I would actually be for disallowing them (although the term "assault rifle" is a bit controversial and lacks definition).  That said, he was running in Texas.  Him standing up (on TV no less) and making that proclamation amounted to conceding.  In fact, he was already losing at that point due to some other stuff.  So it occurs to me that he may have gone to that space in order to give himself a convenient excuse becuase it is a clear political suicide in Texas, regardless of what you might think of the actual issue.   

He's clearly not as bright or quick as is Cruz, although he may be more often morally correct.  Watch a Beto speech sometime, and you'll see what I mean.  He's really not a smart guy at all.  It was a tricky problem because the more people heard him speak the more it became clear that he was/is hopelessly naive.  It was an awful choice for Texans that year. So what's the bright side?  Cruz is absolutely reviled, even by many Republicans.  It becomes more likely each day that he could lose, either to someone in his party or to a more credible Democrat challenger.