All about M4 Feed Ramps
What are they? Why are they there? Do I need them?
Many people often ask about M4 feed ramps, if they need them, why they are on some rifles and not on others and if they are compatible with certain rifles, barrels or ammo. To understand these questions, it is important to know a little history of their development.
History of M4 Feed Ramps
M4 feed ramps were created during the development of the xM4 carbine for the original Army requirement of a carbine rifle that used the then current M16A2 upper receiver, 14.5” Barrel and had featured a collapsible sliding stock. An important thing to note is that the M4 was originally developed as a rear echelon carbine for troops in need of more firepower than a sidearm and to utilize the M855 cartridge (62gr ball w/ steel penetrator in copper jacket). In doing this the xM4 carbine was intended to maintain certain requirements for interchangeability with the M16A2, this was later changed as the M4 was used in more front-line activities.
When Colt modified the xM4 to have a 14.5” barrel with a shorter gas system the carbine maintained a much higher operating pressure which in turn resulted in higher velocities in the gas driven systems (The bolt assembly slammed back and forth faster). With a higher velocity bolt carrier, the carbine was able to achieve cyclic rates as high as 1000 rounds per min. In comparison, the M249 SAW is limited to 725 rounds per min. The higher bolt velocities caused many technical issues that had to be solved.
Failure to Feed Problems in the xM4
There were two major factors helping to cause failure to feed problems in the xM4, the M855 round, and the higher bolt velocities. The M855 round is more pointed and less rounded on the front end. This makes the round more sensitive to feeding issues simply because the bullet is not as round in the front and is less forgiving to feeding issues.
Part two of the ammunition feed problems with the xM4 is the higher bolt velocity. When the bolt is moving at higher velocities it is more sensitive to magazines that are slower to feed. While none of these issues individually cause consistent reliability problems, stacked together they caused numerous issues with feeding ammunition.
What are the M4 Feed Ramps?
The M4 feed ramp is a modification to the two feed ramps that guide the nose of the bullet and casing into the breach of the chamber. The standard AR rifle type feed ramp is a 45 degree cut into the bottom of the breach of the barrel. The m4 type feed ramp is a shallower 52 degrees that extend down into the bottom of the upper receiver. Due to the shallower angle of the cut, it was necessary to extend the cut in the breech down to the bottom of the upper receiver. This is where things start to get a little complicated.
M4 Feed Ramp Compatibility
The M4 feed ramps require that the upper receiver is machined with m4 feed ramps, you can run into compatibility problems between upper receivers and barrels. Simply put, the best thing to do is install a barrel made with m4 feed ramps into an upper receiver that has been manufactured with m4 feed ramps (no milled feed ramps).
There have been cases of individuals installing m4 feed ramp barrels in upper receivers made for rifle barrels, but that is not a recommend due to the fact that this creates a sharper edge in front of the feed ramp. It is also important to note that an upper receiver machined with m4 feed ramps will not work with a standard rifle barrel. This creates a lip that can easily catch and deform the tip of the bullet, preventing feeding and damaging the bullet.
It is recommended to use a receiver and barrel combination that includes the extended m4 feed ramps in a rifle that is using a carbine length gas system or when using ammunition with a longer or more pointed bullet. Although, there are no clear drawbacks to using m4 feed ramps on rifle length gas systems. In fact, the military uses them on the Mk12 rifle due to the longer profile of the sierra match king bullet it fires.