I'm looking to build a M4 forgery. I'm using a pre-ban colt lower as my starting platform. What is better 14.5" barrel or 16" barrel? Can anyone tell me what the M4 feed ramps are and how to identify them and which brand upper receiver should I use? Thanks for the help.

1st of all welcome to the board.
M4 feed ramps are fcut into the upper receiver, at the factory, or by a gunsmith, for those that have older or standard uppers.  Someone will post a pic. for you.  Also w/ those cuts/recesses are slightly larger locking recesses in the barrel extension so that the transition from upper to barrel is a smoother one for the cartridge as it is fed into the chamber.  The pics. are not going to do it justice, they'll demonstrate what the M4 feed ramps are, but not really communicate their importance.  I recomend getting an M4 upper.  I don't have any on any of my AR's but after having seen them up close in person, I wish I did, and was never really impressed when I saw them in photos.
Barrel length is going to be determined by the distance at which you xpct to engage targets.
If at shorter ranges a 14.5" barrel will suffice, as velocity tends to drop off at farther ranges from a 14.5" and w/ most commonly available ammunition tends to make the round less effective.  Not, ineffective, but less effective, according to reports of engagements at longer distances by the USM, in recent engagements.
The 16" barrel will extend that range by approx. 100 yds, roughly, that's off the top of my head w/o referring to any figures.  Sorry if I'm off.
And yet again, the USM hasa been experimenting w/ heavier bullets, 75 and 77grainin a 1/7" twist barrel to make up for some of that distance, but that's going to get expensive for you to shoot recreationally.
If the distance of your engagements against man type targets is not an isue, you may want to consider, looks, and finally the fact that w/o ATF permission, you'll need to "permanently" afix a phantom/vortex/other1.5" flash suppresor/recoil check of your choice. (PITA to service/replace/change) We tend to recomend the FS here , if possible, you've got a pre-ban, you should be good in that dept.
And finally if $$$ is not that much of a problem I would tend to keep the upper a Colt, only because that lower's a colt.  Now that's just me.  Many don't like Colt.  And I own Armalite, Bushmaster and Colt.  They are all great rifles, with their own idiosyncratic differences.  But I try to keep uppers w/ lowers as much as possible.  I'm not saying you need to, and if you got a great deal on a DPMS upper (they use M4 cuts, Bushy doesn't, that I know of, and I just got a factory fresh Bushy upper 2 days ago) fora good deal, you'd probably have no problems, but I'm a lil' anal in that dept. so I'd recomed holding out for a Colt.  Maybe even a complete colt M4 upper.

I wish you luck and enjoyment.  And check here often, many in the indusry could learn a thing or two from the "recreationists" here, I have.  SUL2 out.

Thanks for the info. What a mouthful. I have a baby due in two months so i'm under a budget per the boss. This will be my last toy for awhile.

I recommend you check out the M4 uppers from [url=www.cmmginc.com]CMMG[/url].  They have special DPMS uppers with chrome-lining, 1/7 twist, 4150 and NATO chambers. The have 14.5" and 16" and they are a lot cheaper than the Colts.  I have two and they are great.  I also have two Colt uppers and I like the DPMS ones better.

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