chas, do you mean you have to push the front pin to the open position, or actually remove it from the lower? You shouldn't ever have to remove it, but on one of mine rifles, it's easier to get it back together if I do the back pin first, then the front one.
Well, make sure the bolt carrier/charging handle is all the way forward. Also, pull the hammer back to the cocked position (do NOT pull the trigger, especially with any of your fingers in the way!! you could break a finger or the receiver), then put the two halves together. Try that, and see it that makes a difference.
It's hard for me to visualize what's going on, but it sounds like something's not right. The bolt shouldn't be hitting the lower anywhere...do you mean the bolt carrier instead?
Okay...if the firing pin is not all the way forward into the bolt, it'll hit the front of the hammer when you try and close the upper and lower. Either cock the hammer, or just push the firing pin forward a little. That's about the only thing I can think of, but neither of those should require taking the front pin out...you won't see the problem 'til you swing the back of the receiver down to try and close the back pin.