The answer is simple. US military choose the Sig Sauer P320 over the Glock 19X because the Glock did not offer a weapon that met the requirements. The U.S. military believed that the Sig had a “slight technical advantage” over the Glock; a table in the GAO report suggested it scored a “good” rating to Glock’s “acceptable.”
The Sig also won high points in both ergonomics and ballistic performance. Moreover, to Glock’s single-gun bid, Sig offered a two-gun proposal — the XM17 and compact XM18. The program requested a modular pistol. Glock did not supply one. Modular in this case means that a pistol can easily be re-configured from full size to a compact, to a subcompact, simply by swapping out a few parts.
Sig accomplished this in two ways. First, by some clever engineering. Second, by taking advantage of a weird, old-fashioned quirk in U.S. firearms law. The innovative engineering was in making the entire trigger pack removable. So if you have a full-sized gun and need a compact one for a mission, you can simply open the gun up, yank the entire trigger pack as one self-contained unit and drop it into a compact frame with a compact barrel and Voila!
You now have a compact gun. Sub-compact? No problem! Do the same thing by taking the trigger pack out of the compact and dropping it into the sub-compact.
The weird quirk in U.S. firearms law is that whatever part has the serial number on it is the gun. If a particular firearm model has the serial number stamped on the Outside frame, then the outside frame is the gun. Every other part of that weapon you can buy online and have shipped to your house, no questions asked, but if you buy the outside frame (even with no parts and it doesn’t work), you need to have a background check because that’s the “gun” in the eyes of the BATFE. It’s weird and maybe silly, I know.
Well, this works to Sig’s advantage because guess where they put the serial number? On the trigger pack. So in the government’s opinion, that trigger pack I.S. the frame and, therefore, the gun. And being able to put the trigger pack in different frames makes “the gun” modular.
Glock has their serial numbers on the frame, so the frame is “the gun.” Knowing they didn’t have a truly modular design, they made a hybrid out of their full size and compact models by chopping half an inch off the front of the full-sized Glock 17. So now it has the larger magazine of the model 17, but the shorter slide and barrel of the 19, and they called it the Glock 19X.
I feel like the Glock is a more proven and reliable weapon system. I also feel like the Sig system is inherently deficient in one significant way. Let’s say the Glock costs $500. You buy one of each, a full, compact, and subcompact, so you have spent $1500.
Let’s say the first Sig you buy costs $500 as well, but the drop-in frames for the compact and sub compact are only $300 because they don’t have the trigger packs. Think of them like expansion sets. Well, you have only spent $1100, so it seems like you have saved money.
But in my opinion, you haven’t. You spent $1100, and you only have ONE GUN. You only have one trigger pack. So no matter what frame you put it in, only one soldier can be equipped for your $1100. With the Glocks, sure, you spent $400 more, but for that extra cash, you get TWO ADDITIONAL guns. You can equip three soldiers with guns in the Glock system, not just one.
Regardless, that is why Glock lost the competition. The U.S. Army backed themselves into a corner by writing in the requirement that the gun be “modular,” and the Glock didn’t meet that technical specification.