BTNG

I would not recommend Wiki as a source, as quality control is questionable. But, if you read farther down on the Wiki account you will see that trial appeal (s) did occur. I suspect if credible new evidence is put forward (that are viewed by the judiciary as significant enough), it would be considered,as with other crimial cases.

Here are some more reliable sources, the court documents:US vs LEONARD PELTIER


Check out a trial appeal account of the ballistic evidence. It seems different from what you posted from Wiki

""The ballistics expert testified that he was unable to fire the weapon because of its condition. He was, however, able to remove the bolt, place it on another AR-15, and conduct limited comparisons"".

Appeal document

While you state you have ""no feeling believe either way as to his guilt or innocencehave"" I find it surprising you would base your belief that "" he should be allowed an appeal or retrial"" based on a wikipedia article?