I assume that the cannon you are interested in is the one featured in this logo:
Although the history of Arsenal's badges and logos is fairly well documented (eg here and here) I have no idea who actually designed that particular crest. This is important because copyright protection lasts for the lifetime of the artist plus 70 years from the end of the year of death. Therefore, irrespective of whether the crest is still in use, copyright in it is likely to run for many years yet.
Furthermore, the image above is taken from the IPO's registered trade mark database and that particular mark will remain on the register until September 2017, although of course it could be re-registered by the club before then. This means that the logo, including the cannon part, is doubly protected by copyright and as a trade mark.
Much the same will apply to any other recent(ish) version of the crest, such as this one:
Thanks for your contribution. Copyright doesn't need to be registered in order to become effective. Unless the artist who drew the original crest shown above died more than 70 years ago, copyright will still exist in it. The trade mark aspect is subject to registration (and renewal every 10 years) and as it happens, since my earlier reply, that particular trade mark registration has been renewed and runs until 2027.
You referred to an earlier court case about the sale of unofficial Arensal memorabilia. You probably mean the case known as Arsenal Football Club v Matthew Reed. Although Mr Reed won at first instance, the Court of Appeal reversed that decision and so Arsenal FC was able to enforce its trade marks. As far as I can recall, the issue of copyright was not raised in that litigation.