There are some solid benefits to converting the AR-15 to piston driven operation, but it can be a bit of an undertaking. Here's what you need to know.
Regardless of which conversion system you use, there is some preliminary work. Strip the rifle's upper receiver bare, forward of the upper receiver. That is, remove the flash hider/muzzle device, front sight assembly or gas block, gas tube and handguards.
Only remove the delta ring assembly and barrel nut if you are changing to new handguards. The gas block or front sight assembly has to come off as the piston system replaces it.
The flash hider has to come off because that’s the only way you can get the gas block or sight off. The handguards have to come off because you need the elbow room to position and fit the piston system.
Often, when changing to a piston system, shooters also change to a new, free-float handguard, requiring removal of the delta ring assembly.
Here’s a pro tip. Given the amount of work involved, consider building a new, replacement upper receiver from parts. That way, you’d have two uppers, one DI, and one piston, and you could compare the two at the range. And since you have to strip the old receiver down, we will be looking at the installation of a piston conversion from the stripped-down point, as if you were building it up new anyway, because that gives us a common starting point.
If you are going to change to a free-float handguard, and that handguard uses a barrel nut that isn’t the standard one, you’ll have to remove the old and replace it with the new. At that point, you truly are building up the receiver from scratch.
Not all free-float handguards will clear the hardware for all piston systems. It may take some research and testing on your part to find a combination that agree. I’d offer a chart, but the handguard and the piston system makers are in a constant state of flux and the chart would be out of date before this got to the printers.
Instead, contact the handguard and piston makers. Ask if they have handguards that clear their piston systems, and which ones. Contact the piston maker and ask if they have a list of handguards that clear their piston.
Before You Start
Almost all piston systems require the removal of the entire gas system, including front sight or gas block, handguards and flash hider. I will assume for the purposes of this chapter that you have gotten to this point by means of one of two paths. You have taken your existing DI-system AR and have stripped off everything forward of the delta ring assembly and removed the bolt and carrier assembly out of the interior. Or, you have built the upper receiver and barrel from parts, and are proceeding from there with the piston system.