Differences
Visually, the first thing you notice about the Aptus is the height of the billet upper receiver. It is taller than an AR receiver to make room for the recoil spring above the proprietary bolt carrier. The rear of the spring assembly is capped by a locking piece protruding into the vertical face of the lower receiver, creating a third lock between the upper and lower in addition to the two familiar pushpins.
The bolt itself is a standard AR piece while the M-Lok forend has a raised rail along its top length to provide a continuous plane down the top of the gun. This combination of a slightly bulkier forend and the folding/collapsible Magpul stock make the gun look closer to an ACR or SCAR than a traditional AR.
Cheek weld is something of a universal problem with collapsible buttstocks. The Magpul stock found on the Aptus, however, eliminates the problem with an adjustable check piece. The cheekpiece has only two possible settings, but adjusts quickly and easily without tools. The stock also extends and collapses by pinching two locking tabs on either side and has a QD sling mount reversible for use on either side. It locks up solidly when folded out into place and stayed this way throughout the test.