I’ve never considered fire forming detrimental to a barrel.....and I’ve done a fair amount....am I missing something?
Short answer is ...well, maybe you are but I cannot really answer that for YOU.
Long answer:
If I'm fireforming a metric buttload of brass (300+) and I care about round-count (wear) in the new barrel (22-250AI), then it matters to ME. * my brother formed 300+ in the sacrificial barrel too so it might make more sense to you now.
If I only get 1200-1500 rounds down the tube before top accuracy slips, I would have been 1/4+/- of the way done with the new barrel. That is IF I let it cool down between shots so the metal wasn't as malleable and subject to wear.
If I'm fireforming just a few, and willing to let the barrel cool down between shots, then it's not as detrimental on the new barrel.
I only did it because it made perfect sense, to ME.
By the way, we had a ton of fun shooting 100 rounds each for 3 trips to the range. Way fun to bang away without a care in the world about hot barrel and barrel wear.
And the new rifle shoots like and f-ing champ!
Zeke