Charlie and Dennis,
1. The shot cup SHOULD keep any shot within it due the inertia of the shot and the movement of the wad down the barrel. Shot may however slip between or wear through the petals of the shot cup and score the bore.
2. Any steel shot will score a barrel lengthwise if it comes in contact with the barrel. If you load steel #7 shot in your favorite target wad please expect to scar your barrel. If you do not then consider yourself lucky! Wads hard and thick enough for steel shot are required!
3. First generation steel was loaded with inadequate wads that allowed shot to slip or wear through the wad petals. Roster patented a wad in 1987 that eliminated most of this problem for modern high carbon barrels. Your Parkers have LOW carbon barrels and are at risk if you shoot steel in them! First generation steel loads will likely cause problems in Parkers.
4. First generation steel shot would also rust into a steel slug which split a lot of barrels.
5. Plastic wads will not wear a barrel because the barrel, no matter how soft (low carbon content) the material is, is in fact orders of magnitude harder than the barrel. It would be like rubbing your palm on hardwood. Your skin will wear off before the wood is worn away. This is why plastic wad material must be occasionally removed from older, non chrome barrels. Older barrels are rougher than modern chrome lined barrels.
6. Everything is a spring per Young's Modulus,
http://en.wikipedia.org/wiki/Young's_modulus . Even steel shot will compress under enough force. That does not happen in older barrels. The barrels must strain or temporarily expand to allow the shot and wad to pass through. ALL barrels do this even with lead shot. If however a barrel expands to the point of stress, PERMANENT deformation will occurred. Obviously barrels with less choke constriction will pass steel easier. Small steel shot MAY readjust itself to pass through barrel constriction. Smaller steel shot will adjust easier than large shot. -- Note: The constriction is what most refer to as choke. Choke is the performance of the barrel, constriction, shot, wad, velocity, and perhaps other factors and is measured by the percentage of pellets striking a 30" circle at 40 yards. That is of course another subject.
7. Large steel shot traveling through tightly constricted barrels, even new barrels, may likely cause damage. Something’s got to give!
8. Bridging occurs when the velocity of a column of shot is momentarily is retarded and the expanding gas slips by. This will cut the barrel and cause a split.
9. Ring budges also occur when a column of shot momentarily is retarded. This occurs in thin barrels but could occur in a thick barreled Remington 870!
10. Some of us believe that with enough wad petal thickness and hardness that steel shot can safely be shot in old, low carbon barrels. Research will be conducted over the next few years (Yes years unless someone want to hire me full time to do this!) to provide evidence via controlled tests that certain barrel materials, of certain thicknesses, with certain amounts of constriction, using certain wad thickness/material/hardness, fired at certain velocities/pressures, are safe to shoot steel shot of certain size(s). The results will be published.
ONCE AGAIN: Shooting steel shot in older, low carbon barrels is risky! Certainly the less choke the better when shooting steel in older guns. Wad thickness and hardness is believed to be a significant factor in protecting a barrel from damage. Mylar liners around shot within a really good wad would be best. Do not shoot older factory steel shot loads in Parkers!