Provided herein are buffer systems, bolt assemblies, firearms, and related methods, devices, and assemblies. A bolt carrier assembly for a firearm of an example includes: a bolt carrier body defining a cavity extending along a longitudinal axis of the bolt carrier; at least one weight disposed within the cavity; a forward stop at a forward end of the cavity; and an aft stop at an aft end of the cavity. In some embodiments, the bolt carrier assembly may include a spring, where the spring biases the at least one weight along the longitudinal axis toward at least one of the forward stop or the aft stop, where the at least one weight is configured to be driven against the spring bias in response to a firing action of the firearm.