A power system for a firearm includes a buffer tube adapter that attaches to a buffer tube of the firearm. A buttstock slidably engages with the buffer tube adapter along an axis, and the buttstock is adapted to adjust a length of the firearm. A power source is held by the buffer tube adapter and is at least partially covered by the buttstock. The power source is configured to power electronic accessories mounted to the firearm. The power system is mountable to the firearm without altering the buffer tube of the firearm.