The present invention relates to firearms and, more particularly, to automatic or semiautomatic firearms. In a first embodiment, the gas block of the firearm is tightly bound with the upper receiver rather than the barrel, and the receiver serves as a dampening element for gas block vibrations during shooting such that the impact on the barrel is significantly reduced. Fasteners and barrel nuts are used to rigidly position the gas block relative to the upper receiver to best reduce the impact on the barrel. In another embodiment the gas block is attached both to the upper receiver and to the barrel using a support element. Such fixation of the gas block prevents rotation of the firearm and further decreases vibrations of the barrel. In yet another embodiment, the firearm fits interchangeable magazine wells to change the caliber of the firearm.