The Fighter archetype "Champion" lists the skills "Improved Critical" and "Superior Critical", which state that my weapon attacks score a critical hit on natural 19-20 and natural 18-20, respectively. The description for either of these abilities don't explicitly state whether these attacks have the autohit properties of a natural 20 or not, though they could be interpreted to mean that. Given a single level 3 Champion fighter with no active magical effects fighting an enemy with infinite AC, would that fighter have a 5% chance to hit (nat 20 only), or a 10% chance to hit (nat 19+20)?
No, for the fighter with Improved Critical a 19 is an auto-hit.
According to a tweet by Crawford a 19 (or 18) for a character with the Improved Critical feature is an automatic hit.
http://www.sageadvice.eu/2015/12/21/are-fighter-champion-improved-critical-hits-like-a-normal-20/
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1\$\begingroup\$ Wow, I would not have ruled that way. I would have hoped that would make it into the Sage Advice Compendium by now. \$\endgroup\$ – keithcurtis Oct 7 '16 at 22:12
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1\$\begingroup\$ Well, I guess it doesn't get much more official than the lead designer of the PHB. \$\endgroup\$ – VHS Oct 7 '16 at 23:10
I found this question from a few months ago. It answers the question you have.
Apparently I have been playing DnD wrong when it comes to critical hits.
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1\$\begingroup\$ That reference is for the 4th edition, isn't it? I don't think it is a valid reference since he is tagging the 5th edition. \$\endgroup\$ – Chepelink Oct 7 '16 at 21:43