The SIG 320 was adopted by the US Army as its new M17 full-size service pistol, but that was only have of the Modular Handgun System. The other half was the M18, a compact version of the same pistol, with a 3.9" barrel (compared to the 4.7" barrel of the M17). The Army did also adopt the M18 for MP and CID personnel, but the other service branches chose to go with the M18 as their standard sidearms. Notably, this includes the Marines, who will be using the M18 to replace the M9, M9A1, M007 (Glock 19M) and M45A1 (1911).

Being a highly modular design, the M18 uses the same basic fire control module as the M17, as well as the same magazines. The slide and grip module are shorter to match the shorter barrel, and multiple different sizes of grip module are available to reasonably fit anyone between the 5th and 95th percentile of hand size.

While the initial batch of M17s were made with tan controls and subsequently returned and sold on the civilian market, no M18s appear to have been returned from service. However, SIG continues to run the military M18 production line making "contract overrun" pistols exactly to military specification, and that is what we are looking at in today's video. These have TC serial number prefixes, along with other military features:

Single-screw optics plates
Braided recoil springs
Tool-based takedown
Military slide markings