is that i don’t think Ultron has ever had the opportunity to test it on another sentient machine, not to mention that even G1 Optimus is much more technologically superior than Ultron is. also i think it depends on which Ultron he goes up against, i’m ruling out Ultron Extreme and the one from the big battle with “Kang”, as they are both enormously overpowered.
your normal run of the mill Ultron generally has an adamantium alloyed outer shell (so it isn’t pure adamantium, making it somewhat less “indestructible” but easier to work with). i think the laser cannon would be able to do damage, at least in some amount because Iron Man’s energy beams have done damage to Ultron as well, and the power behind both would be at the very least close to equal