This build is made around the saying "A good defence is a strong offence", but added on is "but just in case..."
The goal is to take down enemies before they take down you, but in case that doesn't work as intended, this build gives you a bit more of a thick hide to resist getting killed.
The mobile cover skill is used to optimize the Big Alejandro's Cover Shooter perk, and the Pulse is used to up the critical hit damage. While primarily designed for PvE content, this build works pretty good in PvP as well.
The perks on the main weapons aren't perfect... Ideally, you want Destructive and Accurate... however, Predatory works pretty good to keep you alive as well.
The "One is None" talent is just useful. When shooting a 1000RPM stream of bullets into some poor heavy's brain, you'll get bullets back, guaranteed.
The Lone Star Classified perk works wonders with the Big Al. When it hits, you just erupt in a massive stream of bullets. For best effect, work with a Reclaimer as a teammate, and stay in the green circle. The Ammo Regen perk on that box will make sure you can keep on firing.
When on your own, however, this build ensures you have 2700 rounds at your disposal. Which should be more than enough to make it from ammo box to ammo box.