N150 and N550 are pretty similiar in burn rate until max pressures are reached and then N550 really peaks and gives much faster velocities.
N550 is a triple base powder that has been known to burn out throats very quickly. I burned out a barrel with it in under 1000 rounds on a cartridge that should have gone quite a few more.
Flame temp is reported to be quite a bit hotter.
Which cartridge?
I found this on a different forum, please tell me what is your opinion?
modern double-base powders like N540 and the Reloder series have additives which bring erosion in line with single base powders. I use a lot of both types and haven't seen any additional erosion in the barrels used with double base powders in 6BR - I'm close to 4000 rounds in one of them and it's still doing very well.
I'm a prone shooter, so like you, I will go about 70 rounds before cleaning, this doesn't present any problems; my last 20 shots are as accurate as my first 20. I use moly coated bullets, which I believe helps maintain accuracy for the 70 shots, but in any event, the double base powders don't behave any less accurately than the single base in this regard. Temepratures here in Phoenix are pretty warm about half the year and that hasn't been a problem either. I take care to keep all ammo out of direct sunlight, however.
As expiper and others have said, find the powder that shoots best in your rifle and use it. In any event, barrels are as expendable as bullets (cheaper too, think about it).
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