Optimal Barrel Time (OBT)

Optimal Barrel Time (OBT) Is a theory that predicts certain nodes of muzzle diameter stability during cartridge combustion. This theory is NOT about “Barrel whip/harmonic” or “Running Vibration”!

Optimal Barrel Time (OBT) concentrates on the diameter at the muzzle at bullet exit time, caused by the combustion pressure longitudinal waves in the barrel material.

The concept of Optimum Barrel Time (OBT) has great potential to predict these oscillation nodes. GRT's “Bullet Lead Time” (BLT) is the predicted time the projectile remains in motion wihtin the barrel after ignition. OBT nodes can therefore be compared with the predicted Bullet Lead Times (BLT) calculated by GRT, and a correction can be applied to force the prediction to match the OBT nodes.

GRT will precalculate the OBT nodes, by using the barrel length measurement the user enters in his cartridge file. The GRT user should save this data to a .grtload file. A loading ladder file is preferred.

Potentially accurate loads are achieved by adjusting the amount of propellant until the calculated BLT matches an OBT node. The true barrel length and actual measured velocity must be used to calculate the BLT/OBT match. This tool automatically performs that matching process, saving user time and frustration.

The Optimum Barrel Time (OBT) concept was developed by Christopher Long in 2003 and the complete document is available here:

http://www.the-long-family.com/optimal%20barrel%20time.htm

• OBT claims to achieve an accuracy of at least 1 MOA.
• From there you have to vary only a little up or down in the charge to get the best accuracy for that node.
• The advantage is that you save time, labor and money instead of performing extensive loading ladders.

Requirement: You have selected a starting charge and have the actual Bullet velocities measured. The starting charge is usually at the beginning, or in the middle range of recommended manufacturer loads. Experienced users can use any load they have confidence in according to their own experience.

• Step 1
  
  Open the OBT tool and your current load file and (set load ladder) powder quantity should be displayed.
  Now enter the real measured bullet velocity of this charge here:

• Step 2
  
  Click on the button
  [Compute OBT & charge adjustment (autom. calibration)]

• Step 3
  
  Calibration of the simulation to the real measured speed is now done by the OBT-Tool automatically.
  (Without this OBT Tool, the users would have to a attempt to manually set the powder coefficients k and Ba until the simulation velocity matches the measured velocity.)
  
  Note: if the calibration fails due to a too high Initial Pressure (IP), then just go back to your load file, lower the IP and try again!

• Step 4
  
  The OBT-Tool now calculates on the basis of the calibrated BLT (bullet lead time at 10% pmax) the valid OBT nodes. You can now select the desired node from the option button to determine the proper powder charge required to match that node.
  
  It is important to know that the lead time (BLT) is inversely related to changes in powder quantity. Also, the user has the option of including so-called half nodes. It has been reported that some users have discovered accuracy around “split, or half nodes”. The GRT OBT tools provides this option for the users who desire to explore this observation. This may be weapon construction dependent.
  
  The Half-Nodes are a GRT-specific extension of Christopher Long's theories by Gordon and are based on the idea of Cassie Nienaber (GRT powder development team).

• Step 5
  
  Click on the button
  [Compute Node Charge Adjustment]
  
  The OBT-Tool then automatically calculates the powder charge, which adjusts the calculated BLT-Time to the selected OBT node.
  
  Note: if the calculation fails due to a too high Initial Pressure (IP), then just go back to your load file, lower the IP and try again!

• Result
  
  This is the amount of powder you would need to load your cartridges with, to get, in theory, a usable precise charge of at least 1 MOA.
  
  Starting from this calculated charge, you should now be within (+/- 0.1 to 0.2 grains) of finding a load that will exibit maximum precision.
  
  You can select another OBT node at any time and calculate the charge quantity again.

IMPORTANT: Please be sure to observe the warnings and error messages, if any!