My build process started with essentially testing the idea. First I built a platform that fired 14 dumbfire missiles every second and a half straight forward. Then I tested several PD builds of my own. While I cannot give any precise figures, after extensive viewing of several builds, I found them all to have a rather consistent 'low' hit probability, given such a plethora of targets.
That done, I built a central barrel that contained all the necessary components for a PD turret barrel, 'plus' four docking rails. I wanted to see if docking extra barrels to standard docking rails instead of 'rail turret axis' would prevent them from trying to rotate, thus preventing collision checks. It does indeed prevent collision checks. It also prevents them from firing. D'oh. Back to rail turret axis'.
I built a very standard PD system into a single block line, seven blocks long.
I attached four of those barrels to the central barrel, having replaced the rail dockers with rail turret axis.
I built a very basic base, after all, this was just for testing the idea:
Then I put them together attached to a firing platform so it would rotate, and tested them. Sure enough, all barrels opened fire. Moreover more missiles were being shot down. It was unambiguous. I can't say it was five times the average, but it was certainly pronounced enough to be noticeable.
Having proven the principle to myself that it could work and did seem to be more effective, I decided to build a proper PD turret with the idea. I calculated that I could get nine firing guns being fired by nine bobby AIs inside a seven block radius turret. So I configured a central core barrel to house eight other slave barrels.
I wrapped the barrels in blocks to make sure they would be constrained the way they would be in a finished turret and did some fire testing. Note too how I added a beam to the central core cannon solely to function as a pointing device, so I could more easily see when it was firing and where it was pointing. Plus I just prefer the look of beams.
Having made sure it was working, I finished the turret build.
Sadly I found that if I put armor in front of the barrels, they would decline to fire. This was the biggest disappointment, as the turret ball itself has only 2000 shield points. Armor would have been nice.
Addendum: I have successfully put armor in front of the barrels. My mistake was in making the armor a part of the turret skin, instead of the barrels themselves. Now each slave barrel has it's own single solitary piece of advanced armor. The weapons fire just fine through their 'own' armor, but not armor of some other entity.
It was very clear that the barrels were still trying to move despite being completely encased. If left to fire for several minutes, a few might even stop firing until reset with ship->entity structure->rail system collective->reset all turrets, whereupon the stuck barrels would resume fire. Despite this, there was no weird behavior from the turret.
The superiority of this design over a single turret was unambiguous. Here is clear proof of multiple missiles being engaged simultaneously. (You can also see the maximum extent of the barrel misalignment that would result from prolonged fire.)
All this said, the block total for this point defense system is 473 blocks, and it has a footprint on a ship's deck of nine blocks, assuming one recesses the base into the hull. That is 53 blocks per firing gun. My own 'Mini PD' gun by contrast uses only 37 blocks and sits on deck in a space only 3x3. (Admittedly the bigger turret has some shielding, but is that paltry 2000 points really going to give it the same or better survivability as multiple smaller turrets? {The base has 3000 shield points the way I built it.})
Certainly the larger turret is somewhat more survivable, but it is also a larger target and a single target. What would be more effective, a larger, multi barrel turret, or a greater number of tiny PD systems? I am not certain.
The only issue is they like to stop shooting for whatever reason and docking to a normal rail causes them to turn into ship settings for some annoying reason. >_>