Build Recipe 7 - Sig Spear LT 9" 300 Blackout

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  • Quiet (Subsonic)
  • Short (under 16" total barrel length with a suppressor installed) 
  • Light weight
  • Capable (consistently hit standard torso steel targets at 300 yards - a magnifier helps)
Build Recipe 7 - Sig Sauer 9" 300 Blackout MCX Spear LT Build Recipe 7 - Sig Sauer 9" 300 Blackout MCX Spear LT


Can you imagine a time that you might want to (or have to) shoot while not wearing hearing protection, without damaging your hearing or disorienting yourself? 

300 Blackout was created as an alternative to 9mm subsonic sub guns like the MP5, but with a subsonic rifle round with higher stopping power than 9mm. 

More stopping power than subsonic 9mm, but still quiet through a suppressor like subsonic 9mm. 


Most people that buy or build a 300 blackout gun do it with plans to shoot subsonic rounds through a suppressor for maximum sound reduction. 

The key to making any bullet quiet is to slow it's speed down slower than the speed of sound which is about 1125 feet per second. Once a bullet moves fast enough to exceed the sound barrier, it creates a tiny sonic boom that sounds like a loud crack. This is similar to the boom you hear when a fighter jet breaks the sound barrier during an air show except much smaller. 

There are two ways to slow the bullet down. The first way is through the internal design of the round itself, and the second way is to shorten the barrel. Typically the shorter the barrel gets, the slower velocity the bullet travels.

Build Recipe 7 - Sig Sauer 9" 300 Blackout MCX Spear LT


Short barrels are a convenient consequence of tuning the speed of a subsonic 300 blackout system for maximum sound suppression. This means you can end up with a 5" to 10" barrel, leaving plenty of room for a suppressor without exceeding the typical 16" overall barrel length that most are used to. 

Adding a suppressor to a barrel that is already 16" long can result in a pretty long gun that can become more challenging to maneuver. So the short barrel length that happens to be ideal for subsonic 300 blackout happens to allow for just the right amount of space to add a suppressor without making the overall length unwieldy. 

Sig Sauer took size reduction to the next level with the Spear LT by using their short stroke gas piston system, which eliminates the need for a buffer tube. This means you can use a folding stock which nearly cuts the overall length of the gun in half. This feature alone could be the deciding factor for whether this rifle will work for the intended use case if space constraints are a concern. 


We added an EOTECH EXPS3 red dot and Surefire M340C Scout Light to this build for maximum visibility. The EOTECH red dot is robust and durable. It allows for shooting with both eyes open, and is compatible with night vision. The Surefire Mini Scout Light is a compact lightweight flashlight that puts out a ton of light for increasing visibility in low light situations. 

Suppressor Compatibility

The Sig MCX Spear LT comes from Sig with their flash hider suppressor adapter. It is threaded to accept sig's line of suppressors. If you prefer to use a different brand of suppressor, the Sig muzzle device can be removed to expose 5/8x24 threads for other suppressors or adapters.

Adjustable Gas Block

Thankfully, Sig Sauer included an adjustable gas block in this model. This means that you can easily turn the gas down without any tools when shooting subsonic ammo through a suppressor. This reduces the amount of gas you get in the face when shooting suppressed, and it increases the guns ability to function reliably. 

NFA Tax Stamp Considerations

The Sig Sauer MCX Spear LT is sold as a pistol without a stock. It is NOT an NFA item and it does NOT require a tax stamp. It can be taken home in pistol form by qualified buyers the same day it is purchased. It can be shipped to your local FFL and be picked up there without a tax stamp.

Build Recipe 7 - Sig Sauer 9" 300 Blackout MCX Spear LT

Once you own the pistol, if you decide you want to add a stock (turning it into a short barrel rifle like the one in this recipe) you will first need to fill out the appropriate paperwork on Silencer Shop's website to request a "Form 1 Tax Stamp" from the ATF. It only takes a few minutes to fill out online, and you can submit your fingerprints digitally at one of the many Silencer Shop kiosks in gun stores around the country (we have one in our store in Sanford Florida). Once the tax stamp comes back from the ATF (usually around 90 days) you can purchase a stock and add it to the pistol legally, converting it from a Pistol classification into a Short Barrel Rifle (SBR) classification. 

This process is easier than you may think, and we can help you through it step by step.