Silver, I've been using Blasers since 1995; they're hard to beat. I'm retired now, but work the Dallas Safari and SCI Shows for Blaser. I hunt Africa once or twice each year, as well as Alaska, Argentina, Mexico, Canada, and the US. I usually have 7 or 8 Blaser barrels sighted in with scopes on, in my safe waiting for a hunt. Just select the scoped barrel, put it in your break-down case with the stock, and off you go.
Although you have a large-caliber double rifle, I would not pass up a large caliber barrel for your R8. I love to hunt with doubles, but because it's challenging and sporting, not because it's a better weapon. With a double, you add gun weight, the use of open sights, and limited range. They're good perhaps for Elephant or early season Buffalo when the cover is thick, but in most circumstances you will limit your capabilities without a large caliber magazine rifle.
I'm not sure what the limiting factor is in your country,...number of firearms owned or number of barrels. If the ownership of barrels is not limited, you have nothing to worry about. I'd get a smaller-mid caliber like a 6.5x57, 260 Rem, or even a 243, then a 300 Win mag, a 375 for larger game (equals the 338 when lighter bullets are used), and a 458 Win or Lott. You're covered!