We investigated the 1-propynyl (CH3CC; X(2)A(1)) plus acetylene/acetylene-d(2) (HCCH/DCCD; (XEg+)-E-1) under single-collision conditions using the crossed molecular beams method. The reaction was found to produce C5H4 plus atomic hydrogen (H) via an indirect reaction mechanism with a reaction energy of -123 +/- 18 kJ mol(-1). Using the DCCD isotopologue, we confirmed that the hydrogen atom is lost from the acetylene reactant. Our computational analysis suggests the reaction proceeds by the barrierless addition of the 1-propynyl radical to acetylene, resulting in C5H5 intermediate(s) that dissociate preferentially to methyldiacetylene (CH3CCCCH; X(1)A(1)) via hydrogen atom emission with a computed reaction energy of -123 +/- 4 kJ morl. The barrierless nature of this reaction scheme suggests the 1-propynyl radical may be a key intermediate in hydrocarbon chain growth in cold molecular clouds like TMC-1, where methyl-substituted (poly)acetylenes are known to exist.