My question is: Is there any problem to break-in new long range rifle barrel with reduced velocity or subsonic ammo or reloads?

I ask because of: Hundred meters from my apartment there is nice warm underground shooting range, but you can't shoot high power rifles there. I believe there is a power limit such as 7.62x39 or .223.

So I would like to use this shooting range now in winter to break-in procedure for new .338 LM rifle barrel and my idea is to use either factory or own subsonic or reduced velocity ammo.

What do you think about that?

Jiri

Factory ammo should work just fine, I'd think. And I see no negatives to reduced velocities. You could contact the barrel mfg for their opinion.