On standard upper receivers, the interface with the barrel extension with what is known as a slip fit; that is to say that the barrel will freely slip in and out of the seat bore in the upper receiver without any mechanical, thermal, or chemical aid. Once the barrel nut is tightened down, the seat bore will constrict around the barrel extension, but this can be inconsistent and leave sundry slop and gaps that can lead to inconsistent performance. Historically, builders have used adhesives like sleeve retainer and gasket maker or aluminum shims to fill those gaps. Instead of relying on an adhesive application that can be just as inconsistent (and a mess to clean up when it comes time to rebarrel) or shims that can deform, our M4P upper receivers are machined with an undersized barrel seat. The only change to the assembly process this requires is a bit of heat to expand the opening so that the barrel extension can slip (or be tapped) into the receiver. Once the receiver cools and the opening contracts, the two parts are sealed together as tightly as they possibly can be before any constriction even comes from the torque applied to the barrel nut. While this may show a very marginal improvement in accuracy or group size, the critical improvement comes to the consistency of group placement.

Most companies churn out AR15 uppers by the thousands. Once they leave the doors of the OEM and enter the market, they are typically indistinguishable from one another. While this may not be critical for your average home build, the considerations for manufacturers are (or should be) very different. Lack of distinguished serialization can cause issues with tracking maintenance cycles and round counts, issuing warranty claims, and authenticating the assembly. While it tends not to be a concern for our customers, this ambiguity can also make secondhand sales dubious at best. Is the URG factory original? Has it been altered? Who made those alterations? In most cases, you have no way of answering these questions with any validity.

Whether it comes to you as a stripped receiver, a complete upper receiver group, or a full rifle build, our upper receivers are marked with our CAGE code to identify their origin as well as a unique serial number that can be audited with us to recover the original configuration that it shipped with and any alterations that have been made by us over the course of its life. While this makes warranty and repairs easier to track on our side, it also assists the end user in tracking round counts, maintenance cycles, and other performance metrics as needed amongst a larger inventory. Car manufacturers have labeled engine blocks for decades, so it seemed like an obvious step to take to ensure quality on a piece of potentially life-saving equipment.

Our receivers are hardcoat anodized per MIL-A-8625 Type III by a meticulous processor that has formulated custom dyes and processes to ensure the most consistent coloration and sheen between parts that is possible, even across both 7075-T6 and 6061-T6 alloys. What this means for you is that all of our parts will have a nearly identical flat black or brown finish that won't glare in the daylight and can easily accept any rattle can paint job you can dream of without the paint slipping off like it can on shiny, slick alternatives done by inferior processors.