I will share this from my own observations and findings, all empirical and no lab testing. I did as the OP mentioned, substituting some assorted 'fiber' wads from earlier times using load data that was published as quite safe in the original CFAA 12 ga. hulls [LUP was the pressure units used specified on some and psi on others and some were just published by the powder or wad maker(s) w/o pressure data] and quickly noticed that the hull length was stretched considerably after firing and also that the crimp folds were ironed out and disappeared! Had similar results w/several types of card & fiber wads, inclusive of Alcan's in several iterations, Federal, Winchester, &c. The worst offender was the Ljutic Mono Wads, which surprised me.
I also had similar results in some plastic 28 ga. hulls, most notably Remington's with the Riefenhouser(sp?) hulls that I had a lot of at that time, though it would tear the hull's mouth off altogether at times.
After giving it some consideration, I concluded that the pressures had to be much higher w/those fiber wads than plastic ones and in looking a bit at the fired hulls and cutting numbers of them apart, it is apparent that the inside of a previously fired plastic hull is considerably rougher than it was when new and smooth, so further concluded that the fiber wad is being compressed into that 'rough' surface, perhaps even before it gets moving good and that's what causes the hulls stretching and probably is also increasing pressures dramatically, though I never sent any to be tested. What I did was to stop the practice and use tested plastic wads in plastic hulls and load the fiber wads only in paper hulls. Paper hulls tubes remain smooth on their insides after firing and only experience burn through at the juncture with the brass after some firings. Have not experienced a problem of stretched hulls or ironed out crimps since.
Doubt it would happen with using a fiber/card wad in a new plastic hull for one-time use. Never noticed a fired RST card wad round loaded in a plastic hull to have stretched after firing. Others here could speak to that issue when using other brands of ammunition loaded in plastic hulls w/card &/or fiber wads.
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Different FWIW, issue encountered several years ago was with some older AA12 wads that had become brittle in their original packaging. The wad fingers would snap off if tested by hand. I was scoring very poorly with them even tho they 'sounded' normal. Went to the grease plate and discovered that they were apparently shattering and turning into molten plastic by the burning powder that was blown up into the shot and leaving the bbl. as a semi-solid projectile, much like the old classic 'whistler' one sometimes got from a paper hull loaded too many times w/with too many pin holes in it and everything from the brass head forward went out the bbl. usually making a whistling noise when it did so. Solution to that issue is to put any wads whose petals are brittle in the trash and do not load them.
