Unless otherwise modified by gear and/or abilities, crits are on a 6+. Most of the time you roll a D6 to hit, so that's just a 6 for crits. When you use a D8 to hit, it should crit on a 6-8.
I can't recall what the Rail Spike uses, but the Splitter is a D6 To hit and D8 for damage, so it can only crit on a to hit roll of 6 (barring special abilities).
What Adria said. :)