Chamber obstruction preventing full case expansion?

[rutilate]

I've been experimenting with lighter loads in an M&P 9 and noticed that most of the spent cases are sooty (not unexpected), but only on one side (unexpected). In the following picture, you can see two horizontal stripes where there is no soot, as though these two areas were touching something in the chamber. I cleaned the barrel ~100 rounds prior to shooting these. What might cause this?

 

[jpm]

chamber, not barrel/bore. Probably made during extraction I'd guess.

 

[EddieCoyle]

It's normal for light loads.

 

[saltydude]

It's normal for light loads.

This, & are you using titegroup? Very common with that propellant.

 

[rutilate]

chamber, not barrel/bore. Probably made during extraction I'd guess.

DOH! You're right, of course. I clearly wasn't connecting my thoughts properly.

This, & are you using titegroup? Very common with that propellant.

No, but I've noticed sooty .45 cases loaded with Titegroup.

It's normal for light loads.

It seems to present on every charge below about 6.1 grains of Power Pistol (i.e. 6.0, 5.9, 5.5, 5.3). Loads upwards to 6.5 grains are not sooty. It is only on one side, no ideas as to which. Is that expected in such a wide range of charges?

 

[soam]

DOH! You're right, of course. I clearly wasn't connecting my thoughts properly.


No, but I've noticed sooty .45 cases loaded with Titegroup.


It seems to present on every charge below about 6.1 grains of Power Pistol (i.e. 6.0, 5.9, 5.5, 5.3). Loads upwards to 6.5 grains are not sooty. It is only on one side, no ideas as to which. Is that expected in such a wide range of charges?

My guess is that the soot is on the side of the brass that's at the top of the chamber when the round is fired, since that side will be ever so slightly farther away from the chamber wall, leaving room for the gases to get into that area. This is based on an assumption on my part (which could easily be wrong) that the round does in fact sit at the bottom of the chamber.

My rounds are light gamer loads for USPSA and do the same thing (147gr Xtreme plated bullet, 3.2gr Titegroup). Now I'm tempted to hand feed some rounds into the chamber to see if my guess is correct. Just keep track of which part of the headstamp is at the top of the chamber, and see if that's where the soot is.

 

[Gammon]

The loads you are using do not generate sufficient pressure to expand the case and seal the chamber.

 

[Sir_Macadoo]

Could be from the extraction/ejection. The casing dinging the slide as its tumbles on its way out the ejection port.

 

[jpm]

I was thinking it drags on part of the round below in the mag during extraction

 

[noylj]

If one must load light, one should use a faster powder. However, 6.0gn of Power Pistol is NOT a light load, unless you are shooting 90gn bullets.
For a 115gn bullet, 6.0gn of Power Pistol is a start load and 6.0-6.2gn produces best accuracy in my 9x19s.