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A perforating device has an elongated tubular housing and a loading device supporting a plurality of shaped charges. The loading device and the shaped charges are located inside the housing. A volume reduction device is adapted to fit within the loading device and to conform to the profile of the sh

A perforating device has an elongated tubular housing and a loading device supporting a plurality of shaped charges. The loading device and the shaped charges are located inside the housing. A volume reduction device is adapted to fit within the loading device and to conform to the profile of the shaped charges thereby filling open volume in the housing.

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1. A method of perforating a wellbore, the method comprising: providing a perforating gun having a tubular housing and shaped charges therein;selecting a desired resultant transient pressure condition at a location in the well bore;calculating a ratio of total explosive mass to free internal volume

1. A method of perforating a wellbore, the method comprising: providing a perforating gun having a tubular housing and shaped charges therein;selecting a desired resultant transient pressure condition at a location in the well bore;calculating a ratio of total explosive mass to free internal volume in the perforating gun that will achieve the desired resultant pressure condition in the well bore;selecting a volume reduction device adapted to fit within the housing and conform to outer profiles of the shaped charges to thereby achieve the ratio of total explosive mass to free internal volume in the perforating gun; andinserting the volume reduction device into the tubular housing. 2. A method according to claim 1, comprising calculating the ratio of total explosive mass to free internal volume in the perforating gun based upon at least wellbore fluid pressure, gun detonation pressure, charge type, and perforating gun configuration. 3. A method according to claim 1, comprising inserting the perforating gun into the wellbore and detonating the shaped charges to achieve the desired resultant pressure condition in the well. 4. A method according to claim 1, comprising selecting one or more additional components for insertion into the perforating gun to further reduce the internal gun volume. 5. A method according to claim 4, wherein the one or more additional components comprise a volume reduction sleeve. 6. A method according to claim 1, comprising selecting a volume reduction device that is a single unified part having a plurality of openings therein to accept a plurality of shaped charges. 7. A method according to claim 1, comprising selecting a volume reduction device that comprises two separate parts, each part being tubular in shape and each part being located adjacent to one another end to end. 8. A method according to claim 1, comprising selecting a volume reduction device that comprises two separate halves that come together along a longitudinally extending plane to together form the volume reduction device. 9. A method of assembling a perforated gun system, the method comprising: identifying a target underbalance condition at a location in a well bore;assembling a perforating gun to achieve the target underbalance condition at the location in a well bore by (a) determining a total explosive mass of one or more shaped charges in the perforating gun;(b) calculating a ratio of total explosive mass to free internal volume of the perforating gun that will achieve the target underbalance condition;(c) selecting one or more volume reduction devices to achieve the free internal volume of the perforating gun, thereby increasing the ratio of total explosive mass to free internal volume of the perforating gun; and(d) inserting the selected volume reduction devices into the perforating gun. 10. A method according to claim 9, comprising generating the target underbalance condition at the location in the wellbore by activating the perforating gun. 11. The method of claim 9, further comprising providing a loading device in the perforating gun, the loading device having a plurality of openings for accepting and supporting the shaped charges. 12. The method of claim 11, wherein the volume reduction device comprises a plurality of openings to accept the shaped charges. 13. The method of claim 12, further comprising locating the volume reduction device inside the loading device and aligning the openings in the volume reduction device and the openings in the loading device. 14. The method of claim 12, further comprising locating a volume reduction sleeve on an outside surface of the loading device, wherein the volume reduction sleeve has openings that are adjacent to and correspond to the openings in the loading device and in the volume reduction device. 15. The method of claim 12, wherein the volume reduction device is a single unified part. 16. The method of claim 12, wherein the volume reduction device is made of two separate parts, each part being tubular in shape and each part being located adjacent to one another end to end. 17. The method of claim 12, wherein the volume reduction device is made of two separate halves, each half being crescent-shaped and each part coming together along a longitudinally extending plane.