I shoot a Disc Extreme, and so far I've been using the Knight 375 gr. saboted bullet. I found a sporting goods store in Minnesota about 5 years ago that invested heavily in the .52 and couldn't sell them. I bought out their entire inventory except for rifles, and still have a pretty good supply of those bullets. This year I counted up what I have left and I can see the end coming, so the search has begun for a good lead bullet. MMP makes the sabots for Knight but since Knight owns the tooling you can only buy saboted bullets from Knight. Who knows how much longer they will be in business.
So, I found a guy on Gunbroker who custom casts lead bullets for the .52 and bought a supply from him. I haven't shot them yet, but they are a great looking bullet and sometime in the next couple weeks I'll get a day free and head to the range. Parker Reproductions also makes a good looking lead bullet in .53 which weighs 530 grains. It's pricey and if I can get the Gunbroker bullets to shoot well I won't bother with the Parkers.
If I get decent results with the lead bullets I'll post my results.
