I have several hundred rounds of 220 gr 300 Blackout that I reloaded several years ago. I used all Lake City brass and Lee AAC 300 Blackout die set .

Brass was all once fired LC that had been converted.  I bought the brass already converted online… don’t remember exactly where from.

I reloaded using Sierra 220 gr Match King.

Ive shot a lot of it through a home built AR using a CMMG barrel with NO issues at all.

I am now trying to test it out in a bolt action rifle and the cartridge will not chamber.  So I’m wondering if it’s a headspace issue or what.  

Any help is appreciated.

I had a similar issue, with converted LC brass. If I recall correctly the thickness of the brass in the neck area was an issue, have you tried converting some commercial brass?

No I have not.

So would turning the neck help if this is the issue?

If I had a piece of brass that was actually fired in the bolt gun I could use it to measure the neck diameter and compare to the sized Lake City brass neck and see difference.

Grab your calipers and measure the neck OD of the loaded rounds.

SAMMI spec is 0.3340" for the neck OD of the loaded rounds.

SAMMI spec for the chamber neck ID is 0.3370"

So you've got just 3 thousandths of wiggle room.

My guess is some of your rounds have a neck OD thats bigger than the chamber ID. I've seen this many times with converted brass.

Check all of them. I have a Sheridan cutaway chamber guage specifically for this purpose.

Take a black sharpie and coat the shoulder and the projectile.  Attempt to chamber and then look at the marks.  This is how I did it before I bought a Sheridan cartridge gauge.

Improper setting of my sizing die was the issue. I learned about cam-over that day.

I have a Noveske barrel with pretty tight chamber tolerances.  I use cases that I cut down myself and have not needed to do any neck turning.  I just emptied a lot of cases last Sunday at the range and plan to use my annealer for the first time this weekend.

My guess is it needs a small base die.  Bolt actions usually have slightly tighter chambers than an AR platform.  

If you don't see any obvious causes with brass length or COAL then try marking the case base/body and then try to chamber it.  IT should be obvious if the base needs a SB die.

This is almost always the first thing to try. I'm willing to bet your bolt action's headspace is at or close to SAAMI minimum, the AR not so much.

Hornady's Lock-N-Load headspace gage is a vital tool when setting up dies or troubleshooting these types of issues. Semi-auto rifles need .003" clearance minimum to be 100% reliable, bolt actions .0015" to .002". If your AR ammo won't fit in the bolt action it's probably tight in your semi-auto, you just haven't suffered consequences yet.

My Lee 300BO sizing die (it’s 7 to 8 years old) does a poor job converting/sizing brass and the problem seems to be at the cartridge base. Now I use a small base RCBS 5.56 die with decapper removed in the progressive press in front of the sizing die. This works since I decap prior with a hand tool, pin tumble & dry, spray lube, then resize with the two dies.  I case gauge then change to a tool head for priming, powder drop, seating, and crimping.  I already had both dies so it was an easy process change.  The 300 Blackout is the only Lee die that has given me trouble.