  Nanocrystalline MgB$_2$ produced by mechanical alloying has been shown to
exhibit enhanced superconducting properties such as increased pinning and
higher critical currents. However, the effects of the synthesis process on the
intrinsic superconducting properties have not been addressed yet. We have
investigated the superconducting gap structure of nanocrystalline MgB$_2$
pellets synthesized by high-energy ball milling employing specific heat
measurements. We found that the larger $\sigma$-gap decreased whereas the
smaller $\pi$-gap slightly increased in ball milled MgB$_2$ as compared to bulk
samples synthesized along the standard routes. The data show that the ball
milling process introduces defects that enhance the interband scattering
similar to irradiation with neutrons. The reduction of the $\sigma$-gap
explains the lower superconducting transition temperature of 33 K.
