Variability in OAL?

[Oldtrader3]

Well I tried! Everytime I try to get people to think outside of the box and think about the physics of something like tangent versus secant ogive pressure curves, they home right back into what they know and start talking about graduated setting depths off the lands, the same conversation that we always have (yawn)! I even referenced the two charts shown because they illustrated totally different curves but nobody took the bait or probably even read my post!

The differences have been quantitified but it requires doing a little reading, research and maybe investing a little time to learn what the quantifiiable pressure curves look like. I don't have the piezo crystals, scopes and equipment to to the work and print the curves at home as I am retired. It would be nice for someone else to look into this and maybe find a better way! There are other engineers on this site and this is a bullet manufacturer's site? For instance, why do some ogives in some cartridges and bullet weights shoot better at factory COAL?

 

[RiverRider]

It did not escape my notice, OT. I noticed that the upper chart is for a round nosed bullet, and I noticed that the lower chart agrees with what I've been saying all along. The pressure is highest in the portion of the PLOTTED curve where the lands are engaged or at least nearly engaged, and as OAL decreases the pressure decreases to a point that appears to be about 6500 psi lower than when the bullet is jammed into the lands---and this is very close the 7200 psi figure that QL specifies. After that point, as OAL continues to decrease, pressure begins to increase. And as I have said, I do not doubt that bullet design matters in the way these curves appear.

I do not intend to continue posting in this thread. Whoever wants to argue that seating ever deeper will result in decreases in pressure can just have at it, but I would suggest that a logical explanation should include the reasons that pressures in a rifle cartridge would behave in a way that is opposite from a pistol cartridge as OAL decreases.

 

[woods]

RR, I am the one who posted the graphs and in several posts I have not said that pressure will not start to increase at some point. My point is that 99% of the time, us reloaders here are dealing in seating depths between KTL and .250" off and in that range pressure will progressively decrease with increased seating depth in almost all if not all of the time.

Your first post

With all else remaining unchanged: seating deeper will increase pressure, and conversely seating longer decreases pressure---until the bullet is in contact with the lands. Pressure will then suddenly become about 7000 psi higher with must jacketed bullets.

was stated as an absolute and is just not correct.

And yes, Oldtrader, different bullets will get different results in how quickly you reach the point where the pressure starts to increase again. But that was not what was in question. If you have some data on the way different bullets react to OAL differences, we can discuss.

 

[RiverRider]

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[Oldtrader3]

RiverRider, you missed my point. I certainly do not wish to discuss incremental seating depth in any greater depth what has already been said here 10,000 times. I have done the work myself while reloading for 50 years and do not believe that is is as simple a model as all that frankly, just my opinion. Plus there is a harmonic vibration component in seating depth as well and that probably has more to do with seating depth choice than anything else.

Woods, I know intuitively, from designing cam pressure angles for 30 years, that different pressure angles give different pressure curves as they engage the rifling and that the pressure ramp up for trapezoidial and secant curves is totally different than for harmonic or tangetal curved ogives. Bullets come with all of these curvatures but only use the last fractional tail of pressure angle for land engagement transition from throat to rifling but the pressure angles are different. That is why you need to redevelop powder loads for bullet ogive profile changes.

All that I was asking about was the pressure angle variations and seating depth effect that these different bullets have in pressure curve development in the throat.

Besides anyone who moves a bullet .010 inches and gets excessive pressure, either hit the lands with the seated bullet and/or was already over safe pressure to begin with, IMHO.