It sounds like you have a very nice rifle. I will have to come back to read the article fully later.

Everything I have heard about the Whitworth rifle points to it not liking cast bullets. The accuracy is not as good as it could be with them. I have heard of swaged bullets being the only projectile that will give the intended accuracy of the that type of rifle.
Those who have tried them vouch for this.

A swaged bullet is consistent where a cast bullet can vary in weight and density. The Enfield .577" was known for its accuracy and this was put down to the fact they used swaged bullets. Other countries cast their bullets so could not achieve the same level of accuracy.