I've heard a number of explanations ranging from excess oil residue in the chamber, to underpowered loads that don't force the case neck to seal against the chamber. What's really behind these occurences?

RSY

I think it is most likely caused by gas and soot makeing it past the neck. There are several things that can be causing this, but are you first time firing brass, or is it from reloaded ammo?

Soot is the result of inefficient burning of the powder. Sometimes crimping the bullet will solve this(work up loads again as crimping may raise pressures). In handguns seating the bullet deeper may help(again work up loads) or perhaps a stiffer charge to seal the chamber.

As far as I can tell, it is caused because the carridge has been subjected to enough pressure to release the bullet but not quite enough to seal the chamber. This does not mean, however, that the load is necessarily a low pressure one.

Slow to very slow burning propellants will also often cause this, even though the peak pressure is right up there. The reason for it is rise time. The slower burners have a slower rise time -- that is, the time it takes to reach peak -- so that the neck may not have enough pressure to seal the chamber for a brief interval of time (and we are talking a few microseconds) but enough for the bullet to be released.

An example of this is one of my 140 gr. 280 Remington loads that develops an average peak of 62,000 psi and produces just shy of 3100 fps (RL-22). The necks show quite a bit of carbon residue almost back to the neck and shoulder junction. This load is certainly not weak and is the single most accurate load in my Browning.

I can switch to a 150 gr. bullet with RL-22 (the powder charge is obviously reduced), and load it to the same 62,000 psi pressure level without much if any residue on the necks. Substituting H1000 for the RL-22, and loading to the same pressure level, will, once again, leave residue on the case necks.

Thanks for the info, everyone. I've most often had this occur with some loads for my .270 using Re-22, so the slow powder theory appears to hold water, in my case (no pun intended).

RSY

Bottom line is the powder is too slow for the case and bullet and you need to step into a slightly faster powder...if just the necks are a little sooty then its no biggie...unless it bothers you....

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Ray Atkinson

ray@atkinsonhunting.com
atkinsonhunting.com

RSY __

The amount of "smut" is determined by the length of time that communication exists between the case neck/bullet gap and the case neck/chamber gap on-firing. As is usual in matters of internal ballistics, this length of time is determined by a whole bunch of variables. The rate of pressure rise (gunpowder "burning rate"), cartridge case and bullet dimensions, firearm chamber dimensions, strength of the cartridge brass, and the amount of burned residue inherent in the gunpowder come to mind.

Good shooting.

Robert

I have to disagree with the comment about the powder being too slow. Barrel lenght, bore biameter, and bullet weight will dictate the burn rate.

A simple explanation may be the chamber is slightly oversize leaving too much room between the neck and the chamber. The case will still make contact but will require more pressure which takes a little longer giving the gases time to pass between the neck and the chamber. Just my $.02

Gard,
The facts are the powder is too slow to blow the neck out fast enough to stop the leakage back...a more violent powder will shut that gap faster..thats as simple as I can explain it...it happens at the exact time of ignition and barrel lenth etc. is not a factor..

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Ray Atkinson

ray@atkinsonhunting.com
atkinsonhunting.com

I get sooty cases and necks with starting loads, but as I increase them and approach max the sooting stops.

The worst case I ever experienced came with a box of Norma 156 grain bullets in new 6.5 Jap factory loads. I was really puzzled until I had one that failed to fire entirely and vented a lot of smoke out of the two gas holes in the receiver ring of my Arisaka. I found the primer folded up and gas went out the openings. I'd say this batch had soft primers, and weren't doing their job.

My eailer post was unclear I guess. Im intention was that the powder should be chosen to match bullet weight, barrel lenght, case capicity, and bore dia. I have never had a problem with a tight neck chamber. Most factory barrels will have some fouling on the neck.