FAQ

Ejection direction alone is rarely an issue or cause for concern unless it’s extremely abnormal (12:30-1:00 or 5:00-6:00) but it can be a diagnostic tool if you are having malfunctions or if the ejection direction changes suddenly when you have made no changes to the carbine. Important note: Do not begin any diagnostics or changing of parts until you have first give the entire upper group and bolt carrier group a deep clean and then test again.

Ejection direction is influenced by several factors:

Extractor
The AR15 uses a spring loaded plunger (extractor) which puts pressure on the case and after firing will flip the empty case out of the upper, and when unloading will flip a loaded cartridge out of the upper. The tension on this plunger can vary and is often neglected during cleaning which leads to the plunger gumming up or sometimes getting stuck. The extractor will also influence the ejection direction. Use of varying power extractor springs or Crane style O-rings can also have an effect, as can wear on the extractor.

Brass Deflector Forging
The upper receiver itself can also affect the direction since MIL-SPEC forgings do vary quite a lot. Those variations in forged surfaces can affect the shape of the deflector where the brass typically bounces (deflects) off of the upper. Much like subtle changes in a golf swing, minor variations of these surfaces can also show up as minor differences in deflection direction.

Ammunition Casing
The weight of cases can also have an effect since a thinner or lighter case head will change the center of gravity of the casing and cause it to deflect differently.

Carrier Velocity
The speed at which the carrier operates will also affect the direction of ejection by varying how the brass bounces off of the brass deflector.
The carrier velocity is a function of:

• Gas Port Size (and gas system)
• Ammo Used
• Buffer Weight
• Action Spring Strength
• Use of a Suppressor
• Added Weight of Metal Spacers on Folding Stocks

BCM uppers are setup to run mil-spec pressure ammunition at a full auto cyclic rate that meets MIL-SPEC requirements of (700-970 RPM) unsuppressed on a brand new carbine. If the new carbine is running less than 700 RPM it is under gassed. If it is running above 970 RPM it is over gassed. With an in-spec extractor and clean and free-moving ejectors, we typically see ejection patterns from about 2:30 to 4:30.

If ejection pattern does suddenly change, or changes after storage, then further investigation is often warranted. Always make sure the action spring is in good condition and in-spec per the M4/M16 technical manual. It’s often a good idea to have spare action springs handy as they can and do wear out. Next, check the bolt carrier assembly. Before continuing the diagnostic and changing the configuration of the weapon the user should do a very thorough and deep clean of the entire upper and re-assemble with proper lubricant applied generously (wet but not dripping) to all moving parts. The extractor should be removed from the bolt and the entire area cleaned and lubricated. The ejector should be given some attention with several drops of lube applied and the plunger cycled several times to ensure it moves freely. If the ejector is soft, crunchy, or drags there is further diagnosis necessary and possibly cleaning or replacing the ejector or ejector spring.

The gas port of a barrel does erode over time. When it does the cycle rate will increase the ejection pattern can change more toward 1:00-2:00. After approx. 2-3k rounds the gas port often grows enough due to erosion that it may be beneficial to run a slightly heavier buffer. Typically going up one step is all that’s required (from H to H2, H2 to H3, etc). This happens slowly though, and so if similar behavior is noted but comes on suddenly then it’s more likely an issue of excessive fouling in the upper, and the system should be deep cleaned and re-fired before changes are made to the configuration.

When the ammunition used is less than Mil-Spec pressures (much commercial ammo is), the carrier velocity will slow down and ejection pattern can again change moving toward 5:00. At that point the end user could use a higher pressured ammo or lighten the buffer weight by one step.

As the action spring wears over time, the carrier velocity will slow down and the ejection pattern can change moving toward 5:00. At that point it would be best to replace the action spring and/or lighten the buffer weight.

Adding a suppressor with Mil-Spec high velocity ammo will increase the carrier velocity quite a bit, and ejection pattern will move toward 2:00. Different types of suppressors will increase carrier velocity to varying degrees. But it’s almost always recommend to run a H2 or H3 when using a suppressor.

(As a note; when running subsonic ammo and some new suppressors designed to relieve pressure from the operating system, there can be a bit of experimenting with buffer weights and ammo choices to get the carbine to perform with maximum reliability)

Folding stock adaptors for AR15 carbines often use a metal spacer that reciprocates with the buffer. This system actually becomes a significantly heavier buffer weight and will slow the cycle rate down and move ejection toward 5:00. If you are seeing short stroking, it is recommended to lighten the buffer weight when using the folding stock.