The solution here is to use an outputFileNameMapping such as this:

${artifactId}-${baseVersion}.${extension}

Using ${baseVersion} for cases where you want to preserve the -SNAPSHOT naming, the plugin retains the ability to use ${version} for the timestamp-buildnumber naming, which is useful for describing the exact library version included in the assembly.

My patch is generally along the same fundamental approach as Baerrach's, but with the following differences:

• aims to fix the problem with minimal changes, instead of including refactoring mixed with the fix.
• applies the fix to the jar indexing codepath too.
• tries to avoid hardcoding the details of how to construct artifact filenames.

In that last point, it is only partially successful - I could not work out how to get hold of an appropriate ArtifactRepository object within maven-archiver, so I resorted to making the twin assumptions:
1. that the existing filepath would contain the correct path, and only the basename need be fixed up.
2. that a DefaultRepositoryLayout object would give me the correct basename.

In practice this seems to work, even if it is a little ugly.