It is totally random, on the weapon pickups. Each difficulty has it's own set of weapons available only on that difficulty, the better ones being available in the last 3-4 levels of the game. So to unlock them all you're going to have to play the last few levels on inferno until you get them all. Now obviously if you start the game out on hard, you'll be able to grab the easy and normal weapons while you're playing hard, making it useless to play easy or normal unless you want the achievement (I do). I'm about halfway through hard, and bygod some of the weapons I'm even getting at this level rock! You love the tortoise? Wait until you get the Prominence M1 (1500 dmg, 5 second reload, travels faster than the tortoise) or the deadly Tortoise ME (4000 dmg, 10 sec reload, 30 METRE blast area!)
I can NOT wait to see what the game gives you on the later stages on the inferno difficulty!
edit: I picked mine up at eb, $39.99 in Canada