This section covers the assembly of the charging handle, the bolt carrier group and checking the headspace of your rifle. Too much or too little headspace can cause serious bodily injury or destroy your rifle. Ensuring that you have the proper amount of headspace is easy and will only take about 10 minutes to check. If you’ve purchased an upper receiver with the barrel installed, the manufacturer should have checked the headspace of your rifle and you may, at your option, skip this step.
The clip that covers the assembly of the bolt carrier group will also be helpful when disassembling the bolt carrier group for cleaning.
It is important that you follow the order of assembly; this will avoid interference of parts, eliminate rework and allow for proper and cumulative function checking of subassemblies.