Powder Compression

[RKRNC]

BUT powder charges will be heavily compressed

Everyone was great with my last question...so here goes another.

1.) How much sleep should I be losing regarding powder compression?

I use a variety of load data for my .308 Win, .223 Rem, and .300 WM loads. My first loads followed the data provided by the dies for my Lee equipment. I started with the Lee Classic loaders (because I wasn't sure reloading was going to be for me), and have moved to the turret press. I have also used the data from the Hodgdon/IMR/Winchester website.

2.) Just the opposite: Why are under-loads (light loads?) no-no's?

I'm not looking to push any envelopes. I'm just seeking a deeper understanding. What is it that makes a short charge bad? Too much air/oxygen in the casing? I seem to remember reading that it was dangerous.

Safety will always be my first concern. Like many shooters, performance comes next, and whereas economy dictates much of what we do (I'm a school teacher in NC, so I know what it means to live on a budget) , I'd rather reload/shoot less than take risks that might endanger myself or those on the range with me. However, I am not imune to mistakes. Last year, I ordered 150 gr Remington SPCL bullets (500). I opened the box, ran to my shop, checked my load data, and reloaded 30. I went to the range the next day and had a great time. I now have no doubt those reloads had powder under compression. It was not until I went to reload those cartridges that I realized that I had recieved 180 gr SPCL bullets. I learned a valuable lesson: Know what you have, not what you ordered.

As always, I appreciate any education you are willing to impart.

Thanks. RKRNC

 

[jim147]

If the load is listed as compressed it normally means you can't put enough of that powder in that case to hurt you. Given you work the load up safely. A compressed load will normally give act like a slower powder or give reduced pressure.

Reduced loads. Check with the powder maker on this. Some mag powders do not like to be reduced. A lot of normal powders can be reduced without any problem.

jim

 

[Tim the student]

Jim147, with respect, I think he is asking why he should not load under minimum, not how to do it.

RKRNC, I don't know myself, but I do know you will get your answer shortly. Then I will know too actually.

 

[mongoose33]

When you go below minimum you may not produce enough energy to cycle a semi-automatic, whether pistol or rifle. You may also, if you're low enough, not get enough pressure to seal the cartridge to the chamber, and you may end up with a squib, a bullet that doesn't have enough pressure behind it to leave the barrel.

And that's bad, because if the gun does cycle another round into the chamber and you try to fire it, that second bullet is going to run into the first, and that'll ruin your day. And maybe your eyesight. Or worse.

I've often started at minimum, and it's common that the minimum load isn't enough to cycle the action, i.e., the slide on my XD won't lock back after the last round.

Don't go below minimum.

 

[Tim the student]

Are there any dangers other than a squib though?

 

[jfdavis58]

Light charges can present a number of problems. Typically no charge is a squib, the primer alone can push a bullet out of the case and lodge it in the barrel. Some very very light charges also classify as squib, but they actually make a small 'bang' (pop) when fired. If it pops, stop!

There is the possibility of a hang fire: the hammer clicks, there is a pause then boom the round goes off; several seconds may pass. It might also fizzle rather than burn rapidly making a whistle sound. The bullet may or may not be launched.

Uneven pressure and erratic results in both velocity and accuracy are possible.

And finally, a bullet my start to move, stop then the whole barrel explodes because it's stuck and the pressure rose rapidly as all the powder finally 'gets going'. This is generally thought to be what occurs in certain slow burning case filling magnum powders when they are below minimum--check any of the big belted magnums, many will have only a single max-use this load-only listing.

Smokeless propellants don't actually need external O2. The chemistry contains both the combustable and the oxidant.

Compressed charges are just that. There are ways to settle powder in a case--tap it, hold a thumb or finger over the mouth and set it on a vibratory cleaner, etc. They are usually maximum loads and don't really cause any problems; except on progressive reloading presses where they may spill. In fact, the compressed powder serves to prevent bullet set-back. One pistol cartridge where this occurs is 357SIG.



Related, but not actually part of your answer:
There are big problems at the loading bench with small charges; namely the real possibility of a double or triple charge. The less experienced reloader should seek component combinations that have the case filled more than half full-especially when charging many cases in a loading block for a single stage press. There is also the invert each case trick (over a clean container) to insure only one charge is loaded.

 

[rcmodel]

2.) Just the opposite: Why are under-loads (light loads?) no-no's?

Going below the listed starting load with any small bore / large capacity case like your .300 Weatherby (or Winchester, can't tell which from .300 WM) is dangerous because of a phenomena known as a secondary explosion effect, pressure excursion, or detonation.

The true cause is subject to much debate, and it cannot be reliably produced in lab testing to get a definative answer.

However, one theory is the reduced charge of slow powder in the large case does not get a good burn started when the primer fires.

That pushes the bullet into the rifling, where it stops, and becomes basically a bore obstruction right in front of the chamber.
When the rest of the charge finally ignites, chamber pressure goes through the roof before the stuck bullet can get out of the way.

Bottom line is = Do Not go below the published starting load for any reason when loading over-bore calibers such as your .300 WM.

As already noted by others, there is no danger from a listed compressed load, because any powder recommended for a compressed load is too slow for the case to hold enough to reach dangerous pressure levels.

rc

 

[RKRNC]

Thanks rcmodel. That was an excellent explanation.

When the rest of the charge finally ignites, chamber pressure goes through the roof before the stuck bullet can get out of the way.

Easy for me to visualize based upon what I think I know about throat clearance. Does it stand to reason that because the throat is smooth, it takes less pressure to start the projectile, and that when the projectile stops in the rifling, the rifling increases friction to the point that the pressure required to move the projectile may exceed the capacity of the chamber/barrel to contain it?

Again, thanks to all who have responded.

RKRNC

 

[rcmodel]

Don't forget the whole bullet bearing surface is inside the case mouth when acceleration normally starts. It has to go at least it's own length before the whole bullet shank is past the leade and engraved by the rifling.

I think that, plus the fact the bullet starts to accelerate smoothly and continues on its way as the pressure builds.

Newtons First Law of Motion.
An object at rest tends to stay at rest, and an object in motion tends to stay in motion.

Also friction:
Static (Stuck bullet) friction is higher then Kinetic (Moving bullet) friction.

rc