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The SIG Sauer P320 is one of the most versatile pistol platforms on the market. As the U.S. military’s adopted sidearm (designated M17 for full-size and M18 for compact size), the P320 has proven itself in duty, competition, and everyday carry. Its modular design lets shooters swap grip modules around a single serialized fire control unit (FCU). However, that modularity also means magazine selection requires a little more attention than a fixed-frame pistol.
At The Mag Shack, you can find a full selection of SIG P320 mags in a wide range of frame sizes, calibers, and capacities.
Choosing the right P320 magazine starts with the grip module. Each frame size has its own standard mag length, which affects how the magazine fits the firearm.
These frames accept the 17-round 9mm magazine as standard. The X-Five Legion and X-Five Max are built on the full-size grip module, so any full-size P320 magazine works. The X-Five typically ships with 17-round mags but also accepts 21-round and larger extended magazines.
The P320 Carry frame features the same grip length as the Full-Size, paired with a slightly shorter slide. It uses the same 17-round magazine. The military M18 designation is simply a P320 Carry in Coyote Tan with specific features, and it utilizes a standard P320 17-round 9mm mag.
The Compact frame is shorter in the grip and designed around a 15-round magazine for a flush fit. It also accepts 17-round and 21-round magazines, which will extend below the grip. Many Compact owners run 21-round extended magazines for range and duty, while opting for 15-round flush-fit mags for carry.
The smallest P320 frame uses a 12-round 9mm magazine. Larger P320 magazines (15-, 17-, and 21-round) will physically fit and function in a Subcompact frame, but they will protrude below the grip by increasing amounts.The universal rule: Longer magazines typically work in shorter frames (with protrusion). Shorter magazines typically do not work in longer frames. This gives P320 owners flexibility to carry a Compact with a flush-fit 15-round mag and keep 21-round mags for the range.
Capacity matters in how the P320 feels and works for you. Every magazine size brings something different to the table, so picking the right one affects both your handling and how comfortable the pistol is.
You can also add extra capacity with the Sig P320 mag extension options. Brands like Tyrant CNC, Springer Precision, and Armory Craft offer aftermarket basepads that add 2-5 rounds to factory SIG magazines. It’s a common move in competition, where both capacity limits and quick reloads matter.
Each caliber in the P320 platform uses its own magazine design, and understanding these differences prevents fitment issues.
The Sig P320 9mm configuration dominates the market. It supports the widest range of capacities, from compact 12-round magazines to extended options and drum magazines.
.357 SIG and .40 S&W share the same magazine bodies. These calibers use the same external dimensions, which allows overlap in magazine construction. Full-size magazines typically hold around 18 rounds, while compact versions hold slightly fewer.
The Sig P320 45 ACP magazine is different in both width and construction. The larger cartridge requires a wider magazine body, which makes it incompatible with 9mm, .357 SIG, and .40 S&W frames. Capacity is lower, with 9-round compact and 10-round full-size options.
The Sig P320 10mm magazine, used in the XTEN platform, is also unique. The Sig Xten magazine is built specifically for the 10mm configuration and does not interchange with standard P320 magazines. These magazines are designed to handle higher pressures and typically hold up to 15 rounds.
If you’re looking into M17 and M18 magazines, the names can get confusing. M17 and M18 are just terms for certain P320 models, not different types of guns.
The standard M17 and M18 magazines are the full-size 9mm magazines for the P320. Both pistols use the same Sig Sauer P320 mags as any other full-size or Carry frame. Usually, that’s a 17-round magazine in the box.
As for the compact frame, it normally comes with a 15-round magazine. But you can use 17-round or even 21-round versions in it, and they’ll fit just fine.
There’s no special magazine that’s only for the M17 or the M18. Any Sig P320 magazine that matches the right caliber and grip size will work.
The Mag Shack stocks a wide selection of Sig P320 mags, covering full-size, compact, and competition setups. Each option is selected with attention to durability and consistent feeding performance.
Yes, within the same caliber. Longer P320 magazines always fit in shorter frames, with the magazine extending below the grip. A 21-round extended magazine works in a Full-Size, Carry, Compact, and Subcompact P320 in 9mm. The key constraint is caliber, as 9mm, 10mm, .357 SIG/.40 S&W and .45 ACP each use different magazine bodies.
The M17 uses a standard 17-round full-size 9mm P320 magazine.
The M18 uses a standard 17-round full-size 9mm P320 magazine
In 9mm, SIG P320 magazines are available in 12-round (Subcompact), 15-round (Compact), 17-round (Full-Size/Carry/M17/M18/X-Five), and 21-round and 30-round (factory extended) capacities. The Sig P320 higher-capacity options come from aftermarket manufacturers. In .45 ACP, capacities are 10 rounds (full-size) and 9 rounds (compact). In .40 S&W/.357 SIG, capacities are 18-round (full-size) and 13-round (compact). In 10mm, you’ll find 15-round magazines. All calibers offer a compliant 10-round magazine as well.
Yes. A P320 21-round mag will function in a compact frame but will extend below the grip.
No. The Sig P320 .45 ACP magazine is wider and only works with .45 ACP frames.
The factory 21 and 30-round mags are popular and usually run reliably. Some aftermarket extended mags can hold more, but it’s a good idea to try them out before depending on them.
Yes. The SIG P250 and P320 share the same magazine design within each caliber. A 9mm P320 magazine will fit and function in a 9mm P250, and vice versa.
The SIG XTEN is a 10mm Auto pistol built on a P320-derived platform with a larger frame and slide. It uses its own dedicated 15-round magazine that is NOT compatible with standard P320 magazines in 9mm, .357 SIG, .40 S&W, or .45 ACP. If you own an XTEN, purchase only magazines specifically marked for the XTEN.
