JAMMED BOLT - 300 BLK AR-15

[Armed Preacher]

Are you sure it was a subsonic load?

I note you’re using converted 223 cases, did you measure Neck wall thickness and calculate the neck diameter with a bullet seated and ensure it fits the chamber neck area with enough room to expand and release the bullet?

[StanCT]

According to the chrony it was subsonic.

I've done random measurements on some of the 223 cases and neck wall thickness was similar to factory S&B brass.

I have not, however, been measuring neck OD once bullet is seated in case, however the cartridge has passed a case gauge check, so I deduced that it should chamber within limits.

Edit:
Factory bullet OD measures .3075"
Factory round neck OD = .331"

My cast & coated bullets have been sized to .309"
My round neck OD = .335
manually ejecting a full round is a bit tight and stiff, compared to a factory round.

[Chris8]

How can a subsonic round develop such dangerous pressures?

Yea I dont get this either.

Maybe there wasn't enough pressure to cycle the action, but enough to expand an already tight case?

[StanCT]

... ensure it fits the chamber neck area with enough room to expand and release the bullet?

As mentioned earlier, I did notice that rounds that were manually tested in the rifle, for easy chambering and extraction, did show noticeable stiffness / tightness with extraction, indicating a tight fit in the chamber, possibly the neck area.

This was in spite of passing a case gauge check.

During case prep, I am using an expander plug to increase neck ID fractionally to prevent lead shaving or swaging down the bullet OD.

Could it be that my rifle chamber has tighter specs than my case gauge?

"Should" they not both be SAAMI specced?

Although, I suppose that even within those allowable tolerances there could be sufficient latitude, that a 4 thou increase in cartridge neck OD could make such a difference.

I'm thinking that next step may be to size bullet and expand case neck 1 thou smaller, for starters?

Any thoughts?

[Armed Preacher]

A case gauge is a headspace gauge, the the neck diameter after loading will only fail if grossly out of dimension.
I see that Hodgdon state 8.5gr max load on a 220 BT bullet, your is 250gr and probably flat base, I think you should be starting at least 10% (or more) below the max charge for safety and work up from there. I also think that their loads are developed in bolt action rifles and those will handle greater pressure than the semi.

On the extraction issue, blacken the case neck area and check where it may be binding, it could be that you need to size the case a bit more for your chamber (typically measure a fired case to shoulder datum line and size 0,002 - 0,003 lower)

[Stevin]

According to the chrony it was subsonic.

I've done random measurements on some of the 223 cases and neck wall thickness was similar to factory S&B brass.

I have not, however, been measuring neck OD once bullet is seated in case, however the cartridge has passed a case gauge check, so I deduced that it should chamber within limits.

Edit:
Factory bullet OD measures .3075"
Factory round neck OD = .331"

My cast & coated bullets have been sized to .309"
My round neck OD = .335
manually ejecting a full round is a bit tight and stiff, compared to a factory round.

SAAMI Max Neck OD is = .3340 so it appears you are over that.

[StanCT]

Thank you, Gentlemen - help greatly appreciated.