I’ve been reloading for a number of years and just encountered a problem I’ve never experienced before. I was reloading for my 6.5 creedmoor and opened a new box of Hornady brass. Trimmed and chamfered it but did not resize it. The seater die would not close completely. I tried a once fired case and the die worked perfectly.

I’ve been inconsistent over the years and not always resized new brass but this is the first time I’ve encountered this problem.

How many of you resize your new brass and have any of you experienced this problem?

I have run into big bore brass that needed to be sized before loading.

Never assume that you don't need to size new brass. Sometimes you do. Tolerance stack up between brass, dies, and chambers is huge, relatively. It is a miracle that any of our systems work together. Remember, SAAMI specs are Voluntary.

I used to load and shoot new brass without benefit of running them through a sizer die but don't do it with rifle brass now because the brass being made now just isn't as uniform as it used to be. Maybe the expensive stuff is but the cheaper brands that I use aren't. I still will just load and shoot new straight walled pistol brass.

ALWAYS run new rifle brass through a sizing die FIRST, before first firing. I always use a FL die. Mainly this will even up the internal neck size, giving more consistent tension holding the seated bullet and generally produces rounds of greater accuracy potential. In newly produced brass, even the higher quality brands, neck diameters can vary considerably to the extent sometimes that seated bullets are barely held in place.
Also, it's not unusual to find small neck dings or out of round brass sold in bulk packaging and these definitely need to go through a sizing die.

the only problem I had with HORNADY BRASS I couldn't get the primers to go in. I broke my primer arm on my RCBS, I had to ream all my primer pockets in new brass

Thanks to everyone for your comments and suggestions.

I have shot thousands of rounds yearly in years past and now shoot between 1000 and 1500 rounds a year. I have never resized new brass. I prep the necks inside and out. I use Winchester brass mostly with some Nosler. I do find the Nosler brass primer pockets needing reaming. Good Shooting.

take nothing for granted, most new brass is pretty much a semi facsimile of what the case should look like.
your chamber will fix it, but you gotta get it to that stage yourself.

I've rarely seen new brass on which the many of the necks were not slightly dented. New brass at least needs to be neck-sized, then chamfered (unless you're a Luddite who unnecessarily crimps in bullets.) It rarely needs FL sizing, but might for some chambers.

I always resize new brass. I don't trust they are the correct size. Just like pre gapped spark plugs LOL, trust but verify.

about 17-20 years ago, i noticed that "all" new brass was undersized, but i had worked in the habit of resizing everytime .. this is when i found that nickel plated brass runs larger.

this "problem" went away when i went to progressive presses