If you look up the webpage of the people who made the proposal, it's kind of hard to decide if they're serious or just some weird kind of crackpots.
There is definitely not 740 grams of polonium 210 in the solar system, since it has a half-life measured in months. The only way to get it is to manufacture it. This is done all the time, but nowhere near enough to power even a single rifle. This is "just a matter of engineering", of course.
The radiation probably wouldn't kill the soldier - it's all alpha, and those are trivial to shield. Neither would the radiation kill the soldier if a bullet struck the casing - the massive explosion caused by the 270 atmospheres of pressure would kill him. The rest of the squad might die from the radiation spread from it, though.