Most of the commercial Plastic shot cups are about 13 gauge in size, to fit inside the thick wall of modern plastic 12 ga. shotshells. In a true 12 gauge MLer, they simply are too loose! Using a card wad under them, putting a cardboard collar inside the cup to further protect non-toxic shot, and then an Over shot Card on top to hold the load in the barrel makes them work. Then grease the bore with a greased cleaning patch to keep the plastic from rubbing off against the bore, and you have cured all the ills, sealed gases properly for consistent MV, and improved the pellet count in your patterns.
I personally prefer NOT using plastic shot cups, and the recent information here on making cups using heavy Fabric Strips is very promising. I do not like having to clean either lead or plastic out of my MLing shotgun barrels.
Taking only OS cards into the field simplifies what you have in your pouch, and that also appeals to me. Other than dove hunting, Hunting around here is a cold weather proposition, where fingers get cold, and picking through Stuff in the pouch is a ROYAL PITA!
Oh, at the very short range of most Sporting Clays shots, blow- by is not much of an issue. At the one or two stations where full choke is needed, the choke tightens the patterns enough. I don't find too many sporting clays folk picking up their spent plastic shot cups- nor Skeet or Trap shooters, for that matter. If they did, they might be more alarmed at the condition of the shot cups, and what is going on in the barrel.
I have picked up hundreds- if not thousands of wads-- my own, and others left on the field after a shooting match, and examined both good and ball plastic shot cups. You are correct that many modern cartridge guns are back bored and vary widely in bore diameters. But, Back Boring actually helps most of these wads seal better.With Smokeless powder, most of the powder is burned inside the casing before the Shot cup exits it and enters the bore. The soft skirt on the bottom of the cup is generally wide enough to open to bore diameter, even in most of the back bored barredls.
My guns have the throats relieved using a tapered reamer, that gets rid of the shoulder at the end of the chamber before the throat begins. It also gets rid of the factory-cut,steep-angled, shoulder, and replaces it with a long taper. The back boring, and the long tapered Throat does MUCH less damage to the plastic hull, and to the shot load inside.
The condition of my After throat-relieving plastic shot cups are much improved over the "Before" samples. The patterns have lost their "Strings" and are more round, and have more pellets in the patterns now.
The last time I had problems with plastic wads or cups was with a deer "slug" barrel I had ported at the front sight to keep the muzzle rise down in recoil. The sharp edges of the ports were cutting plastic as it passed the ports. I had a friend round the edges of the ports on the inside of the bore, and that eliminated the cuts. I get nice, one-hole groups with deer slugs out at 50 yards, now. :thumbsup:
