Peter was one of two 53rd Mass soldiers to volunteer for a "Forlorn Hope" regiment to make a third and "final" assault on the works at Port Hudson. Fortunately the assault never took place, as the Vicksburg garrison surrendered on July 4th, the planned day of the assault, thus delaying and effectively cancelling the attack. Henry Willis opines in the regimental history that it would have almost certainly failed. Papers at the Warwick library indicate that Peter reenlisted after the 53rd mustered out. Indeed, both the 7th and 27th Massachusetts regiments list a Peter Dyer amongst their ranks. The 27th was from Springfield and seems the more likely of the two for Peter's reenlistment. The 7th was from Taunton, in the eastern part of the state.
Image from the collection at the Warwick, MA Public Library