Complex sentence: Bump fire stocks (gunstocks specially designed to enable bumpfire) allow semi-automatic weapons to mimic the firing speed of fully automatic weapons, achieving rates of fire between 400 to 800 rounds per minute.
Split into simpler sentences: Bump fire stocks are gunstocks that are specially designed to enable bumpfire, which allow semi-automatic weapons to mimic the firing speed of fully automatic weapons. They can achieve rates of fire between 400 to 800 rounds per minute.