Witnesses reported seeing a rifle. casing were found in the building obviously. They are easy to figure out. If pistols were used, you would think at least someone who witnessed the crime would come forward and mention that fact.
Your premise is based on ALL the witnesses being part of the conspiracy which is ludicrous. Plus you are dealing with a small town police dept, not a secretive federal acronym. Keeping them quiet is almost impossible.
The known surviving adults that actually saw the shooter did not see the rifle. You can find interviews online. one of them only saw his legs.
Take a look at the following link
https://www.youtube.com/watch?v=oJkvB9goPAA
Do you think this report where they stated they heard from federal officials was wrong? I doubt it. I think it was the truth until the Holder led justice dept got involved, that's when the narrative changed to a Bushmaster. Research the interview with the coroner, see how uneasy he is when questioned about the weapon involved. It's very clear he is not comfortable with what he is saying.