Assembling an AR-15 is a fairly easy task. Even if one starts with everything disassembled, and has never done it before, the process should take no more than an hour or two, and requires only a few basic tools, a few specialized tools that are readily available, and a vise. However, it’s also fairly easy to mess up.
One of the more common modifications to an already assembled rifle is the installation of a free float tube or free floating handguard that covers the gas block area. This is also a common task when assembling a rifle from parts. However, a very important step during installation is skipped over in many of the installation instructions I’ve seen: gas block placement.
The AR-15 rifle was originally designed with a fixed front sight base that held a metal handguard cap against a machined “shoulder” in the barrel. Standard handguards reside behind/inside this cap. A majority of AR-15s are sold with standard handguards…however, standard handguards are not normally used with low-profile gas blocks designed to be placed under a tube or rail. Therefore, when Joe Rifleowner removes the standard front sight base, handguard cap, etc to install his new free floating handguard, he tends to forget about that handguard cap, and slides the gas block along the barrel until it’s against the shoulder in the barrel.
The only problem is, the majority of low profile gas blocks are manufactured in a way that allows them to be used with handguard caps, and sliding the gas block against the shoulder means that the gas port isn’t properly lined up with the gas block, which holes the gas tube, which allows the weapon to function properly.
As a result, it’s important to place the gas block about 1/16″ from the shoulder on the barrel – give or take. The port in the gas block is oversized, leaving some room for error, but I can’t count the number of rifles I’ve encountered that didn’t function as a result of improper gas block placement. See the photos below for shots of proper gas block placement…
If you’ve been having “short stroking” issues after assembly of a rifle with a new gas block, you should definitely check gas block placement as a potential culprit.