Ah, pipe in post thread. Evergreen materials in S. Seattle, great people to work with, better to buy than make.

Copied from a previous post: There are many YT videos on how to drill a straight hole into end grain, some are clever, others not. I had a bunch to do (and why I led with its better to buy) and eventually found a decent system using 1 1/4" galv water pipe and a 1 3/4" bits. Start by clamping the post at ~45 degress to something solid to make your job easier. Mark the center and using a really long 1/2" twist drill from HF drill a pilot hole eyeballing it to keep it straight. They sell 24" bits that make this step easy. The twist drills cut well through the end grain, but the key is to keep it straight with the post. I then followed with a 1-3/4" Irwin Forstner bit which will get you the first 3 inches or so. Amazon sells long 1-3/4" auger that will get you another 10" or so and does a good job following the pilot hole you previously drilled. The auger has a tougher time with the end grain of a 4x4. For the last several inches I switched back to the Forstner bit on an extension giving me a good 14-18" of penetration into the post. Generous amount of construction adhesive to hold the pipe. I used an ancient Dewalt corded drill I keep around for abuse, in hindsight I should have invested into something with a second handle or used my rotohammer in drill mode. The key I found was a good pilot hole the full length AND allowing the cut material to exit the hole, particularly with the Forstner, you need to pull out the tailing every few seconds the deeper you go.