JDBraddy......
sorry that this may turn into an argument but I'll continue to attempt to answer your question. Because it's a good one. and there IS AN ANSWER, you have given the necessary information and THERE IS AN ANSWER.
You ask "WHY PPC GOOD" and WHY others NOT good all same-same??"........Right? And the answer is, "Because Your PPC Die Fits Better"
But all of this is begging the REAL question which is "WHY STRAIGHT AMMO?"
And again, there is an ANSWER.
Let's change gears for a second. Let's ask "
why those big fat tires on a drag car?" WHY??? The WHY is what's important. The reason for those big fat tires is to put more rubber on the road, to promote "hookup"...... and any racer knows that once you get the hookup you got all sorts of other stuff to deal with but you will always need those big fat tires.
So WHY "straight ammo?" Why all this talk of concentricitty? WHY does it matter? And there is an ANSWER to your question, not an opinion, not "something alwina made up" but an ANSWER. The answer to your question is "to promote alignment of the bullet to the bore" and furthermore "to promote consistent VIBRATION PATTERNS in the system." Now many here may disagree with this last statement re vibration but one must ask those who disagree, "have YOU ever managed to achieve true. tunable accuracy from something OTHER THAN A PPC???"
I have.
And many others have.
And those who HAVE actually done it will tell you that they achieved this accuracy through FIT of the loaded rounds to the rifle system. It has been shown many times and in many ways and by many people 'not alinwa' that FIT is important. And any of you shooters out there who care enough to actually TRY IT can easily make sloppy, loose reloads for your accurate rifle (probably a 6PPC) and see for yourselves that the most accurate rifle will not shoot to potential with sloppy, loose ammunition.
Here's where your problem lies, here's where your inaccuracy originates, here's the DIFFERENCE between your 6PPC and the three other rifles mentioned.
FIT.
That's it.
Your 223 and 338 have big sloppy chambers, they do not have the FIT of your PPC. And herein lies your problem.
Let's just say you brought your stuff over to my house and we "straightened" all your reloads until they mic'd out and ran out on the gauges like little machined missiles. (I have all the equipment to do this...... I've got two concentricity gages with freaking LEVERS on them so's you can bend the ammunition like Savage straightens their barrels. I've got 11 presses and dozens of dies and blocks and gadgets and gizmos AND a lathe and a mill..... we can MAKE your ammo straight!!!) or let's just say that you modify your own stuff (or, most likely, CULL your own stuff, it's cheaper!) until you get a bunch of "straight like your PPC cases.
Now you have cases just a straight and perty as you PPC rounds. They should shoot gooder eh???
Ya THINK???
nope.
They will not shoot any gooder.
Because they don't FIT any better..........doesn't matter how STRAIGHT they are they're still just laying there in that over-sized chamber with air all round them and when you drop the 60,000lb hammer all HELL will break loose. Long before that bullet moves that case will be slamming and jumping around like a caged weasel......because it's too small for the chamber.......
So, here's where the rubber hits the road,
you want to try something that will actually WORK??? First of all you must test to see if the 223 and 338 are actually CAPABLE of good accuracy. And here's how you can do that. First problem is FIT, so we make FIT........MAKE some good cases by doing whatever you have to do to achieve a crush-fit fireform and produce a set of cases that are STRAIGHT and that fit your chamber perfectly. It won't cost much. You can absolutely make cases just as straight as your PPC.
Proper fireforming MAKES straight cases. It's the only way.
Now we get to the second problem, neck diameter. Accuracy is dependent on a series of alignments among other things. One of the necessary alignments is that of the bullet to the bore. If your neck is oversized you will not be able to achieve really good alignment. Your bullet
WILL BE canted as much as the neck clearance allows. This is a simple mechanical fact. The bullet doesn't magically float into the hole..... it's DRIVEN in by 600lb of turbulent pressure. And it will cock to one side. But all is not lost! There is a reason short/fat bullets with stubby ogives have the reputation for being accurate. They will "self-straighten." Starting a football straight is much harder than starting a soup can straight....... So shoot soup cans, get some short Match bullets, something like 50-52gr for the 223 and maybe some MatchKings for the 338 (you're kinda' stuck here) but do what you can to use light/short bullets.
Or build some tight-neck cases.......
And now you've got STRAIGHT cases that FIT as good as your PPC. Your system can achieve it's full potential. Get some necksize-only Wilson hand dies, using these dies you can load and shoot using straight, fitted cases...... just like your PPC.
For a while.
Until the brass gets too tight to re-chamber.
Which brings us full-circle as to WHY FITTED DIES EXIST! Fitted dies exist to MAINTAIN straight cases. Fitted full length dies exist because they allow you to shoot as accurately as your rifle allows, more than once or thrice.
I sincerely hope this gives you something to think about.
If not, I meant well
al