modifying the barrel extensions to M-4 feed ramps. If someone already owns an M-4 upper receiver why spend the money for an M-3 upper when he can probably modify the feed ramps himself.

I am just curious about the logic. You may get me to change my opinion.

17

Kasenit to case harden the surface, but I can see your point. Thanks

ETA: After thinking about it overnight I would not use Kasenit on the barrel extension because you have to heat it up and may effect the case hardening in the rest of the extension. That’s what I get for posting without thinking it through.

18

why ? would have to do anything to the gas system ?
if all your doing is swapping the barrel extension ?