The Iron Dome system – like every other computerized system – has it’s strengths and weaknesses.
After the PIJ tunnel inciden last year, the PIJ was able to set off the sirens by simply shooting off high caliber machine guns at a certain trajectory. They then we’re able to shoot 10 mortars at the fence, while the ensuing panic was underway.
Hamas was trying to figure out the Iron Dome breaking point and they found it. By sending in anti aircraft machine gun fire, as they did last night, it actually confuses the system. Once the system is confused with the anti aircraft projectiles, which it thinks are rockets, Hamas can then throw in rockets and bypass the Iron Dome.
The same problem would apply if there was a major barrage of rockets. At some point, the system will get overwhelmed.
By using smaller arms, Hamas avoids using expensive rockets as bait, can use the smaller projectiles as bait, and save the bigger projectiles for the real damage.
In addition, Hamas can use the small arms to cause panic and mayhem without firing a single rocket – and could hope to avoid IDF retaliation by claiming they were simply practicing shooting their machine guns in the strip without launching a single rocket.
Hamas knew exactly what it was doing last night. As part of their “drills” (as reported here), they were intentionally testing the system.
We know this is the case, because in the subsequent hours, additional heavy machine gun fire was used by Hamas as part of their training – and no sirens were activated.