Colt Rifle Users Guide

Hammer & Trigger Pin Sizes

Colt is the only AR-15 rifle manufacturer to use both small and large diameter pins in their trigger groups. Most Colt AR-15 rifles manufactured before 1991 and after fall 2009 are probably small pins while rifles manufactured between 1991 and fall 2009 are probably large pins. Because Geissele Automatics offers several triggers in “large-pin” versions, the customer will need to measure the hammer and trigger pins in their Colt rifles to determine if they need a small or large pin trigger.

Note: If your rifle is not a Colt, it uses small pins exclusively. Complete removal of the hammer or trigger is not required to measure the pins. Each pin needs to be pressed out of the receiver far enough to measure the pin’s diameter. Small pins will nominally measure 0.154 in.  and large pins will nominally measure 0.169 in. as shown in the figure.

Small Pin Measurement

Large Pin Measurement

Sear Blocks

About 1989-1990, Colt began installing sear blocks in their lower receivers to prevent the later installation of auto-sear components. The photographs below show a standard receiver without a sear block (Fig. 1), a receiver with a pinned-in sear block (Fig. 2), and receiver with an integral sear block (Fig. 3).

Figure 1

Figure 2

Figure 3

Sear Block Removal

The following table shows which Geissele triggers will fit into Colt receivers and which receivers require the sear block to be removed. All Geissele small-pin triggers will fit into small-pin Colt receivers without a sear lock (Fig. 1) or with an integral sear block (Fig. 2) and no modifications to the receiver are required. All Geissele small-pin triggers will fit into small-pin Colt receivers with a pinned-in sear block if the sear block is removed. Very few small-pin receivers with pinned-in sear blocks were made). The large-pin versions of the Geissele SSA, SSA-E and S3G will fit into all large-pin Colt receivers even if it has the pinned-in sear block. No modifications to the receiver are required.

The large-pin version of the Geissele Hi-Speed will fit into all large-pin Colt receivers without a sear block or an integral sear block and no modifications to the receiver are required. The large-pin version of the Geissele Hi-Speed will fit into all large-pin receivers with a pinned-in sear block only if the sear block is removed.