APA Fat Tuna

Thread Diameter


  • Increase Accuracy 
  • Tune Factory Loads
  • Reduce Blast and Concussion 

Accuracy Tuner

The APA Fat Tuna is a product of a partnership between APA and Adaptive Tuning Systems (ATS). This tuning device attaches directly to the APA Fat Bastard. Simply replace your existing jam nut with the Tuna nut and you’re ready to start finding the node your ammo wants. Whether your barrel is speeding up post break-in or temperature swings have influenced your load at a match. Or maybe you really just want to shoot a specific factory ammo that your gun doesn’t love; the Fat Tuna is your guy.


Blast Deflector

The geometry used on the front end of the Fat Tuna is specifically designed to counteract the blast flow of the Fat Bastard port. The initial gas flow that has already generated the directional force to reduce recoil is redirected, creating a gaseous shield reducing sound waves and percussive energy. This dramatically reduces the concussion to the shooter without sacrificing recoil reduction. No more brake flinch.


How To:

Start by removing the jam nut on your APA Gen 3 Fat Bastard Muzzle Brake. Install the Tuna nut onto the brake just like the original nut. Now re-install the brake onto the barrel as before. Slide the Fat Tuna weight over the front end of the brake until the threads engage and thread the weight all the way down until it bottoms out. Tighten the set screw down (not much pressure is needed here). Begin testing by shooting a 3-5 shot group. You may see an immediate change in POI and accuracy. If the accuracy does not improve right away it's time to start adjusting the weight by rotating counterclockwise. We recommend two notches or 90 degrees of rev per group in the beginning stages of testing and smaller increments for fine-tuning. We also recommend not going past 1080 degrees or 3 full revolutions. If 3 full rotation is exceeded, the Fat Tuna system may require more frequent cleaning.


We recommend cleaning the Fat Tuna system every 200 rounds or between matches. The Fat Tuna only functions on APA muzzle brakes. The Fat Tuna is 1.7” long and 1.73” in diameter. Can accept barrel contours up to 1.25"

 The adjustable weight is 5.3oz and uses a 3/32 Allen key wrench.