The closest example we have to a conflict within the U.S. that we could compare to today is the Civil War. Obviously the technology is far more advanced than 150 years ago but there could be some similarities.
Confederates were able to get their hands on weapons that they already had, captured, smuggled in, or traded from foreign powers.
Not all of the union military stayed loyal and many future generals joined the confederacy. So counting on the American military to remain undivided isn't a guarantee.
Those that remain could be loyal or have a hidden agenda.
I still feel the average American couldn't last a month against the government because of their military capacity but there definitely could be some potential issues.
If we're talking about a citizen getting their hands on an F22, might as try just be a bit of scrap metal. An anti tank weapon, possibly a different story.