I'm a bit confused, I was playing for about 5 hours in total and then the end credits came on at the end of a level as if the game was over. But when I go back to the menu it says 50.3% completed.

What exactly am I missing out on?

Mine's the same way, I think it's becasue you have to beat campaign on every difficulty and probably get all the enemy intels to get it to 100%.

Umm yeah...You need to complete it on everything to get 100% xD

Recruit/Regular
Hardened
Veteran
And get all 45 pieces of intel xD That is how you get 100%

wrong, completing it on veteran and getting all the intel gets you 100%
so for example, complete the game on hardened and you've completed it on recruit, regular and hardened

Its just that you didnt beat it on all difficulty yet

And from everything I have read, 5-7 hours is about the standard to get through the single player game.

(and from other threads many many hours to get to level 70 on mp....)

It took me about 6 hours for campaign on veteran. Just memorise where they come from and you gonna start easing your way through it.

Is the museum the end of the game? Do you just run around blasting people or can you get out of there?

Its just for blasting around. You cant get out or anything the guys will just keep spawning after you kill them.

But if you press the button and then manage to kill everybody you get to hear a satisfying noise. Isn't that great?

I miss the MP5

Edit: Very wrong thread post. I had multiple tabs open, you see...

Veteran isn't that hard. Play it on an easy dificulty, see where they are, quit, restart at Veteran, and then play every level like that. Quick Tip, In No Russian, pick veteran, and skip level after it starts, and it counts as if you did complete it.

Same here man its because u have to go back and get all of the acevements and enemy intel in order for the 50% to change

Finishing Game on Recruit/Regular gets you 50%.
Finishing Game on Hardened gets you 75%.
Finishing Game on Veteran gets you 90%-ish.
All intels get you the last 10% or so.

I don't actually know this but its an estimate based on how much value each is worth.