1 in 24 twist is not a fast twist for shooting big heavy conical bullets. 1 in 18 would be very much the norm and the trend is starting to go towards 1 in 16.
For a rifle like this I like the bullet to be close to bore dia. It should not start hard, and should go down the bore with just a little more than the weight of the ram rod.
You did not mention the use of a wad. Try a card wad, or wool felt wad, or a combination of both.
You are using a lighter bullet, with a slower twist barrel. I would drop the powder charge.
You did not mention what you intend to do with this rifle or what ranges you are wanting to shoot.
I would drop the charge down to 70 grains, use a wad with a properly sized bullets and see what you can do. I am assuming that you are not going to shoot it past 500 yards. 70 grains is plenty of powder.
Your paper on the bullets can make a difference in your groups. I do not lube mine and I tried 5 or 6 different kinds of paper before I found one that grouped well. The paper I settled on is air mail paper that is really old, bought it on ebay.
I do not have any idea on what type of steel that Oregon uses for their barrels or if it is safe for what you are trying to do.
Rick Weber in TN is a buddy of mine, he makes the platinum nipples. Not sure what he is charging but they will be around $45 each.
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What are you using for sights? peeps, scope or open?
Fleener