Redshfit Automation adds a few weapons and other fun things, such as a custom boss, music for the vanilla bosses, and more.

Here are some of the major additions:

Gliders: Fast flying vehicles that can drop non-destructive (but highly damaging) bombs. The bombs create smoke, obscuring view, and inflict "Shell Shock" that prevents one from regenerating health for awhile. Bombs deal much additional damage to rank-and-file hostile mobs and animals (excluding bosses, defined by max HP, and a few hard-coded mobs like villagers and tamed wolves).
Dragonslayers: Handheld anti-air flak weapons that can shoot down Gliders and also flying mobs, including some from the Ice and Fire mod
Boss overhauls for the Ender Dragon and the Wither to make them challenging despite Redshift's powerful weapons
A custom boss with music and the ability to call in hostile Gliders
A sword that gives invisibility when held and doesn't reveal your identity in your victims' death messages
Much more in the same vein

Redshift has been extensively playtested before release, but it is built for a certain kind of game. It is great for messing around with your friends. It is not great if you want an extremely well-balanced, difficult survival experience.

A mod like JEI is highly recommended to view the crafting recipes.

The Redshift Core, which is used to craft all of Redshift's tools and such, is obtained by killing the Automaton. The Automaton is a custom boss made by placing a redstone block on top of a smooth stone block.

The Automaton will usually attack by spamming arrows. (Its boss track is called "Wall of Arrows" for a reason.) Prepare accordingly. It can also call down glider raids and summon Operatives, dangerous semitransparent mobs that carry Electron Swords.

It will also drop lots of useful items other than Redshift Cores. It can Redshift items, materials used to make them, and boatloads of iron.

Redshift Weapons