I've read m855a1 has higher pressure and reduces barrel life.
I've read that too, but once you start digging a little, it's one of those "everybody knows" things that everybody doesn't in fact know. There are a lot of assumptions in that statement, one of them is that every round fired will cause the same amount of barrel wear and that just isn't the case. Barrel temperature has a much bigger impact on barrel wear, so if you fire 100 rounds a day, it's very different doing a 100 round "mad minute" versus one round a minute for 100 minutes.
So yes the M855A1 has higher pressure than the older M855 load, but by how much? Is it significant? Is it outside SAAMI spec? Is it anything to worry about?
M855 is rated 2,916 fps from an M4 (3,113 fps from M16)
Mk318 Mod 0 is rated 2,925 fps from an M4
M855A1 is rated at 2,970 fps from an M4 (3,150 from an M16).
The M855A1 gains a whopping 43 (from M16) and 54 (from M4) fps over the older M855 load, and only 45 fps over Mk318.
The answer is that if we use the M855 as a baseline, both Mk318 and M855A1 have more velocity and you can expect a reduced barrel life because of that. At least in purely theoretical terms, you could have a lower pressure load that you shoot a LOT of with the barrel really hot and cause way more wear than shooting M855A1 judiciously and keeping the barrel temperature lower.
One of the other myths is that the M855A1's copper slug doesn't deform as easily as lead and therefore that will cause increased wear. But the Mk318 projectile also has a solid copper back end, so there is no gains in barrel wear from going with that bullet.
So yeah, which is better? The one that your rifle shoots better. Or maybe the one that you can get for cheaper in bulk. All bullets from a 5.56 platform are sufficiently lethal to get the job done. Even the yaw dependent M193 and M855loads work just fine (Mk318, M855A1, and Mk262 loads are not dependent on bullet yaw for terminal effects).
Doesn't matter what you shoot, you shoot it hot you'll have premature barrel wear. But AR barrels are pretty cheap and easy to replace.
Jimro