The absurdity of the video is beyond description. However, here are a few points which come to mind.
As Rachel stated in her debunking segment last evening, the weapon was not, as Cruz suggested, a "machine gun" which, of course, is an automatic weapon. In warfare, barrels of ground-mounted machine guns have become so hot due to repeated firing that they caused the weapon to fail because the steel of which they were made began to melt.
The hand-held automatic weapon shown in the archive video which Rachel presented displayed a flame plume from the top of its barrel. It is not known if that flame was from burning weapon oil, gases, or some other source. I am thinking that, after the weapon had been fired for about fifteen seconds, the barrel of that hand-held automatic weapon would be hot enough to sizzle a piece of pre-warmed bacon, but repeated firing would be necessary to thoroughly cook it.
The weapon which Cruz used in his video was, according to Rachel, an AR-15 semi-automatic rifle. I think that it would have required many consecutive firings to cook the pieces of bacon which Cruz wrapped around the rifle's barrel and caused to be held in place with the over-wrapped aluminum foil. At the end of the video, Cruz is shown unwrapping the foil and using a fork to pull a morsel from the end of the wrap and eat it. I think that the morsel which Cruz ingested was cooked more than by just the barrel heat generated by Cruz's firing. I think that the bacon which Cruz nibbled from was pre-microwaved, wrapped on the barrel, and maybe even subjected to an outside heat source.
In any case, I think that the humor which Cruz aimed for by using the weapon in that stunt was sophomoric at best.