Obviously you don’t know what an assertion is.
An assertion is something stated as truth or fact, generally offered without any underlying or supporting evidence or proof.
That's how I put it.
The American Heritage Dictionary put it this way:
"Something declared or stated positively, often with no support or attempt at proof."M-W is a little kinder to you:
"A declaration that something is the case"But notice that it's a "declaration" and supporting evidence is not mentioned.
Dictionary.com follows the AHD:
"A positive statement or declaration, often without support or reason"Wordnet defines it as:
"A declaration that is made emphatically (as if no supporting evidence were necessary)"And Collins follows along:
"A positive statement, usually made without an attempt at furnishing evidence"Now, let's go back and look at what you wrote. First there is:
"a throw-down gun that McDonald brought"This is a positive statement that you provided no supporting evidence. That is to say, an assertion. You followed up with:
"It’s [the alleged McDonald throw down gun]
no less plausible than the evidence-less official claim about the revolver"This is also a positive statement provided with no supporting documentation, argument, or evidence. That is, another assertion.