Saddlesore has it right!
Even when kiln-dried, timbers the size you want will still check and shrink.
A goof friend did a timber framed section of his new house, had a local supplier do the kiln-drying, moisture meter read okay................while sitting on horses in a heated shop, you could hear the timbers crack. He used a new system of machined fasteners that were interlocking. One year after assembly, there were fairly large gaps between the joints. While the fasteners kept things from moving, the shrinkage was pretty unsightly.
You must always remember, even though wood is cut and "killed", it is still alive and will shrink and expand. Saddlesores recommendation is spot on, and is the only method I will use. It simply works. I've been doing Finish Carpentry and High-End Architectural Woodworking for forty two years.
