I had a dabble with these and a few others, and they have without doubt a very consistant weight and also, pretty damb useless past 15 yards, they all group like sh*t, and dissapear when wind is in play.
but the worst thing is, they will clog up the rifling, which can take an age to remove, (phosphor bronze brush recomended) and you wont get anything to shoot straight until you have removed all the zinc thats been welded into the rifling.
Good Luck, and Happy Scrubbing
PS. be very careful not to damage the Crown of the barrel when cleaning.