Better at WH40K than WH40K

Space Marines in the WH40K universe are bad ass. They come, they kill, they dominate, and they crush their enemies like bugs beneath their feet. Seven feet tall, hugely strong, wrapped in battle suits that can shrug off cannon rounds and armed with guns that can take out tanks, buildings and anything in the way.

Then you get the official WH40K books, like Deathwatch,

After you've spent an hour building your awesome Space Marine player-character and are ready to start striding across the battlefield laying waste to the Emperors enemies, you get to your first encounter with the enemy and ...

Referee: "Roll to hit the enemies."

You: "So I've got my base to hit chance that's 32%. You said they were grouped together, so I can use my 'hordes bonus', that gets me to 36%. They are the declared enemies of my chapter, so that's anther 2%, so that's um... 38%. I was blessed by the chapters chaplain that puts me on 39%. Rodrick is suppressing them so that gets me to 42%. I roll ... 56. Gosh darn it I missed."

And that's where the official 40K games leave me. I'm looking at the beautiful books filled with glorious details and lore of a universe that is hideous and alluring, a place I want to game, but with a bittersweet taste. The actual game play especially combat, crawls at a snails pace. Instead of wiping swathes of enemies to jelly in an instant, I'm adding up small percentages that if I'm lucky allow me to hit one bad guy and maybe taken him out.

It fails, blocks your might say, the story you are trying tell. Its simply not fit for purpose.

Contrast to the game: Three Sixteen: Carnage Amongst the Stars

After you've spent the full 30 seconds creating your character you dive into the action

Referee: "Roll to hit the enemies."

You: "7 on the D10, that's a hit, rolling 2D8 for kills, I kill 9."

Clearly you see the difference. 

Deathwatch is a traditional detail orientated , methodical, plodding game, where as Three Sixteen is a rapid, rules-as-tool system that's almost entire story focused.

The background of Three Sixteen is almost a null, so combining the rules of it with the lore and background of Deathwatch is the way to go if you want that "feel" of Space Marines". Go ahead, mash them together, two great tastes in one!