The middle barreled action in the photo is a 450 Rigby on a Brno 602 action. I will be completing the rifle this spring. Barrel is a special contour 24 inches long. 0.850” at the muzzle, 1.3 inches at the breech for a length 3 inches in front of the receiver ring. (I ordered the barrel with a 4 inch 1.3” diameter breech, trimmed and threaded to leave 3 inches out in front of the ring) By sheer luck, this is about the same contour as the one in 450 Ackley's post. The 450 Rigby is simply a 458 on a necked up 416 Rigby brass, or a 460 Weatherby brass without the belt, however you want to call it. So basically the same case as your 510 Wells. I would use the same contour on your 510 Wells. Top barreled action is a 404 Jeffery in a No. 6 contour 24 inches long. Bottom is a 458 Win in a Mark X factory barrel, cut to 20 inches. This is about a 4-1/2 contour, this is a light 458.

Where are you in B.C.? I am 30 miles south of Victoria, if you are ever across the border this way let me know in advance and you can see the barreled actions in person.