Its a supressor with external muzzle brake custom made for me by Robbie Tiffen of GunWorks in NZ: http://gunworks.co.nz
He's a great guy and prepared to listen to new ideas.
Without the brake, in cases of this large capacity there is a lot of rearward movement, making it hard to get back on target to watch the bullet strike. Like a rocket if you like, all that gas still has to come out that same front hole! The brake minimalises this rearward movement, is a pussy to shoot and is quiet, about like a 223 which ain't bad for a 180gn 7mm Berger doing just under 3400fps I reckon!
Yea, acheiving accuracy with a gun as light as the Fatso can be challenging. Everything must be true and straight and then the shooter's got to be up to it too! With out the muzzle brake the Fatso was fairly violent off the bench, particularly when shooting the 180gn Bergers, but braked its no problem at all, just loud! It averages about .4" at 100 yards for 3 shot groups and 1/2 moa at 600 and 1000 yards with the 162gn A-Max load we settled on for the longrange work. This is off the fixed Harris bipod and hand under the butt rest we use in the field. But boy is it great to carry round our steep country! Its about to get even lighter now the load development work is completed as we're gonna replace the Nightforce with a lighter Leupold hunting scope. Ideally the new VX7 4.5-18x56L if they're available in time, but otherwise either Mark4 4.5-14 or 6.5-20 M1's. Then it'll weigh 7lbs all up in ready to go hunting mode, that'll be a joy to carry!
I'll write more on it soon.