First of all, you bought a sabot gun with a fast twist. Normally, 350 gr conicals are fond of slower, medium twists from 1-38 to 1-48". Your rifle will shoot a conical well out to 100 yards, but the optimum conical bullet size to accomplish this is 430-460 grains.

Despite all this, you should still be getting pretty decent groups out to 50 yards with your combination of twist & bullet. Cayugad's mention of around 70-75 grains 777 is sound advice. So is a clean, dry barrel with no patched lubes like Bore Butter in your barrel. The lube that's on the Maxihunter is all the lube you need.

Swab your bore every 2nd shot with Windex to start.... one dry patch after each swab using both sides of the patch. The idea is to keep your bore just a tad dirty. Most muzzleloaders shoot better/more consistent with a fouled bore.

Also make sure your powder is not old or moisture-driven.... that will affect consistency & accuracy. Powder must be poured slowly for best results & bullet seated with the same hand force every time. Make sure your open sights or scope is tight. If the scope is a used model, remove & replace it for troubleshooting purposes.

You should also consider another bullet to try & possibly a wonderwad between the powder & bullet. Lead Powerbelt conicals are cheap to purchase at ...
http://www.midsouthshooterssupply.com
Your Bl. Diamond shoiuld shoot the 295 grain good. If you desire a sabot, start off with the T/C Cheapshots 240 gr. with around 90 grains of powder.

Lead Powerbelts are around $11.50 for 20 at that site. Click on "sabot/bullets" to find them after clicking on muzzleloading. Why they are listed under sabots instead of conicals is a real mystery to me. Also make sure your new bore is really clean. Most blackpowder solvents are weak.... use Birchwood Casey Bore Scrubber Two In One, Butch's Bore Shine or Shooters Bore Cleaner. Use one patch of solvent - one dry patch - one patch of gun oil - one dry patch..... keep repeating this step until clean.

Accomplish all this & you should be on the right track to success. Good luck & keep us informed!