Take the advice I learned way too late:Any tips for heavy Bowgun by chance as every guide tot hat one I see people getting pummeled more than anything
Guard, guard, guard. You can get away with taking the initiative once in a while on the light bowgun. You really can't do that with the heavy.
However, do not rely on the auto-guard with the heavy bowgun. Get used to the active guarding (on an XBox controller that's hitting Y+B) and do it right before you get struck. Do it too early and you'll still take scratch damage but you may still be able to react. Do it on time, and a lot of attacks will do nothing at all. I'm still wary of trying to guard against eruption attacks but I think it's doable - but only if you're guarding against the last eruption. There feels like a bit of downtime between guards.
From my experience, your optimal ranges are quite a bit farther away with the heavy bowgun than they are with the light. Spread ammo, despite being shotgun range, is actually not going to be effective if the monster just attacked and you're point-blank with their thighs.
For the Ignition modes, I can't say much about the Wyverncounter vs Wyvernblast yet as I never got good with either. However, your choice of Wyvernheart (the machine gun) vs Wyvernpiercer is going to depend on the boss and your angles - namely, you want to use Wyvernpiercer against long bosses like Jin Dahaad, and Wyvernheart against shorter bosses like the Ebony Odogaron or any of the primates. Something like Guardian Rathalos or Arkveld, you could go either way. Watch for long attack animations, that's the time to let loose - and if the monster's coming your way, stop immediately as there's a bit of delay after stopping your attack.
I would also say that stocking ammo types, while still important, is actually less of a concern on the heavy bowgun than it is on the light. Ignition Mode relies on its own ammo pool and doesn't benefit from your ammo pouch, unlike the Light Bowgun's rapid fire mode, so you're not going to be burning through ammo quite as fast.