Trespassing on federal property is usually considered more serious than simply trespassing on someone else's property.
If you get on federal property and go past signs that say no trespassing then you might only get a warning to not do it again or if you do then you will get arrested.
However I've seen some people on Youtube go on Federal Property and get arrested right away and then they had to go to court.
Usually the person gets off with a fine or 6 months in jail for the first offense of trespassing on federal property.
If you do then trespass on federal property again after the first time then you could face up to 5 years in prison and/or a fine of $20,000.00 or more.
If you trespass on federal property with a weapon then it usually almost certainly becomes a felony even for the first time and if you trespass on federal property with a weapon you'll get arrested.