Burn smudges on one side of the case - new reloader's question

Folks,

Today I tested the following load in my Pardini GT45.
Bullet: Zero JHP 185 Gr
Powder: N310 3.8 Gr or 4.0 Gr - twenty rounds of each
OAL: 1.205"
Crimp: 0.469"
Primer: CCI LPP
Case: Winchester - multiple reloads

The ammo shoots very fine - on call and soft. Rounds loaded with 3.8 Gr N310 gave a very tame recoil that was a pleasure and 4.0 was only slightly hotter.
However, the spent cases have a large burn mark or a smudge on one side of the case (please see picture). Some cases have a very pronounced mark, others a much smaller one but it is there. The mark is present on the cases with both amounts of powder, which was the only variable.
I checked my spent cases from the lead reloads and the mark is almost non-existent.
Is this something that I should worry about and if so, what may be the cause?

Best Regards,
Oleg.

Perfectly normal my friend.

I suspect the blowback from lead bullet lube etc reduces the gas passing, that, or they have less due to lower pressure.

My interpretation:  At very low pressures, The case does not fully seal, and some of the hot, dirty gasses may intrude between the case and the chamber.  The inside of the cases was probably equivalently blackened.

NuJudge, I think you're right on the money there. I think the only cure for it would be increase the pressure or anneal the brass so it's softer.
 Oleg, I wouldn't worry about it, if they shoot accurately.
Stan

NuJudge wrote:My interpretation:  At very low pressures, The case does not fully seal, and some of the hot, dirty gasses may intrude between the case and the chamber.  The inside of the cases was probably equivalently blackened.

I agree. My softer loads both 45ACP and 9mm do the same. A little hotter and burn marks are less or almost none.
Oleg, don't worry too much about it. Find the load that shots most accurate and if its on the soft side with some blackened brass, so be it.

Thanks, folks. This load shoots. So, that's all that matters.

Best Regards,
Oleg.

I think some of you have the physics wrong. If you watch hi speed photography of guns firing, you'll see a puff of smoke well before the bullet exits the barrel. Then no smoke, followed by the bullet exit. This is due to flow over the bullet before it has sealed the bore. Jacketed are higher pressure. This gas also flows back around the case just after ignition. THEN the pressure seals the case. Lower pressure lead loads have less tendency to flow back and what does go back also has unburned powder and bullet lube which plugs up the chamber.

Either way, the marksvare normal.

Тhat's all that matters

Boris - fully agree!

Cheers.

For indoor, I use n320 5.8gr with a coated 200gr swc most of the time, and haven't seen my brass discolor. Now, with Titegroup, it scorches no matter what bullet I use, 185, 200, 230, lead, coated of FMJ. I have 1-3/4 pounds just sitting on my loading bench, I just don't like it. I wet tumble my brass, so it does clean up nicely.