For this purpose it is modified with several holes drilled in the side, which are used to pin spacers in place to hold the smaller 9mm magazine in the standard 5.56mm magazine well.Dr.

The full fence lower (both A1 and A2) is also the receiver used on Colt's 9mm SMG. However, due to the difficulty in identifying them from a distance and the fact that different manufacturers use slightly different designs (such as strengthening around only the front push pin or omitting the selector markings on the right side of the weapon), these have been treated as one variant for the purpose of identification. The design of this lower was upgraded with the adoption of the M16A2 featuring strengthening around the front and rear receiver push pins and fire selector marking on the right side of the receiver. It can be identified by the raised fence around the magazine release button on the right side of the weapon which is designed to lessen the chances of accidentally releasing the magazine. The full fence lower is the standard receiver used on all AR-15s since the M16A1. The preferred term for identification, be it the model name or simply the model number, is highlighted in bold. Due to the large number of companies that make AR-15 pattern rifles that are nearly indistinguishable from the original variants, all rifles should be identified using Colt terminology in the absences of any information to the contrary, such as unique parts or visible receiver markings. This is not a complete list of all variants produced by Colt, and only contains those variants that have appeared in media. This page is intended as a guide to identify the various Colt manufactured M16 variants and derivatives.

1968 colt sp1 serial numbers