Major differences to 1.7 - pWn3d1337/Techguns2 GitHub Wiki
Guns
dual wield / two-hand weapon system
improvements in effects/visual and faster bullets. Bullet speed is now a stat that is different per gun.
new Weapons
new Tool
ammo types: some guns can load incendiary rounds. More types to come in the future.
Armor
Damage system is reworked, damage formula is like the new vanilla formula. Damage Types still exist, and armors have different armor values against types
Armor-penetration is now anti-toughness.
Turrets
Turrets can now be places upside down, hanging from the ceiling.
Techguns Inventory
new Slot: Hands
two additional slots for ammo
Icons
Most icons got downsized to 32x32px to better fit the minecraft style, also all non-binary alpha channels have been fixed, no more holes in item models that are generated from textures.
Worldgen
some buildings removed, new stuff added. Will get more in the future.
new spawners that dissapear after a specific amount of monsters have been killed.