SIG's CAL-X kits are what make the MCX platform genuinely modular rather than modular on paper. In SIG's words, this kit "unlocks new possibilities for your SIG SAUER MCX-RATTLER LT chambered in 5.56 or 300 BLK by seamlessly converting it to the powerful 7.62x39 caliber." Everything you need for the caliber change is in the box: a 7.75" barrel and gas block assembly, a 7.62x39 bolt carrier assembly, and one 28-round magazine. SIG part number 8901525, UPC 798681687015.

One Host, Three Calibers

If you already own an MCX-Rattler LT in 5.56 NATO or 300 BLK, this kit adds 7.62x39 to the same lower and the same handguard. That means one registered host, one manual of arms, one set of controls — and a genuinely different ballistic option when you want it. The 7.62x39 delivers substantially more bullet weight and frontal area than 5.56 from an equally short barrel.

What's Included

• (1) 7.75" 7.62x39 barrel and gas block assembly — 26.4 oz
• (1) 7.62x39 bolt carrier assembly
• (1) 28-round magazine — 6.1 oz

Adjustable Gas

The kit retains an adjustable gas system, so the rifle can be tuned between suppressed and unsuppressed use rather than being over-gassed in one configuration and undergassed in the other — which matters a great deal on a 7.75" barrel.

SIG CAL-X Kit 8901525 Specifications

Frequently Asked Questions

Is this a complete upper receiver?

No. This is a barrel, bolt, and magazine kit. It does not include a handguard, upper receiver, or charging handle. SIG sells a complete MCX-Rattler LT 7.62x39 upper assembly as a separate item if that's what you need.

Will it fit an MCX Spear LT?

No. This kit is specified for the MCX-Rattler LT. SIG makes a separate CAL-X kit for the MCX Spear LT in 7.62x39 with a 16" barrel.

What do I need to install it?

Standard MCX barrel-change procedure — consult your MCX manual for the handguard and barrel retention hardware and torque specs.