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§ 2. Any person who shall commit or attempt to commit a felony when armed with a pistol, revolver or gun, as defined in section one, shall, in addition to the punishment provided for committing the crime, be punished by imprisonment for not less than two nor more than five years within the discretion of the court. § 3. The court shall have power to sentence any person who may be convicted of a second offense to double the addition penalty imposed under section two thereof for carrying such concealed weapon without a license. § 4. In the trial of a person for the commission of murder, assault with intent to do great bodily harm, robbery, larceny, or any attempt to commit any of such offenses, the fact that he was armed with a pistol, revolver or gun as herein defined and had no permit to carry the same, shall be prima facie evidence of his intention to commit the crime with which he is charged[.]