The New testament reinterpreted Psalm 118:22 to define Jesus in Matt 21:42, Mark 12:10, Luke 20:17, Acts 4:11, Ephesians 2:20, 1 Peter 2:7. If you read NIV, you get: "The stone the builders rejected has become the capstone". If you read NASB, you get: "The stone which the builders rejected Has become the chief corner stone". What is the difference? Well, Capstone refers to "one of the finishing or protective stones that form the top of an exterior masonry wall or building. It looks like this:
Cornerstone however refers to the "first stone set in the construction of a masonry foundation, important since all other stones will be set in reference to this stone, thus determining the position of the entire structure." It looks like this:
I think both translations are possible and depending on which one you choose, you will derive a different meaning of the phrase. If you think of Jesus as the Capstone, he is the stone that presses down and holds down the entire structure. When the Capstone is removed, the structure implodes. Willimon's description of Jesus as the Capstone brings out this nuance, (although his title is Jesus the Cornerstone). This is his sermon: Jesus the Cornerstone.
"Now at the Chapel at Duke, where I usually preach, the chapel is built in the style of true gothic buildings, with no structural steel, with each stone resting upon stone in the fashion of true medieval architecture. High above the crossing there is something that's called a capstone. All the soaring arches thrust their weight upward and meet right up there in the center at the capstone. If that capstone, which I’m told weighs over a ton, were removed, the whole building would collapse. To expand Ephesians’ architectural metaphor, if you were to remove Jesus, the capstone of the whole Church, it would collapse." - Willimon
On the other hand, if you think of Jesus as the Cornerstone, he is the stone at the bottom corner that the entire building plan aligns itself with. Everything is an extension based on that stone. This is especially important for religious buildings because that stone marks out the original position of the previous temple or shrine or holy place. Tim Gallant's sermon brings out this nuance with his focus on Christian unity. This is his sermon: God's new temple under construction.
"So verse 20, then, helps us see if we have indeed discovered the temple of God. If, as Paul writes, this temple is founded upon the apostles and prophets, with Christ Himself as the chief cornerstone, then the features of the temple come into focus. If a society calls itself a "church" but has moved off the foundation of the apostles and prophets - you are not looking at the temple of the living God. If a society calls itself a "church," but is not founded upon the Person and work of Jesus Christ - you are not looking at the temple of the living God. The Church of Christ does not worship a goddess. The Church of Christ does not worship humanity. The Church of Christ does not worship the flag. The Church of Christ is founded squarely upon Christ and His cross and resurrection." - Gallant