I intend to build a prototype version of the Harkonnen Mk1 from Hellsing.



The sprite itself is public domain and is quite good. This will be a first mod. So far from what I understand I want it to weight enough to where you can only hold it with a pistol, as well as

Piercing explosive tiped rounds
Metal Slug Explosion sprites
Faster reloading than piercer rifle, slower than PAR.

any idea on what I should be studying. I'll work on this as long as it takes to get it right

It's been done a lot. So don't expect anyone to be polite to you if you put a finished version in the mods section.

It's essentially a resprited long cannon (flak), so get the base code for that, change the sprites, and figure out the simple essential things like offsets, and if you want the weight to be anti-jetpack, or anti-movement. Also, think about, how the inability to use a jetpack while using your mod might be fun for the more tactical player, one who wants challenges, it will be frowned upon by a lot of people.

Thanks you, I know there will be alot of people that won't like it because there are similar mods. (all the grenade launchers) though I don't think there are many balances ones. Thats why I intend to do this. God knows we have enough super weapons.

Well, I got a little piece of advice: If you want it to be a one hit-KO gun, make it fire grenades, not necessarily exploding on impact, but standard grenades will do. If you're not sure what I mean, you could take a look at the KYA WTF sniper module. It fires grenades like the flak cannon, at a sick velocity, which vaporizes actors and might take off a dropship engine. The grenades also dig a little into the ground, which makes it look less like a high-velocity grenade launcher, even though that's what it is...
Anyways, just an idea and bragging about my mod.
I wish you luck on this project, every Harkonnen mod I've seen looks nothing like the image you posted, just a long stick with über-sharpness rounds, so thanks for coming up with a better idea.
Of course, if you want the rounds to pierce, it sucks a little that it's hard to apply sharpness to grenades, unless you're giving them high velocity, which means higher recoil unless you lower the mass of the bullet.
So, that's what I can offer of help. If you need a suggestion on whether it'd be best with impact-explosion or timed, I'll need to know more about it. Like, what kind of gun is it? (Sorry if I seem uninterested, but anime really isn't my thing). A sniper would IMO be best with timed rounds, so you can kill more, but a machine gun would be best with impact explosions. That way, it wouldn't go through so many people, but hit and detonate, instantly killing one or two actors, so it wouldn't lag so much. Imagine the lag caused by 30 timed rounds being sprayed in a 90 degrees area. On hard, it might kill up to 20 actors, including dropships, which would make your game lag to the extreme.