Alright. I admit I am tired and have a headache so maybe I am just stupidly overlooking something.

I resized 6arc brass. Put it in my chamber and it fits and feeds and ejects.

Prime it. Still fits and feeds and ejects.

Seated an 85 sierra hpbt. Bolt won't close.

Tried several other pieces of brass. All ok.

Seated bullet lower several times. Still no go.

Seated a 60 grain Sierra. Feeds and fits and ejects. Wtf?

No marks on the bullet to indicate I was into the rifling. No marks on primer to say it was not seated completely.

Wth did I do wrong? Been reloading for 15 years so not a newbie.

Also put a factory round in. Fits and feeds. Arghh

I would try a sized, un-primed brass. Place a 85 hpbt in the case mouth. Do not crimp. Run this "dummy" cartridge into the rifle chamber slowly until bolt locks.
Withdraw slowly as not to disturb the bullet. See what you get.

2nd choice is get a 6 ARC OAL gauge.

[link]https://www.youtube.com/watch?v=fW6H9HJzsvQ[/link]

I seated deeper several times until the thing looked silly and still no.

I'm at work so many gun related sights are blocked for me.

What pics I can find of the 6mm 85gr HPBT that appears to be a stubby bullet.

I would bet the bullet ogive is contacting the throat.

AR or bolt gun?

Is it possible you slightly crushed the shoulder on the brass you seated the 85 gr bullet in somewhere along the line of seating it deeper ?

It's a bolt gun. I will try another piece of brass but I tried several already.

I will mark the bullet with some permanent marker and see if it might be hitting the lands...but it's not leaving a mark

If there are no marks on the bullet then I'd pull the bullet and see if the empty brass chambers.

It does. Weird.

Guday, trimmed to length? Maybe the bullets deforming the brass? But you tried one that wouldn't chamber with bullet, then tried the SAME case and it chambered. If all the cases chamber without a bullet, that only leaves the bullet. Mike the bullet, actually a sample of bullets. Try another bullet?????? GWP. 👣🐾👣🐾👣🐾🇨🇦 ??????

New rifle? How’s neck clearance?

Those are typically 1.5 degree throats so it’s likely to engage way dow the bullet vs. close to the neck.

Short throat maybe?

Yeah. Not sure on necks. Also have not loaded the 6 arc before so maybe some quirk.

I dunno. Will try later when I am in the mood to be annoyed

This.

Did you measure the neck of the loaded round and compare it to the factory one that fits?

I bought some “bargain” 243 brass that had uneven necks, very thick in spots. Wouldn’t chamber in a Ruger 1V.

Hmm. Good thought. But why would the 60gr Sierra fit

Missed that part (I’m old). Still, measure the one that won’t go and maybe you’ll know what, then maybe measure your bullets for why.

Ok. Just a matter of me being tired and not focusing. I went back and tried to make each piece of the ten I sized and primed fit.

Only three fit. So I must have grabbed one randomly and confused myself.

They are once fired from an AR, so the chamber must have been bigger than mine and they need sized more.

Dog-tired and playing with stuff that goes boom. What could go wrong?

What’s your 6 built on?

Congrats on solving the mystery.

It's a savage axis 2. Wanted to try the round out before building something expensive.

I quit trying to figure out the quandary when I got tired. Just meant it was at the end of the day.

I wouldn't be spending time putting powder into cases if I couldn't think clearly, for damn sure lol

I'm just getting into the arc thing.
First I'd turn your FL sizing dies down another 1/8 to 1/4 turn. AR's expand and stretch brass bad.
Next, get a oal casing. I just bought 1 this morning for under 10 bucks shipped.
That will tell you if your jamming the lands.
Have you measured the neck with out a bullet, and with one seated ?

Hope I helped...

Load q 60 grain and a 85 grain.Mike the outside diameter of the necks.Or you can just mike the diameter of the bullets before the ogive.

Was just normal variation due to being fired in a big chamber. All good

Just have to resize them again and quiet my anxiety about now having to anneal earlier. Lol