In the flashback, the handgun David's brother loads and cocks is a Beretta. After young David comes back and gets the gun from the drawer, it changes from a Beretta to a Colt.
David's brother was killed shortly after the Vietnam War. That means that his body should have been in a high state of decomposition, but in the closing scene, he appears "fresh".
Philip Murch's tombstone lists his rank as Lance Corporal. When he returns from the dead, he showing wearing an Army uniform. Lance Corporal is strictly a rank in the Marine Corps.
The guy David initially loads and puts to his head is a Beretta M9, which did not exist until 1985. Also, the safety is on (pointed down), which is why the hammer is not cocked after the gun is loaded.
The talk show host Marty Clark says that David Murch's brother Phillip is buried in "Silver Springs, Maryland." Although commonly but mistakenly called "Silver Springs," the city is actually "Silver Spring."
The first Marine to return from the dead tells the Air Force sentry, "At ease, soldier." Air Force personnel are properly addressed as "airmen".