Free: Contests & Raffles.
get a honed barrel, skip the lapping
Buds has one I believe. I just looked it up. XBOLT Stalker Long-range 300 wm with muzzle brake. Is that the one, it's there.
I went to the Browning catalog to see what the difference between the Pro and the Pro long range was. If it was me I'd be more concerned with how the barrel was chambered, screwed on and bedded over being lapped. On the topic of "proprietary lapping" I don't know what other method there could be other than hand lapping or fire lapping? Only other method I could think of would be using a laser to lap. I've shot Shilen hand lapped barrels and don't think one could shoot better than that one.
I don't think the 1:8 twist is going to be better than a 1:10. If you check out the Berger stability calculator you can see that 1:10 twist is fine for even the longest of their target bullets. I'm surprised Browning is using 1:8 twist in the 300wm...........I do agree though, that the heavier barrel would be nice.