I don't think this is correct. Making a gun that shoots a certain caliber most likely does not infringe on copyrights or patents. Making an identical rifle MAY infringe on them, but making one that shoots that caliber most likely does not. That being said, it seems that no one has any copyright or patent hold over the AR15 design, so one could be manufactured to shoot that caliber. Calling it a "Beowulf" rifle could get you into legal trouble as far as trademarks but creating an AR15 calibered in .50 Beowulf most likely would not. It would be very difficult to have a patent granted to protect that.
Also, late to the party on the LAR questions, but the RCMP also have the bulletin (or a similiar one) that they sent questar on their website. This clearly states that the magazines capacity is in regards to the caliber it was manufactured for (regardless of it being for a pistol or rifle - it just so happens that pistols are allowed to legally hold more). This was also directly mentioned in the verifier's course I took from the RCMP.
http://www.rcmp-grc.gc.ca/cfp-pcaf/bulletins/bus-ent/20110323-72-eng.htm