Who says it's supposed to be the same trooper saying both lines? There are about six of them chasing Han and Chewie.

Also, "Close the blast doors!" is the setup to the punchline that is "Open the blast doors!". It got a laugh from the audience that I can remember. It was in the "Story of Star Wars" record too, so it's burned into the brains of us old coots. ;)

I personally believe the 44 magnum blaster is the actual blanks being fired on set. They simply goofed, and forgot to put a regular blaster sound in the mono mix.

The story I read ages ago was Lucas went out to eat while working on one of the foreign dubs, when he saw the line around the block at Grauman's Chinese and wondered what movie it was.

I personally own a Japanese LD from 1983 with the original stereo mix. Non time compressed. Curiously, it has subtitles for lines only present in the mono mix. I verified this with a fellow board member who could read Japanese a few years back.