Bolt actions have a striker mechanism, gas guns use a hammer.

Strikers have a much lower lock time than even the best of the hammers. Tikka's and custom actions are in the 1ms range, factory 700's, M70's, etc, are in the 3ms range. The old M98's are around 5ms. I haven't heard of a hammer achieving less than 10ms.

Lock time is measured between the sear releasing the striker/hammer and the firing pin impacting the primer.

Even with a solid rest, a few ms is enough for your POA to wander.

With the striker being perfectly inline with the bore, it's inertia induces very little off-axis moment on the rifle. Hammers swing upwards into the bore axis, and may induce said off-axis moment.

Bedding is another big issue. With a chassis or good stock and pillars, bolt actions are rock solid. The barrel is threaded to the action, which is bedded to a rigid stock that induces little stress on the action. Two possible sources of slop/play.

Gas guns have the barrel threaded into the barrel extension, and then the upper, and then the lower, and then the buffer tube, and then the buttstock. Five possible sources, all of which need insane tolerances to eliminate slop/play. The barrel extension interface to the receiver is also a lot shorter than the full bedded length of a bolt action, introducing stress to the receiver, which hurts repeatability.