Case Discoloration Comparison

T

TunnelRat

New member
Hi all,
Two weekends ago I was shooting a new M&P 9mm Compact next to an HK P2000. Both in 9mm, both firing 115 gr Blazer Brass from the same case lot (I am not making any claims that Blazer Brass is somehow a Gold Standard for ammo). What I noticed was that the cases from the HK P2000 were being ejected noticeably cleaner than those from the S&W M&P. Both guns were cleaned prior to the shooting. Essentially it looked like gases were escaping passed the edge of the casing (towards the bottom where the feed ramp is) and causing the discoloration of the brass on the M&P. After 100 rounds through each pistol, the M&P was also noticeably dirtier than the P2000 on the feedramp and around the mouth of the chamber.

Could this be a result of the HK's stepped chamber allowing a tighter gas seal? Any thoughts?

S&W on the left, H&K on the right.


Thanks,
-TR
 
I would bet thats a good guess. the Hk cases look noticeably smaller at the front and seem to have an mark around the case where the step would be.
 
IMHO, you have a combination of a somewhat oversize chamber on the M&P an a tight one on the HK. The result is usually somewhere in the middle, but here there is a sharp contrast.

FWIW, a fellow named Georg Luger designed the chamber for his 9mm pistol with a step exactly the same way and because he thought it gave better obturation. The pistols were made with stepped chambers until WWII, when the chambers were made without the step to speed production. Looks like old Georg had something.

Jim
 
IMHO, you have a combination of a somewhat oversize chamber on the M&P an a tight one on the HK. The result is usually somewhere in the middle, but here there is a sharp contrast.
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Sounds reasonable. The brass from the P2000 almost looks new, though as Venom pointed out you can see the line from the stepped chamber and the slight difference in case bulging at that point.

One thing worth noting, is that while I would agree the P2000 has a tighter chamber, for some reason it feed snap caps that are beat up that the M&P chokes on. Frankly those snap caps are now out of spec and I don't blame the M&P for not feeding them, but it is interesting.

FWIW, a fellow named Georg Luger designed the chamber for his 9mm pistol with a step exactly the same way and because he thought it gave better obturation. The pistols were made with stepped chambers until WWII, when the chambers were made without the step to speed production. Looks like old Georg had something.
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What silly guy that Luger, naming himself after a pistol. :p
 
You wanna see some wacky brass? Find some that was launched from a gun with a fluted chamber!
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I was gonna say. My HK brass was always a lot dirtier than anything else Ive shot, pistol or rifle calibers.


Is that line about a quarter of the way down from the mouth on the HK brass, the "step"? It seems just forward of it, the brass is showing a little color.
 
When a gun is fired, one of the first things the pressure does is to expand the thin case neck to release the bullet and seal off the chamber. That puts almost the full power of the gas behind the bullet and keeps gas carbon from dirtying the chamber and the brass. But that sealing, called obturation, takes an instant to complete and in that instant some gas and dirt can leak backward. How much depends on several factors. Generally, the less pressure, the less obturation and the more blackening of the case. Other factors, like chamber size and the flexibility of the case can also be factors.

Since all the examples shown were from the same ammo lot from the same maker, I would have to assume that the difference here is in the chamber, not the ammunition.

Jim
 
James K, Thanks for that info. I thought I was pretty knowledgeable about most things 9MM, but did not know about the step in the earliest 9MM chambers.
 
FWIW, a fellow named Georg Luger designed the chamber for his 9mm pistol with a step exactly the same way and because he thought it gave better obturation. The pistols were made with stepped chambers until WWII, when the chambers were made without the step to speed production. Looks like old Georg had something.
Click to expand...
My understanding was that the tapered chambers are actually a little harder to machine, but that the stepped chambers were causing problems with the wartime steel cased 9mm ammo.
 
The S&W is a compact and therefore it may begin the extraction process a tiny bit sooner than the P2000. That can make a difference in how much the gun fouls in the chamber area.
 
It's only a difference of .16" in barrel length.

But the M&P's action design may unlock earlier in the cycle than HK, with more residual chamber pressure.
 
I was thinking that there may have been a little oil left in the chamber of the S&W as well...

tends to glom burnt powder in the chamber...giving the brass a funny look...
 
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