The M4 feed ramps that extend down in to the aluminum is how we (USMC M4 Program) got through the extreme cold weather chamber testing in the mid 1980's. Once the feed ramps were extended in that manner, the Carbine met the reliability spec for cold wx.
Unfortunately, (logistically) this leads to the Upper Receiver of the Carbine not being interchangeable with that of the rifle (Std 20-inch M16A2 barrel). At that time, the Army (Rock Island Arsenal M16 Guys) did not want the ramps added to the rifle's tech data package for obvious non-interoperability issues.
So in worst case, one puts an A2 barrel extension into an M4 ramped Upper Receiver only to find that the barrel extension's ramp "overhangs" the upper receiver's.
On one point I agree that most probably don't need it, however, we found out later it actually speeded up the cyclic rate of the Carbine. This told us that the original feed ramp was "slowing down" the gun, and this would be true of the rifle as well then. And if this is true, then every round fed out the magazine and up the ramp is probablty being bumbed pretty hard on its way into the chamber.
So for shooting a variety of bullet types, and to lessen the possibility of "bullet set-back" during the ramp feeding moment, I go for ramping, especially if my gun shows signs that rounds are hitting the flat face of the upper receiver directly below the feed ramps formed in the barrel extension.
For example, I have been testing some Federal Tactical Bonded 62 grain LE ammo. These bullets look moly coated, and this kind of hides that fact that its lead tipped. Feeding these up a non-ramped rifle, and then extracting the round before firing, showed that the lead tips were being deformed by striking the feed ramp. This affected grouping slightly at 100 yards, as dropped-in single loaded rounds had smaller and more uniform groups.
Firing the same ammo through a normal mil spec ramped M4 showed less bullet deformation.
Another hint, The way I check ammo companys that submit new ammo for acceptability in our SR-25 sniper rifle system is to number and measure for overall length (OAL) of a sample of say 20 rounds. Then load them in the magazine "by the numbers". Immediately after each round feeds, I unload it and re-measure OAL. If in your gun, these rounds all measure less OAL than before feeding and chambering, than they are being "set-back" when they impact the feed ramp.
In the SR-25/Mk11's, my baseline ammo for these tests is M118LR (Government Issue). It never set's-back. Its has the right combination of case neck tension and a waterproof sealant.
All that being said, my best hint is "if it ain't broke, don't fix it". Or as the government so often does, "fix it until it is broke".
ColdBlue sends