YOu can get away using plastic cups with BP IF you first Grease the barrel before loading the cup and shot down it. The grease allows the plastic to slide OVER the metal, rather than rubbing off against it. Ergo, NO plastic residue. :wink:
I use a cleaning patch liberally coated with Young Country 101 Lube, a predecessor to Bore Butter, and Wonderlube, to grease the bore. Its a natural lube made with vegetable oils, and wax. I stumbled on this fact by accident. I had seen and heard of shooters at the Sgt. York Memorial Chunk Gun Shoot, held every March at Pall Mall, Tenn., greasing their bores after loading the PRB down on the powder. When we tried the before and after of greasing the bore in a rifle, we got higher velocities, and lower SDV. Hadn't expect the lower SDV at all. Tried it in several guns to see if it was unique to the one gun tested. Worked in other rifles, and other calibers.
So, one day when my brother and I were shooting a trap match at my club, we decided to try the technique in our smoothbores. I was not using plastic cups, then, but found that the bore became " slicker" ( if that is a word) after every shot, and was easier to clean. So, one day, for clucks and yucks, I greased the bore before putting a plastic cup down the barrel. It eliminated the need to use a bore brush to remove the plastic, with a patch and modern solvents over the brush to dissolve and scrub the bore clean. I was impressed. I was all prepared to spend a good half hour scrubbing that set of barrels with my bore brush and solvents, as I have done before when I used plastic cups during some testing. ( I still have not used them for hunting, yet.) :shocked2: :hmm: :thumbsup:
Oh, just to be clear, I use an OP wad over the powder, but I have not yet cut off the bottom assembly of the modern shot cups, and just pushed the entire cup down the barrel. The OP wad acts as a firewall, and protects the half cup that faces to the rear of the shot cup, from being burned, or melted. I do not rely on that half cup to seal gases. When I tried that years ago, the cup was badly burned. Those shotcups are probably actually 13 gauge, and not a true 12 gauge, so they can't seal gases well behind the skirt, or reverse half cup. I am going to have to hit the pattern boards again, using cut off cups, and then use tape, and index card stock inserts to see if I can control the size of the patterns at 25 yds., and longer ranges.