While looking in the Bushmaster catalog I noticed that they have a product called a Expandable Tensioning Pin for the AR15.
According to the description it cures the looseness between the upper and lower.
Would anyone here know if this takes the place of the front pin or the back pin?
If it is the front pin,  I have a Colt upper with a large hole and the receiver has a small hole, will this work?

It's the back pin.

You would need to get a gunsmith mill out the holes to the same size for it to work.