thinking of upgrading

[BPTactical]

So it's just the force of the gas pushing back on the BCG that causes it to unlock, then I can see how both systems works pretty much the same. Why does DI need the gas to actually enter the bolt carrier instead of just acting on the face, like piston?

The other common method for rifles, other than piston or DI, is roller delayed blowback, like the H&K G36, ja?

Thanks for the schooling, and the video is helpful too.

H.

As far as the gas entering the carrier it acts as an expansion chamber, allowing chamber pressure to drop to a safe level before unlocking the bolt. It also is doing the same thing as a piston, hence the gas rings on the bolt. You have to contain the gas somehow, otherwise the pressure drop will not cycle the action.
With a piston rifle this "buffering" is accomplished by venting a certain amount of gas off before the piston.

You also have weapons systems that operate purely off of recoil, the 1919 and M2 come to mind.

[Not_A_Llama]

So it's just the force of the gas pushing back on the BCG that causes it to unlock, then I can see how both systems works pretty much the same. Why does DI need the gas to actually enter the bolt carrier instead of just acting on the face, like piston?
H.

BPTactical covered it, but another angle on DI: The force generated by the gas acting on the piston face of the bolt is linear, acting to push the bolt assembly backwards. It's the cam pin that creates the rotation necessary to unlock the bolt. The reason the gas goes into the carrier itself is so that the pressure can act against the larger surface of the bolt, and so that the gasses can be vented outside the rifle when the bolt has reached its full extension.

[BPTactical]

BPTactical covered it, but another angle on DI: The force generated by the gas acting on the piston face of the bolt is linear, acting to push the bolt assembly backwards. It's the cam pin that creates the rotation necessary to unlock the bolt. The reason the gas goes into the carrier itself is so that the pressure can act against the larger surface of the bolt, and so that the gasses can be vented outside the rifle when the bolt has reached its full extension.

You bring up a good point Llama-linear force/ bolt carrier tilt on piston AR's. Linear force applied by gas is applied equally, meaning you get an "even push" straight back on the boltcarrier meaning the carrier is coming back in a strictly linear fashion.
With a piston AR that recoiling force is concentrated in one area, the top of the boltcarrier where the gas key is usually located. That causes the boltcarrier to "tilt" somewhat and if pronounced enough, binding or shortstroking of the boltcarrier can result. In extreme cases the buffer tube can be deformed/gouged by the carrier. This is where using only Mil-spec and top quality components should be utilized if converting to a piston rig.
Upper end conversion boltcarriers will have "skids" built in to the bottom rear of the boltcarrier to "minimize" carrier tilt.
Another case of reinventing the wheel when it really isn't needed. You create a new set of problems when running a piston system in a firearm that was not originally designed for it.
Beware of conversions that have you replace the gaskey with a solid faced key for the pushrod to bear on. You WILL shear the gaskey screws.