"Rambo" (The last one where he goes in after the missionaries) has a .50 caliber blowing people in half with one round. The .50 is an awesome round but I think that is a bit extreme.
Don't forget in "Rambo First Blood Part 2" he fires a LAWS rocket inside a helocopter. Did they not realize that that weapon has a signifigant backblast. They made a similar mistake in "Deathwish 3" staring Charles Bronson.