Heavy pistols in Shadowrun are already heavily favored in comparison to other, larger, weapons (submachineguns, assault rifles, etc), in terms of what their damage code/AP is versus their realistic caliber size, barrel length, etc, etc.
I don't think Shadowrun really needs a super mega customized pistol that's almost on par with a full-on sniper rifle. You can already get a ridiculously powerful hand cannon just by fiddling around with the existing, canon, firearm creation rules and starting with something nasty.
In both your proposed ammunition and your proposed handgun, here, you are drastically upsetting the balance of weapons, damage, armor, capacity for soak, etc, in the Shadowrun universe. That is probably a bad idea.
You will be marginally -- perhaps not substantially, but at least theoretically -- better off using canon rules for firearm customization. You can still build some really nasty pieces of work, there, but you're at least a bit less likely to completely throw things out of wack that way.
And I'm not just making these suggestions to bust your chops. I know you're pretty new to the game, and excited, and all of that's awesome. But whenever you design a new piece of gear like this, you have to ask yourself "What if someone shoots it at a PC," not just "Wow, won't it be awesome when my PC shoots this?" Because all these suggestions you're making, for new guns and new ammo, are pretty nuts, and -- on average, over the course of a campaign -- no one takes more hits than player characters, right? No single character in an adventure gets attacked as many times as a PC. So what happens when every weapon is a .79 caliber monstrosity shooting min-grenade bullets that explode inside you and ignore armor?
You get dead PCs and not a lot of fun, that's what.