[Unclenick]

I took awhile composing this, so I apologize for duplicating information posted while I was at it.

I would suggest the OP run the search for threads on the topic of pressure signs. There are many and none are wholly reliable, so you watch for the first one(s) to show up and decide whether or not it is safe to try to keep going? There is experience and judgment involved as some signs appear way too soon sometimes and don't show up until it is too late at others. In the specific case of a sticky bolt (one sign) the closest corresponding behavior in a gas gun is usually the extractor pulling too hard on the rim off the case because the case isn't coming out as easily as usual. Look for an extra-deep extractor mark in the case rim or a bent case rim. Some brands of brass are softer than others, so don't be surprised if the same load that bends the rim of a Hornady/Frontier case leaves a Lake City case looking fine.

All the common pressure signs below are unreliable in isolation, but all throw up a caution flag. Accumulating pressure signs is like accumulating evidence. Most modern guns withstand more pressure than SAAMI max, as the proof loads have to demonstrate at the factory, but I can't see beating the metal up for no reason. Be conservative. But if you detect, say, casehead expansion, yet your primers are very rounded at the corners, figure the CHE was premature (it can happen bellow 40,000 psi one time, and not until 70,000 psi the next). It's a question of looking at the whole picture.

• Primers really flat
• Primers crater (raised rim around firing pin strike; but also caused by oversize fining pin tunnel or undersize firing pin).
• Primers mushrooming (fired primer head wider than sides of primer pocket, forming a slight mushroom head)
• Primer piercing at firing pin strike (also caused by too much firing pin protrusion or striking force)
• Primers leaking around primer pocket (also caused by under pressure loads, though usually accompanied by high primer if low pressure is to blame)
• Primers fall out when you try to re-prime the case (case head grossly expanded)
• Casehead expands more than half a thousandth of an inch on firing*
• Case pressure ring (the shiny area just ahead of the case) expands more than two thousandths*
• Case primer pockets get loose in 5 reloadings or fewer
• Cases are hard to extract or bolt is sticky
• Case rims look like about to be torn off due to sticky case on extraction (also caused in gas gun by using too slow a powder raising gas port pressure)
• Cases swell into extractor groove (primers usually are loose and/or fall out)
• Cases can't be re-chambered after extraction (does not usually apply to gas guns owing to case stretching on rapid extraction; also can be caused by off-axis chamber if case is not reinserted with same orientation)
• Groups on target start to open up with charge increase
• Velocity fails to increase when charge is increased
• The gun sounds or feels different or the shock you feel in the stock gets worse
• And the obvious: Blown cases, smoke appearing where it doesn't belong, broken metal. You went way too far if you get here.

*see: http://www.shootingsoftware.com/ftp/...%2019%2004.pdf

I'm sure this list is not complete, but those are the common ones that come immediately to mind.


Pressure verses velocity.

Unless the barrel is very long relative to the powder charge size, the bullet is accelerated to some degree all the time it is in the barrel. Most during the pressure peak, least as it approaches the muzzle. That acceleration is determined by the average pressure applied to the base of the bullet during its time in the barrel, minus the friction that opposes that pressure. A pressure gradient develops between the bullet and the breech end. It is small when the bullet is near the chamber and not moving fast, but the pressure drop from the chamber to the bullet base grows as the bullet speeds up and the gas has to chase it to keep pushing.

So, pressure measured at the chamber always exceeds what is actually pushing on the bullet. Peak chamber pressure is not what determines final bullet velocity. The average pressure at the bullet base minus the friction is. That average is the result of integrating the curve of the projectile base pressure minus friction to determine the area under the curve, which, as mentioned earlier proportional to actual acceleration.

where: 
a = acceleration
P = pressure
Fb = friction
t = barrel time
m = bullet mass
A = bore cross-sectional area

      t
a = m ʃ (P/A)-Fb dt
      0

Here is an example of the same pressure in the same gun giving different velocities with the same powder:

.308 Winchester, IMI brand case with 56 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with Varget in gun with SAAMI minimum chamber produces:

18" barrel MV = 2575 fps

24" barrel MV = 2755 fps


.308 Winchester, Winchester brand case with 59.5 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with Varget in gun with SAAMI minimum chamber produces:

18" barrel MV = 2610 fps

24" barrel MV = 2797 fps


Here is an example of the same pressure in the same gun giving different velocities with the different powders:

.308 Winchester, IMI brand case with 56 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with IMR4198 in gun with SAAMI minimum chamber produces:

18" barrel MV = 2555 fps

24" barrel MV = 2712 fps


.308 Winchester, IMI brand case with 56 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with 748 in gun with SAAMI minimum chamber produces:

18" barrel MV = 2668 fps

24" barrel MV = 2856 fps


Note that you have to have the same barrel length as the factory or manual test load was fired with for listed velocities to come close to applying. Moreover, your bore cross-sectional area, which depends on groove diameter, bore diameter, and rifling configuration all affect the amount of bore friction as does the roughness of your bore. That friction is subtracted from pressure at the bullet base, as shown above, so it affects bullet acceleration. If any of these factors in your gun is different from those in the test gun used to develop the load, pressure and velocity achieved will be different in your gun. Add to that, the internal volume the case expands to in your chamber determines peak pressure, and with it, powder burning speed. That probably isn't identical in your gun either. I have fired the same lot of M72 match ammunition in different M1 Garands and found average muzzle velocities that varied from 2495 fps to 2610 fps. And that was with a statistically insignificant group of six guns. If' I'd had more samples to test, I'm sure that would have opened up.

Warning!:
Taken together, and ignoring the fact you likely have no way to independently determine the accuracy of your chronograph, the last paragraph illustrates why you must never rely on a chronograph to determine a load level. Your gun won't get the exact same velocities for the exact same peak pressure that the test guns did. Every set of chronograph instructions I have ever seen warns what an unsafe practice that is. The only time a chronograph is useful in indicating load limits is when its average readings, whatever they may be, fail to increase when your increase your powder charge. That is a pressure sign.

Below are some QuickLOAD plots to satisfy curiosity. The first is chamber pressure vs. distance the bullet base has traveled (red trace) and bullet velocity gained vs. distance down the tube (blue line). The second is bullet base pressure for the same load vs. distance the bullet base has traveled.





Nick