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Scharfschuetzer · April 13, 2012, 11:06 AM

TFL,

As noted above, each manufacturer has their own equipment for measurement of pressure and velocity. Variables in this equipment and its calibration can all cause slightly different reading from lab to lab.

No two rifles shoot the same it seems and variables in just the rifle barrel and components can and do make a difference in the results, even in the tightly controled test barrels of ballistic labs.

A few variables:

Barrel length
Chamber dimmensions
Throat or leade dimmensions (I.e. 5.56mm v. .223 Rem)
Rifleing (polygonal or conventional)
Actual bore dimmensions down to the 10,000 of an inch
Powder lot
Primer lot and manufacture
Brass manufacture and lot (a heavy for calibre case will have less internal volume than a lighter case of the same diameter or length)
Bullet design for a given weight. A 180 grain VLD (low drag) bullet will have less bearing surface than a 180 grain round nose bullet.

Given all these variables of reporting pressures and velocities, you can easly see why it is advised to start low and work up to your weapon's maximum load.

The pictures of the primers posted by Will Lee show primers that exhibit what looks to be a safe pressure. While primers are not an exact measure of pressure, the fact that they have a rounded radius around the face and that there is no cratering (flow of metal into the firing pin hole) indicate that his load is within a safe operating pressure even though he states that it is slightly over the book maximum load.

Of course primer manufacture can and does change these indicators. A hard primer (those for semi or full auto weapons) will usually show pressure signs later than a primer made of softer metal.

All good loading manuals will state the make of primer and case used in their loading data as they are an important part of the pressure measurement.

April 13, 2012, 11:06 AM	#6
Scharfschuetzer Senior Member Join Date: January 28, 2012 Location: Puget Sound Posts: 293	TFL, As noted above, each manufacturer has their own equipment for measurement of pressure and velocity. Variables in this equipment and its calibration can all cause slightly different reading from lab to lab. No two rifles shoot the same it seems and variables in just the rifle barrel and components can and do make a difference in the results, even in the tightly controled test barrels of ballistic labs. A few variables: Barrel length Chamber dimmensions Throat or leade dimmensions (I.e. 5.56mm v. .223 Rem) Rifleing (polygonal or conventional) Actual bore dimmensions down to the 10,000 of an inch Powder lot Primer lot and manufacture Brass manufacture and lot (a heavy for calibre case will have less internal volume than a lighter case of the same diameter or length) Bullet design for a given weight. A 180 grain VLD (low drag) bullet will have less bearing surface than a 180 grain round nose bullet. Given all these variables of reporting pressures and velocities, you can easly see why it is advised to start low and work up to your weapon's maximum load. The pictures of the primers posted by Will Lee show primers that exhibit what looks to be a safe pressure. While primers are not an exact measure of pressure, the fact that they have a rounded radius around the face and that there is no cratering (flow of metal into the firing pin hole) indicate that his load is within a safe operating pressure even though he states that it is slightly over the book maximum load. Of course primer manufacture can and does change these indicators. A hard primer (those for semi or full auto weapons) will usually show pressure signs later than a primer made of softer metal. All good loading manuals will state the make of primer and case used in their loading data as they are an important part of the pressure measurement. __________________ Scharfschuetzer US Army Distinguished Rifleman Washington State Distinguished Rifleman NRA Police Distinguished Expert Last edited by Scharfschuetzer; April 13, 2012 at 11:12 AM.