Building an AR15 is a lot different than some other rifles. Most of the components are able to be easily changed and upgraded.
Some special tools are useful for keeping tool marks off the gun and parts and being sure that the job is done correctly. You will want a vise, an AR15 vise block, snap ring pliers, punches, an AR15 armorers wrench, grease, and of course the AR15 parts you will need.
Why do this?
In this case, the original barrel was quite large and heavy. Also the twist was not what the shooter wanted (1 in 9). So a new lighter barrel is being added to this AR15 upper receiver with a 1 in 7 twist
First thing is to put the upper receiver in the vise block, which is in the vise. Remove the handguards and expose the gas tube
We changed over the muzzle break too, so it came off next with a simple crescent wrench
Next the gas tube pin is removed and the gas tube is pulled free from the front sight assembly. These are cheap, it is just tight, so it should come loose. But if you damage an old one removing it, swap it out with a new one. Have a few of these around anyway, these can break depending on the type of barrel and handguards you use
Use the AR15 armorers tool to loosen the barrel nut
The barrel has a notch to align it with the upper receiver. The new barrel will be much lighter than the first one and has a preferred twist (1/7)
Then use the snap ring pliers to remove the delta ring, spring and snap ring. Move these items to the new barrel set and reverse the process to assemble it again. Remember when tightening the barrel nut, don't over-do it but make sure it's snug. Align the barrel nut and delta ring assembly so the gas tube isn't crooked
This new barrel set had no front sight so these were also installed
Last step was to add the muzzle break to the new barrel using a crush washer and checking to be sure it is indexed to the top of the gun
It helps to have some spare parts around in case you drop something or break a part along the way