The reaction between ground-state carbon atoms, C(P-3(j)), and methylacetylene, CH3CCH (X(1)A(1)), was studied at average collision energies of 20.4 and 33.2 kJ mol(-1) using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of C4H3 at m/e = 51 were recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular flux distributions isotropic at lower, but forward scattered with respect to the carbon beam at a higher collision energy. The translational energy flux distributions peak at 30-60 kJ mol(-1) and show an average fractional translational energy release of 22%-30%. The maximum energy release as well as the angular distributions are consistent with the formation of the n-C4H3 radical in its electronic ground state. Reaction dynamics inferred from these distributions indicate that the carbon atom attacks the pi-orbitals of the methylacetylene molecule via a loose, reactant like transition state located at the centrifugal barrier. The initially formed triplet 1-methylpropendiylidene complex rotates in a plane almost perpendicular to the total angular momentum vector around the BC-axes and undergoes [2,3]-hydrogen migration to triplet 1-methylpropargylene. Within 1-2 ps, the complex decomposes via C-H bond cleavage to n-C4H3 and atomic hydrogen. The exit transition state is found to be tight and located at least 30-60 kJ mol(-1) above the products. The explicit identification of the n-C4H3 radical under single collision conditions represents a further example of a carbon-hydrogen exchange in reactions of ground state carbon atoms with unsaturated hydrocarbons. This channel opens a versatile pathway to synthesize extremely reactive hydrocarbon radicals relevant to combustion processes as well as interstellar chemistry.