(Quote from AJ from the old AR15 Forums)
Author: AJ
Topic: Colt sear block removal
Junior Member posted 01-10-2001 20:12

Before you drop $200 to have somebody cut on and refinish your $800+ receiver to remove the sear block consider this:
As an engineer and perfectionist I like to keep my stuff looking factory. I removed the block on both my ARs because I could and they look exactly like they did before except the block is gone. Here is how to do it.

1. Place the receiver in a padded vise
2. Drill a vertical hole in the block(correct size for a #8 machine screw with hex head) 3/8" to 1/2" deep and centered at the intersection of the "L" in COLT (centered over the block pin).
3. Thread the hole with a bottom tap.
4. Fill the hole to the top with Marvel Mystery Oil and start the screw.
5. Slowly tighten the screw and you will notice the pin begin to protrude out of the receiver.
6. When the pin is about 5/16" protruding from the receiver the block should wiggle fore and aft.
7. Grab the block with a pair of vice grips and gently wiggle it while pulling vertically.
8. Once the block is out the roll pin may still be in the receiver. Repeat step 7 if so.
9. Knock the block pin back in flush with the receiver side.
DONE..isn't hydraulics just great!

Use a carbide tip drill, the block metal is hardened.

This is only information. I am not responsible for your mistakes. If you don't feel comfortable doing this send me $30 and your receiver and I will do it for you, however since you don't know me from Adam I suggest you take these instructions to a competent gunsmith and have him do it.