You ever played Space Marine 2 and thought "You know what this game needs? What if we took out your backup and made your enemies a lot tougher, then threw them at you in overwhelming numbers?" You didn't? Well, too bad! Because that's what you get with Warhammer 40k: Boltgun.
Boltgun is an interesting homage to old-school shooters, particularly from the days of Wolfenstein 3D. And it absolutely nails the look of those games. All the hallmarks are there, right down to the 2D sprites that always turn to face the camera. Granted, movement is a bit more fluid in this one. The player can actually look up and down unlike the original Wolfenstein 3D.
Like Space Marine II, Boltgun is a straightforward game about playing as a Space Marine and purging heretics in the name of the Emperor. The basic mechanics are pretty easy to learn. You have access to different types of weapons and three main attacks (melee, ranged, and throwables), and go through waves of enemies. It's nice to get a 40k game that can immerse you in the world without bogging you down with a bunch of convoluted rules.
However, in addition to replicating the look of old-school shooters, Boltgun also replicates their difficulty. This means it's a bit more challenging than Space Marine II. For one thing, you're working alone and don't have Chairon or Gadriel providing support. This can make it a lot more daunting when the forces of Chaos come out in overwhelming numbers.
This isn't necessarily a bad thing, but players should be ready to die a lot. It's very easy to lose health while also being taken by surprise. Of course as they move forward, the player character can get access to better weapons, but obviously, some enemies are difficult no matter what you come equipped with.