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Yep, as above, chambers factory ammo.
I couldn't find any marks on the case when I used the permanent marker, I might have to try another marker.
If it looks like it is tight at the base I will...

Tried that.
It's a brand new Daniel Defence barrel. Bolt is spikes tactical and both are clean.
It would seem a simple problem but it's not!

Well it's got me buggered, definitely not a projectile seating issue, I have now run some of the loaded rounds through a small base sizer without the decapping stem and the bolt still won't go into...

Yeah, pretty sure it's not the projectiles, I will try pen around the base when I get home.

So what will be stopping it chambering? The size of the base or the length to the shoulder?

I recently sold a AR15 and have all the ammo I had loaded for it (about 600-700 rounds) the ammo won't fit (bolt won't fully close) in my other AR, can I resize it without pulling them all apart?...