Sig P320/M17 9mm 21-Round Mag: Up Your Ammo

The Sig P320 has become one of the most popular handguns on the market. And the US Military contract didn’t just benefit our military members, it benefited the civilian market as well. When Sig produced the Sig P320 they had to meet specific requirements the US Military was looking for. Designated as the M17 and M18 (the shorter version) the military wanted high-capacity mags to come with it. They met that demand by producing a 21-round mag for the M17. As with other guns, mags, and gun accessories, they are available to the civilian market.

Sig M17 Mags - Hold more ammo
The baseplate on the M17 mag is easy to remove.

When I first bought a Glock 17 back in the day, I was excited that it came with a 17-round mag. Over time, aftermarket mag extenders increased that amount, and eventually, they started producing higher-capacity mags. When you buy an M17, it comes with two of the 21-round mags and one 17-round mag. Because the M17 is a variant of the P320, the mags for the M17 are interchangeable. This means you can buy the M17 (21-round mags) to increase your ammo carry.

Simple and Reliable

One of the good things about buying a weapon the military is carrying is that it has already undergone numerous tests. While nothing is completely perfect, this saves time from looking up reliability issues. If it is something the US Army has already tested and purchased, there is a good chance it is reliable. Another thing I liked about the M17 mags is that they are easy to load. Some mags have heavy springs making the last few rounds difficult to get in. I had no such issue with the M17 mags.

Sig 21-Round Mag Assembly
The Sig 21-round mag is very simple and consists of only 5 parts. It can be disassembled with only a flat-tip screwdriver.

Simple is the keyword for the M17 mags. I don’t need any special tool to get the base plate off the mag. I use a small flat screwdriver and push slightly on the base plate takedown button. It is easy to remove, and the spring and follower come out without issue. The metal frame of the mag is slick and has a nice finish to it. At 3.6 oz (empty), it is lightweight but very durable. The base plate has a textured grip on it making it easier to pull from the mag holder when loading. In total, there are five pieces to the M17 mag. In the event the spring does need to be changed, it would take about 90 seconds to replace.

Are high-capacity mags better?

The most obvious answer to this question is the more ammo, the faster you can shoot. When you have your sights on a target and are ready to engage, more ammo allows the shooter to stay on target longer. Performing a mag change causes a momentary pause in shooting. In some cases, it will also cause a loss of target acquisition. When preparing for a hunt, self-defense, or just going out to the range for some fun, the more ammo carried the better. Most self-defense shootings only involve a few shots being fired. But the problem with relying on statistics with your life is that the bad guy doesn’t care about statistics. You never know what situation will arise and with it, the need for self-defense.

Get more ammo with the Sig 21-Round Mag
The 21-round mag is slightly longer than the 17-round mag but carries four additional rounds. The textured grip at the bottom allows for quick and secure mag changes as well.

The standard in high-capacity mags for handguns has been 17 for a while (for 9mm). With the M17 mag, four more rounds can be held in a slightly longer mag. Because they are made to be 21-round mags, the peepholes allow you to see the last round. With most aftermarket mag extensions, there is no way to see how many rounds are in the mag. The base plate makes a significant difference when drawing the mag from the holder during shooting drills as well.

Are longer mags harder to use in tactical situations?

The magazine does stick out of the bottom of the gun further than I am used to. But that doesn’t mean it is not functional. I like to carry the 17-round mag in my M17 because it sits flush with the gun, and then carry the 21-round mags as my two spares. Not only do they have more ammo in them, but they are easier to draw from the mag holders.

The agency I work for issues three 17-round mags along with a handgun. We qualify with 51 rounds, which has become a standard amount of ammo for most cops to have on them. With the Sig M17, that amount can be increased to 59 with the same number of mags. After running numerous tactical drills, I found the longer M17 mags easier to pull from the mag holders during mag changes from the standing to prone positions.

As I said earlier, the military has done extensive testing on the M17 and M18 pistols and mags. But I can say during my time training with the M17, I have never had any feeding issues with the mags. They are durable and robust. In the past, I have even dropped one of my mags in the creek while jumping over it. By the time I found it, mud had made its way everywhere. It only took a couple of minutes to take it apart, clean it out and snap it back together.

Sig M17 with 21 round mag
Sig’s 21-round mag (pictured above with the 9mm Sig M17 pistol), is a must-have if you own the P320, M17, M18, or P250 gun.


The Sig M17 extended mag is a great addition to your inventory and is a great way to carry extra ammo on your belt. It can reduce your mag changes on the range and the texturing on the bottom of the mag allow for an easy grip. The spring tension is at the perfect spot where the ammo is easy to load but still feeds perfectly through the gun. If anything, the longer mags were easier to use during mag changes.

Regardless of if you have the Sig M17, M18, P320, or the P250, the Sig 21 round mag will work for it. So don’t just increase the number of mags you carry, increase the amount of ammo in each mag.

Sheriff Jason Mosher is a law enforcement generalist instructor as well as a firearms and tactical weapons trainer. Jason graduated from the FBI-LEEDA (Law Enforcement Executive Development Association) and serves as a Sheriff for his day job. When he’s not working, he’s on the range, eating steak, or watching Yellowstone.

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