I seen myth busters try to mushroom the barrels...They plugged them with dirt and even welded a bolt in the barrel,,nothing signicant happened at all to the barrels....The most that happened was from the welded in bolt was the end of the barrel swelled up...
Have you ever looked at the arbor on a boresighter?Most are expandable with a wedge inside fingers that are forced against the barrel.Most fixed arbors have hollow centers.In either case,when the bullet strikes them,they are forced open rather than out of the barrel.In fact this is howhollow point or ballistic tip bullets expand so rapidly.The cartridge chosen as well as the bullet usedare also factors.
But if youare so surethat this is fake,I am sure that I can come up with a bore sighter arbor for you to install in your rifle,fire it while holding it yourselfand prove everyone wrong.