I am considering using some of the Flat Line 255.5gr in .338 caliber which will push my max effective range out several 100 yds. Any experience with these?
I haven't worried about 'claims' for a couple of decades. It's a waste of time worrying when a some short testing sessions will reveal the truth. Given the advent of the internet, many manufacturers limit their stretching of data due to the fact that they can be called out to a potentially huge audience.
The first thing I will point out is the OAL of the cartridge when you start to plan your rifle. Using the venerable

.338 Lapua as an example, you will be over 4.0" when loaded to the suggested seating depth. This can indicate a need for single shooting instead of magazine feeding. Expense of testing powder/bullet combinations will be high unless you get lucky right off the bat or you are blessed with a greater funding potential...
I have not experienced an increase in coppering in the Bartlein barrels I've been using and I don't expect any increase after break in. Initially, I thought about buying into the Lapua mystique or as I prefer 'A certain something,
je ne sais quoi (intangible quality that makes something distinctive)'. But I backed away, deciding instead to pursue other venues and cartridges. Now, as it turns out, I find no need for this cartridge at all since it has been bypassed, for my pursuits, by other cartridges better suited to the chosen tasks. In particular, the .338 Norma Mag. for general long range shooting beyond 1,000 yards, the .338-378 Weatherby for extended long range shooting and a yet to be named variant for the 2 mile and beyond category which is still in development.
I haven't found a downside to using the .338 Flat Line bullets, in fact I enjoy shooting them at the extended ranges I have available where I shoot. Once my loads were established my only real concern was the wind which could be a little twitchy on occasion. Norma MRP2, VVN-570, IMR-7828, Re-26, 23 and H-1000 make up the base powders which work well with this combination. Note that the barrels I use are in the 30" -33" range. Weigh is of no concern whatsoever as long as I can carry it from the truck to the pad.
I still have the 6.5mm and the .375 Flat line bullets on my agenda if I can ever get a little more free time to experiment.
Regards.