  This work compares computational methods for laser pulse propagation in
hollow waveguides filled with rare gases at high pressures, with applications
in extreme nonlinear optics in the mid-infrared wavelength region. As the
wavelength of light \lambda=2\pi/k increases with respect to the transverse
size R of a leaky waveguide, the loss of light out of the waveguide upon
propagation, in general, increases. The now standard numerical approach for
studying such structures is based on expansion of the propagating field into
approximate leaky waveguide modes. We compare this approach to a new method
that resolves the electric field in real space and correctly captures the
energy loss through the waveguide wall. The comparison reveals that the
expansion-based approach overestimates losses that occur in nonlinearly
reshaped pulsed waveforms. For a modest increase in computational effort, the
new method offers a physically more accurate model to describe phenomena (e.g.,
extreme pulse-selfcompression) in waveguides with smaller values of kR.
