Decrease powder charge could increase pressure? What?

[robert101]

Never decrease this charge as an increase in pressure could be encountered.

[Ahhnother8]

That is why they have starting loads in the manuals. I have experienced it with a 7mm Rem Mag. Began with the starting load in the manual, and it was very apparent that it was overpressure. It kind of hang-fired and was just an odd recoil. Kinda freaked me out at the time. Stepped back, went home and thought it through. Then went back out and fired the other test loads with MORE powder without an issue. So now I just start at the max load and go up from there. It is much safer.

[Fjold]

Undercharging a case is dangerous. It is thought that the powder has too much free space in the case resulting in inconsistent ignition that causes the powder to detonate instead of burn. It is very common to see this warning with slow burning rifle powders in large capacity cases but examples have been seen in pistol cartridges.

[buffybuster]

Some powders are very sensitive to having too much empty space. Instead of burning progressively they become erratic and explosive (detonation). That is why you must be very careful NOT to got too far below the Starting Loads. Some powders are very forgiving of this and some are not (usually the slower burning ones are not as tolerant).

[robert101]

thank you for the input. Do you think I'll have a problem with Blue Dot, Unique, or Bullseye? I don't reduce the charge below the 1/4 theory explained in Richard Lee's book.

[ar15barrels]

A name for this is a pressure wave.
The theory as I understand is is that you have less than half the case full of powder and all the powder is laying on the side of the case as the gun is held basically level.
The primer flame ignites the TOP of the powder all at once instead of lighting the front or rear of the powder column as it would when the case is full.
Since so much powder is ignited all at the same time, you get much higher pressure.

One solution for this is to use a case filler material that will keep all the powder back against the flash hole when the primer goes off.

[robert101]

I have read some literature on the need for case material with lower powder charges. I believe a Dacron Filler wad 1/4" thick is what was used. Just to be safe, I will try adding the filler on future loads. I had always used a lower level of about 900fps as the lower indicator and never had a problem. Oh well, better to learn the right way than to live a mistake.

[freakshow10mm]

If you want mouse fart loads, use a powder that isn't position sensitive like TG or one that was designed for situations like this, TB.

[5hundo]

What's really going to fry your noddle later on is why when you are loading heavier bullets, using the same powder, you throw less powder for the heavier bullet than the lighter one...

[Bug Splat]

Heavier bullets = more pressure with same powder load.

[tamalpias]

AR got the basis of it right. When the powder is laying along the side of the case and the primer flash ignites all the powder at once you get a pressure spike. It is NOT a controlled burn when you have all the powder burning at once vs. powder burning down the case.

[5hundo]

Heavier bullets = more pressure with same powder load.

It took me the longest time (and numerous explanations from my Dad) before I took hold of that concept...

(kudos on getting the "Matrix" reference too...)

[robert101]

5hundo, yea I understand that a heavier bullet (which is typically longer) requires less powder. I believe this is caused by more bullet setback into the charge area thereby decreasing the capacity/volume of the case. I'm sure getting that additional mass of a heavier bullet moving might have some impact on pressure as well. Thanks for all the help.

[Ugly Dwarf]

I'm sure getting that additional mass of a heavier bullet moving might have some impact on pressure as well.

Actually, I think this is the more significant of the two factors.

I agree that seating a long bullet, deep into the cartridge will impact case volume, but I suspect this is less relevant in standard 10mm loadings.

[rimmer]

I would think the spike in pressure would just send the bullet out the barrel quicker. I wouldn't think that the pressure is going to rise a lot higher. I might guess it turns to detonation. Who knows what really happens in a few milliseconds.

Oscar