Bright harmonic generation in a long Ne gas jet achieved using self-guided and chirped femtosecond laser pulses has been analysed. Three-dimensional modelling was performed to look into the details of laser pulse propagation, harmonic generation and attosecond pulse characteristics. The theoretical analysis and the comparison to the experimental results showed emitted harmonics form a bright attosecond pulse train with good synchronization in time. In the space domain the synchronization is especially good in the self-guided region.