Here is another fun debate... Rather than take sides, I will try to do the Fox News "you decide" sort of thing. And now, for your viewing pleasure, the M4 Feed Ramp.
So what are they? Well, all ARs have feedramps cut in the barrel extension... the bottom two lug cuts in the barrel extension are widened and ramped to guide the round into the chamber during loading.
So what is different about the M4 ramps? Well, on a regular AR type platform, the ramps are cut into the barrel extension only -- the M4 style ramps are cut so that the cut "breaks through" the barrel extension and cuts slightly into the upper receiver. To be honest, an M4 feedramp is really a two piece arrangement... part of it is in the barrel extension and part is in the upper receiver.
Here is a great picture that shows the difference, shamelessly stolen from bigbore (thanks Steve).
As you can see, the feed ramps on the regular rifle barrel are cut just to the edge of the barrel extension... but do not extend beyond that edge; while the M4 ramps continue slightly "outside" of the barrel extension.
As stated, the rest of a properly cut M4 feedramp is in the upper receiver... while the cut in the receiver is small, it matches up with the cut in the barrel extension and makes for a smooth, continuous ramp.
So why are they there? Ughhhh... here are both sides, the extremes at that:
1. "M4 feedramps serve no purpose at all, they are nothing more than an attempt by Colt to get a patent continuation based on redesign."
2. "M4 feedramps are the greatest thing since canned beer... in order to have a reliable rifle, you must have them. It is a miracle that the platform has functioned this long without them."
Here are some of the facts that lie between those two arguments. With the proliferation of the the carbine gas system, there did come some unique problems -- it is believed that with the M4, there are times when a cartridge being loaded is more "tip down" than normal, and that the bullet tip could strike the flat surface of the upper receiver, just below the standard rifle feedramp.
So, the ramps were extended down slightly, in order to catch these.
There is a suggestion that the use of heavier bullets also exacerbates any potential problem... this idea seems to have some weight (get it) -- and of course the use of soft or open tip ammunition can help make things snag.
So... Do I need them? Being married has taught me that there are varying degrees of need. The honest answer would be that unless you have a select fire carbine... no, you can get by fine without them. Truth is, everyone that really "needs" them has been given a rifle that has them... I certainly would not run out and replace an upper, just because it does not have M4 feedramps. Note: With the increased use of larger diameter cased cartridges (6.8 SPC, etc), the M4 feed ramps are proving to be of value when using these cartridges... the magazine lips can not move, so with these larger diameter cartridges, the tip of the bullet sits lower in relation to the centerline of the bore (half of the diameter increase to be exact) -- this puts the cartridge in a position where feeding is improved by the extended feed ramp.
Okay, but do I want them? Well, sure, why not. As they are becoming more and more the standard and they certainly do not do any harm -- why not look for them on your next purchase. Again, I wouldn't consider them a deal breaker and would not pass up a good deal on an upper only because it did not have them. Our military is doing just fine with a whole lot of rifles that do not have them...
And all of this is in a perfect world. Feed problems can be caused by several things... the fact that magazine quality is all over the map is most often the first suspect and there is more than one lower receiver in the wild that has an out of spec mag well.
My gun has an M4 barrel, but it looks like someone cut the ramps in the upper receiver with a dremel tool! That is most likely exactly what happened. There are a lot of uppers out there that do not have the cuts... hand cutting the upper receiver with a dremel is a quick way to make the conversion.
Granted, it may not look so great, but odds are (if they are actually doing anything) they will work fine. The most obvious fault with this method is that the cuts are not refinished and you see bare aluminum -- while there may be some surface hardness that is not there... you will wear out a lot of other things on the gun before you have to worry about any problems with "wearing" on the hand cut ramps.
If you are the kind of person that lays awake at night and obsesses over the dire thought that your feedramps were not refinished... well, hit one of the dealers on the site up for a new upper so you can relax.
Can I run a M4 barrel in a regular upper receiver? Sure, there seems to be no issues with this... or you could just dremel in the receiver cuts.
Can I use a regular barrel in an upper receiver with M4 cuts? I don't recommend it... there is a chance that the round can jam on the little "lip" that is caused by the barrel extension extending into the cut; however, there are guns out there that have this setup and work fine -- this tends to go towards the thought that the ramps really do very little? Again, it is not a setup that I would feel all warm and fuzzy about. Here is why...