HP uses The same magazines as regular Vulcan 3.
AAO tuning kit gets You about 11,9mm to 12mm length. I believe there isn't much to be gained having the front of magazine machined from the inside but I'm tempted.
If You end up using AAO kit there are a few things to watch for with the magazine covers: Do not just throw them in and start tightening them up. I did.
They are not too good of a fit and so I messed the threads up really bad. You should patiently size them up and only when they drop in nicely after some very likely filing, you should gently take on the bolts. They probably won't fit well either. I had to use a Dremel tool to have them sunk in far enough not to stop magazine fitting in to the gun. The final step to have them working was to start wondering why the cocking lever was very reluctant to move forward and it did so with metallic screech. Sure enough I had a slug going in because You cannot have the slug probe going through the magazine when the magazine is empty and rightly so. Well I had to use some force to load the slug because dismantling the rifle would have been the only other way to make sense of it all and get rid of the slug jammed half way through. I found out that the cutout on the magazine cover was way too low, and the probe was more than touching it. Maybe it was made for .22? Dremel tool once again to the rescue. Now all is well and I look at this the way that I bought innovation from AAO, not so much a ready product. I have 0,5mm stainless steel sheet to make my own magazine covers in the future.
But yeah, I wish to point out once more that the magazine covers were a bit oversize and hence did not fit. The small bolts are 2mm or 2,5mm and unnecessarily short to my taste. The threads won't take much abuse so after messing them up I used thread locker and only tightened them down with the appropriate sized Torx bit (with no handle) and firmly pressing the covers with my fingers, not putting stress on the threads.
Live and learn, and naturally I was in a hurry. You will do better.