Good discourse here, thanks for your question Hagen! BTW, I too think you have made a good choice for your intended purpose w/NP6's, just not too close if you intend to eat those birds;-) To continue the conversation ..
The Bornaghi 36 gr 'Helice' loads that I had were specifically marked as being 3% Antimony. I was first introduced to them by Dr. Marsala who became aware of them through shooting the world cup helice matches in Italy and serving on the same committee & being friends with Giancesare Bornaghi who is a brilliant shooter himself.
Personally, I like the Remington 'Heavy Dove' loads as well as anything I've used on helice. They are readily available in #7.5 regular lead shot and carry a 1 & 1/8oz. payload.
B&P also makes a helice load. Not positive on today's formulae, but it used to utilize a different & softer shot than their Star Rosa flyer loads.
Hevi-shot has to fly much like a teardrop once loosed as there is no other logical nor likely position. In spite of its irregular shapes, I suspect it also has slighty less drag than spherical shot in flight.
Lead shot sphericity does matter, to the extent that it has now been proven that the shot column 'turns over', i.e., the 'round' shot flying in the middle ultimately overtake and pass the shot that started out in front doing all the 'pushing' and meeting with & breaking the air's resistance much as a NASCAR drafting pass manouevre. It has also been proved that deformed shot does not fly as true, e.g., flatened/deformed shot being purpose loaded for close in or spreader loads.
