I've shot two different manufacturers ammo out of my custom 260 Rem and had no problem. I just started reloading and got some Lapua brass and loaded a bunch up. Went to the range and they wouldn't chamber. At first I thought I seated the bullets too far out. Eventually, I figured out it was the thickness of the case necks. I trimmed the necks and they now 'fit'. I hope I didn't trim them too much but I tried to get to the same outer loaded round case neck dimension as one factory loaded round I had that fit fine. (BTW, I have a Forster hand trimmer that I used for the first time and the mandrel was very tight and difficult to get started which I didn't like). However, I don't won't to have to trim down Lapua case necks all the time. I don't know if I have a tight chamber since the factory ammo cycled just fine. Am new to reloading and I am amazed how some things can apparently come down to a couple of thousandths in dimensions. I now have a bunch of bullets I have to pull and reload. Any other brass I need to worry about before I get some new brass to avoid having to trim necks?