Using loads for heavier bullets

[zacharoo]

I have lots of W571 powder. I also have a good bit of HS7. I like the powders for 255 gr cast 45 colt loads. I have a lot of cast bullets that are 290 to 295 gr. but no data on it with either powder. Can I use data for the afor mentioned powder for a 300 gr. bullet(all cast). How do I go about it safely? Start with the lightest load and work up. What if the powder charge is to small for the lighter bullet. Any of you ever try this? I NEED A LITTLE ADVISE!  HELP!!!
Zacharoo

[helotaxi]

If the 290-295gn bullets are a similar design to the 300gn, you can consider it essentially interchangeable.  If you thumb through the Speer, Hornady and Sierra manuals you'll see that they lump jacketed bullets with 5gn differences in weight together quite often.