I am running the large 5 port on a .338 LM build. The brake is incredibly efficient. Muzzle blast is non existent to the shooter but anyone on either side of you will get hammered. They are big and impressive. Extra machining will need to be done before installation as the exit port is left to be opened to the proper caliber you will be putting through it. I personally would purchase it again if I were to build another large caliber gun.
I have one on a 308 Win to make it shootable for my son. It is very effective, bringing recoil down to roughly a 243, or a little less. And Kirby is good to deal with. I would recommend the PK brake without hesitation.
We use these for our 375 Cheytac builds. VERY affective.
We shot that particular rifle recently with a Harris BRM-S and rear bag. Rifle barely moved at all. This is our friend Jim shooting it. He described the recoil as "very mild".
I'll second the comments about Kirby. E-mail him. He is fast on service and the product is terrific.