work in an interesting way, especially a partition style muzzle brake
like the Painkiller. If you take a chambering such as my 338 Allen Xpress(338 Lapua Improved), you will see roughly the following performance with my PK muzzle brakes. This is using a powder charge of roughly 95 to 100 grains of powder(H-Retumbo) under a 250 to 300 gr bullet weight. This is also using a barrel length in the 26 to 28" length. This is somewhat important which I will explain more on later.
The first port runner, the one closest to the muzzle does about 60% of the work. By that I am saying that roughly 60% of the muzzle gas gets bled off through this first port. About 20% of the escaping muzzle gas is bled off through the second port and the third port removed less then 10%. Its a pretty good rule of thumb that each runner will remove roughly 1/3 the gas as the one before it for the first 4 ports, after a 4th port, you are only looking at less then 5% of the escaping muzzle gas that actually get to that distance away from the muzzle. Again, this is for a case with roughly a 100 gr powder charge.
So what does this mean, simply put, you will get no more effect at reducing felt recoil by adding more partitions unless the cartridge powder charge is increased dramatically OR barrel length is reduced significantly.
This is easy to see just by looking at a muzzle brake that has been used. The first runner will have a heavy build up of carbon fouling, the second partion will have a noticably lighter carbon build up. The third port will have VERY little carbon build up.
I have fitted a few of my large 5 port PK muzzle brakes on some conventional rifles and the last two paritions will often stay clean from any carbon build up with cartridges of 100 grains or less, especially if your using a modern stick powder which burn very clean. Some of the ultra slow ball powders will show more carbon build up but thats simply because they are Dirty burning.
Now, dramatically shorter barrels will change everything about what I have just said because of the dramatic increase in muzzle pressure because of the shorter barrels. For example, I have built several custom XP-100 bolt action handguns chambered in my 338 Allen Xpress. These are SERIOUS handguns producing 2700 fps class performance with the 300 gr SMK in 15-16" barrel lengths. With this class of chambering in these very short barrels, the large 5 port brakes can be used to full effect.
Why explain all this, simple, You could add another partition to the medium and small PK but you would not increase the effectiveness of the muzzle brake at reducing felt recoil but you would increase the manufacturing time (COST) with no improved performance level.......
Thats why I do not have a four port PK in the medium and small sizes, no advantage from what I have seen in my testing. When I was developing my PK muzzle brakes, I had several different prototypes including 4 and even 5 port small PKs, they showed no advantage at all over the 3 port PK and added another 3/4" to their length and also added 40% in manufacturing time over the 3 port PK design. If I am going to charge my customers more, it will be for a reason, so customers actually get a good return for their investment so the 3 port Medium and Small PK was the best of all prototypes tested.