i built a 17 ackley hornet this winter. My load has been using 10 gr of lil gun and a 20 gr bullet. i thought i'd lower the load a bit because i usually get better accuracy, so i dropped it to 9.5 grains. at 10 grains the primers were starting to flatten and things were working smoothly. BUT at 9.5 gr. the primers were flat as a pancake and showing excessive pressure. HUH??

I think you have dropped the load to a point where the primer blast circulates the powder in the case as it is igniting. Not quite a detonation but shaking the powder does make it burn faster.

square sharp shoulders on a case have a tendency to make powder burn faster.

You are down in the 78% fill range. You might be getting an ignition jump across too much powder giving you a spike.

Chronograph the two loads then get back with us. Interpreting pressure via primer appearance is typically iffy and often misleading.

Are the primers from the same lot?
Just asking.

muck

Although Lil Gun works great in 22 Hornets....it is bad juju in 17AH's. There are too many unexplained pressure variations. Smaller sectional density, temperature variations, load density???....who knows for sure?! Switch to 1680 powder, and don't look back. Hope this helps. Kevin

same primers & i'll try 1680 - providing i can find some - otherwise i'll try some 4227

The phenomena, I have never bought into the detonation, pre-ignition and or the two flame front theories, many years ago, before the Internet it was believed the pistol will not blow up every time due to bad habits, from overseas came a research study that identified the ‘phenomena’, we rejected the ideal because the researchers could not blow up the pistol every time.

It was suggested reducing the powder could cause the powder to spread out in the bottom of the case from the case head cup to the base of the bullet. Spreading the powder out and flat allowed the primer to fire/flash over the powder and cause the bullet to move into the forcing cone, it is believed the bullet stopping in the throat of the barrel took additional pressure to start it moving, then the ‘time is a factor’ kicked in as in the bullet being lodged in the throat could not move out of the way before the burning powder built up enough pressure to render the pistol scrap. Then there was the flame front theory, the powder laying in the case had a larger flame front causing the powder to burn from the top down instead from the rear to the front. And that added to the lodged bullet, caused all of the excitement.

Then came the powder sensitive position of powder and those that used toilet paper, etc., with reduced loads so on and so own. My opinion, reduced loads are a bad habit. Some, in the old days would point their firearms skyward to position the powder in the rear of the case and in front of the primer, JIC, just in case there was any truth in the foreigners research. The company that did the research has moved on, we haven't
Then there was the reloader with the Weatherby that used a steady diet of of reduced load, one day it let go, suddenly and without warning, the rifle swarmed on him, the consensus? A double charge, I do not believe Weatherby built that rifle for all that sudden shock, it was not the last round fired that rendered the rifle scarp it was the accumulation of all those sudden shocks starting with the first one.

F. Guffey