Varies with barrel length AND barrel width - if a straight profile barrel, like T/C or Lyman uses, OR profile if a swamped barrel.
But to give you an idea, usually if of the SAME model and length, a 5-cal would be a good 1/2-pound or more LESS in weight that a comparable 50cal model.
The Kibler Colonial Rifle of 42" swamped barrel wighs a hefty 9.0 pounds. I had a Don Bruton 54 Boone rifle of slimmer 38" barrel that was a dream to carry, at less than 8-pounds. The weight of the Colonial Rifle is why Jim came out with the shorter and lighter 'Woods Runner' model.
As someone else offered, the school or year of model, also determines the weight. Like early rifles (the Colonial Rifle certainly being one) had much heftier and wider buttplates, so that addes overall weight. Golden Age longrifles slimmed down and are trimmer and lighter.