gas blowback and H380

[blackpowderbill]

Yesterday at the range I noticed a problem with some of my loads. I was shooting a 7.5x55 loaded with 130gr mk319 bullets over a middle wt charge (as per manual) of H380. At one point I noticed a blast of gas on my forhead and thought I popped a primer or split a case. The fired case was blackened so I wiped it off to find the flaw. I found no failure of any sort and then noticed that most of my brass from that lot was similarly sooted up. The inside of my rifle was sooted up also.
  I was shooting other loads using H380 and heavier bullets in the same rifle that showed no soot upon the fired cases. I believe that what happened was that the light bullets were started down bore without the pressure in the cases building up enough to fully expand them prior to the bullet leaving the case mouths. That would allow some gas to come bace around the cases untill sufficient pressure had built up to seal them. What do you think, am on the right track or totaly lost.?
  What I did learn was that the k31 does seem to offer good gas protection as none of the gas hit my glasses.

[Old Syko]

Soot on cases = too light a charge.

[stimpylu32]

Yep , your on the right track , you may want to up the charge or better yet change to another powder for the lighter bullets , something with a quicker pressure curve .
 
stimpy

[blackpowderbill]

Thanks guys

[BBF]

Sometimes a different brand of brass may eliminate that problem as well.