I've am aware that there are differences in feed ramp cuts for the M4 rifles as opposed to a "full-size" AR15/M16.  I've seen pictures of the deeper M4 feed ramps cuts in both the barrel extension and in the upper receiver.

I am puzzled, however, as to why this is necessary.

Since (correct me if I'm wrong) the "geometry" of the relationship between the barrel, the bolt, and the magazine seem to me to be precisely the same in both an M4 and a full size rifle, why doesn't the same feed ramp design work in both?

Another question:

Are there any differences between a lower receiver for an M4 (outside of the obvious buttstock difference) and that intended for a full size rifle?  In other words, if one owned a single lower receiver, could that lower be used for a 20" HBAR upper, for instance, as well as for an M4 upper by merely swapping the uppers and the buttstock?

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Quoted: Since (correct me if I'm wrong) the "geometry" of the relationship between the barrel, the bolt, and the magazine seem to me to be precisely the same in both an M4 and a full size rifle, why doesn't the same feed ramp design work in both? Another question: Are there any differences between a lower receiver for an M4 (outside of the obvious buttstock difference) and that intended for a full size rifle?  In other words, if one owned a single lower receiver, could that lower be used for a 20" HBAR upper, for instance, as well as for an M4 upper by merely swapping the uppers and the buttstock?

1. the M4 is a later design if you will, the M4 cuts where kind of an upgrade to help with feeding with full Auto guns, from my understanding, the lower cut ramps, help direct the rounds into the chamber with less chance of miss feed or jamb in full auto mode

2. there should be no real diffrence with the lowers, and should match up to any upper that you want to use, with the exception of some colts that have larger take down pins, they require adaptors normally

I'm not sure but I think the carbines were having trouble with full-auto fire for some reason, might be because of the shorter gas system but someone who knows for sure will post.  I've not had any trouble with non-ramped semi M4geries.

The lower receivers are interchangablebetween carbines and rifles.

Colt developed the M4 ramps to aid in reliable feeding for use in the M4A1 (auto). A 5.56 lower is a lower & can be used for carbine types or for full size 5.56's. Barrel length has nothing to do with it as long as you are using ammo that can fit in the same mag demensions as the type was designed for. AR10's use different lowers.

The extended feedramps in carbines were necessary because of the shape of the M855 round compared to the M193 round.  The M855 is more sharply tapered than the M193, if you compare the two bullets in the casing, the M193 is slightly fatter and more rounded.  

While rifles seemed to be able to continue to feed M855 without problems, however, it was found that in the carbine, particularly on full auto, there were occaisional failures to feed, due to the different profile of the M855 round.  

Colt discovered that by lengthening the feedramps into the receiver, they were able to satisfactorily eliminate the tendency of M855 to FTF.  Note that Colt has used two different feedramp configurations, "carbine" feedramps in the 700 series, and the current "M4" feedramps.  

~Augee

ETA: Primary source is TBR II.

Is it friday already?