One day in Jan '68 we got the first revision (M-16A1, I think). What they told us about the improvements was increased carrier/receiver clearance (dirt jams); a revision to the cross-carrier pin so that you could not put it in backwards and jam solid upon firing the first round; and the carrier ram in the stock now had a damper in it to slow the firing rate (maybe less spring tension, too). Other than the new 5-slot flash suppressor that replaced the 3-slot, I can't recall anything further.
From what the vets I had the privelege to work with told me, the only thing one could do was throw a cleaning rod down the barrel forcefully and hope it either loosened the problem or forced the impacted round to fire upon contacting the pin. I heard fingers were lost due to carelessness.