Shovel: Good advice from ivobb, and others. Don't try to re-invent the wheel! Be sure to follow the good advice already given and stick to published shotshell load data...only.
You can have some success with gas seals. Ballistics Products are well known for this product. But, if you want readily available wads for both buckshot and slugs, go no further than Federal. Or, their Claybuster equivalent. Then, load them in combination with straight-walled hulls, ie: Federal, Cheddite, etc. This combination works very well, for both buckshot and slugs..
You can use tapered wads/hulls too, if that's what you have available, ie: WW, Remington, etc. But, these are better suited to smaller shot sizes and may present some challenges when loading buckshot, or slugs.
As far as buffer is concerned, stay away from cornmeal, flour, or any other organic filler. Use only inert(plastic) filler. Be sure to include the weight of the filler as part of your load.
Here's a tip: Add a thin card wad to the top of the buckshot load column as the final step, before crimping the hull. This helps keep buffer from leaking out of the loaded shell. You can add this step when loading bird shot too. Helps in keeping the crimp tight and firm.
Also, if you don't have one already, pick up a copy of the Lyman Shotshell Reloading Manual. This will have any number of good load recipes for both buckshot and slugs. It won't matter whether this is a new, or or one of the older copies. The data should still be present.
Another good, online resource: Hodgdon Reloading Data Center. Plenty of data available, there.
Finally....don't overthink this! Buckshot and slug loading isn't that difficult. A lot of the data, components and technique used in loading regular birdshot, etc., readily transfer over.
Hope this helps:
Al