He was afraid of that. Once he saw the Kessek drawing their weapons he reacted.

"Protego!" He ducked as he said the spell. He didn't know for a fact that the shield would stop the bullets- plasma- whatever they were shooting. What he did know was that he had to render them useless.

The spell barely blocked the shots in time but the real tricky part would come next when Sirius had to lower the shield to go on the offensive.

"Confringo!" He aimed at one of the Kessek's rifles. If the spell hit, the weapon would explode.