AR PLATFORM eJECTION PORT DUST COVER SIZING, FITMENT, AND DIMENSIONS

A detailed overview 

ar15 Dust cover details

When you are getting ready to upgrade, replace, or simply install your first ejection port dust cover, it is a good idea to take a moment and make sure you have a standard Mil-Spec size ejection port in order to ensure any cover you purchase will operate perfectly in your upper.  There are hundreds if not thousands of manufacturers of uppers and AR’s, and most do use the Mil-Spec standard, but even in that, there are tolerances that may be different at each factory that may inhibit your ability to just drop in a new part without having to fit the part (by fit we mean polish/sand/grind) to make it work.  If you are a DIY’er and good to take a file to your port then by all means go for it, but we recommend a certified gunsmith install any parts that may require fitment or have issues.  On top of the standard fitment issues, some of the large calibers such as 450 Bushmaster, 50 Beowulf, etc…. use enlarged ports that have been tuned for ejection of the larger brass cases.  Some manufacturers will only alter the part of the port that is not essential for the detent on the dust cover to catch in the groove on the upper, so standard covers will work.  However, some upper manufacturers of the larger calibers have their own proprietary dust covers that are larger.  Obviously the standard Mil-Spec dust cover will not fit in these applications.  The list constantly changes on these so we do not keep a list of what works and what doesn’t, but we have provided some detailed dimensions below to help you determine if your port is standard mill spec or other, and if the standard covers will work in your upper.

AR10 Dust Cover Details

Since there is a nice standard most manufacturers use when making AR15 parts, most people assume the AR10 is the same.  Unfortunately as many have found out the hard way, there is no set standard for the AR10 platform.  Many manufacturers do use what is considered the original standard referred to as DPMS Gen 1 Specifications, but not all.  Manufacturers regularly use proprietary parts and sizing and still call it an AR10.  Some notable exceptions are the Sig 716, and the DPMS Gen 2, both of these use proprietary sizing and dimensions not compatible with standard AR10 dust covers.  Fitment will not even help with these two, the Sig cover is larger by a significant amount, and the DPMS Gen 2 has the detent housing offset to fit the groove in the bolt that is not centered in the port.  Again, since the list is constantly changing, we find it easier to provide you the details of the standards (or as close as we can get to a standard for AR10) so you can do your own measuring and make sure parts will fit.  We have included detailed dimensions below so you can determine if the parts will work for you. 

Concluding Thoughts

In concluding, we do our best to make sure our follow Mil-Spec dimensions where available for the AR15 platform, and the most common DPMS Gen 1 spec for the AR10 platform.  If manufacturer tolerances are tighter than Mil-Spec, minor fitment may be required to use certain parts, but this is extremely rare (1 out of 500 parts may need fitment help) in which case we will work with you to make sure you end up with what you need, sometimes even going to the extreme and engraving your OEM parts that came with your upper/lower.  We stand by our products and will make sure you get what you ordered if we can at all possible.