Accurate 1680 - large fireball in 7.62x39

[ciwsguy]

Shooting Ruger American Ranch 7.62x39 at indoor range. Handloads of Berry's .311 diameter 123 Grain spire point plated bullets over 20.1, 20.4 and 20.6 grains of Accurate 1680. The max velocity of the Berry's bullets is listed at 2000 fps. Using Federal Large Rifle primers (not magnum). In the scope, I see a large flash when shooting any of these rounds, which I presume to be a large fireball at the muzzle. I have not noticed this with Hodgdon CFR BLK powder over 123 grain Hornady bullets. The RAR has a 16+ inch barrel.
Anybody else notice anything like this or can help say whether this is normal or not? Do you think Accurate 1680 powder is too slow for a 16 inch barrel?
Wondering.

P.S. There is no muzzle device on the rifle. Shooting as-is out of the box. Any brass cased factory S&B, Hornady or Fiocchi ammo do not exhibit the muzzle fireball.

 

[mehavey]

1680 is 1st cousin to H110/w296.
It doesn't like low-volume/reduced loads and is therefore
unsuited for what your're limited to w/ plated bullets.

In fact that loadout dumps almost 25% unburnt at bullet exit
(But you saw that)

 

[jetinteriorguy]

1680 is 1st cousin to H110/w296.
It doesn't like low-volume/reduced loads and is therefore
unsuited for what your're limited to w/ plated bullets.

In fact that loadout dumps almost 25% unburnt at bullet exit
(But you saw that)

I have a question for you, what powder would you recommend for reduced loads using plated bullets? I’ve never tried them but would like to see how they work.

 

[mehavey]

Accurate 2495
Accurate 2015
IMR 4198
Alliant Reloder-7

Any of these give ~2,000fps in a 16" barrel 7.62x39/123gr for 30,000 psi with decent case fill/burn completion ( > 85% in both cases)

But you need an internal ballistics program (like QuickLoad) and a chronograph to predict/check what you're doing.

 

[ciwsguy]

So in a previous posting about the RAR 7.62x39, a responder stated that AA1680 was made for 7.62x39 was wrong? That statement and Hodgdon load data supports it are apparently meant for longer barreled rifles. Would you concur with that assessment?

 

[GeauxTide]

As Mehavey stated and Hornady X confirms, use the recommended powders. My Grendel is based on this case and it shoots .3" with CFE223 at 30gr. Beginning load with 1680 comes in at 2200, according to Loaddata.

 

[TX Nimrod]

The other poster wasn’t “wrong”, AA1680 works well in the 7.62x39 - if loaded to its design pressures. The OP’s load is ~20% below Accurate’s recommended minimums, it should be no surprise that it performs sub-optimally.

 

[ciwsguy]

Berry’s 123 plated bullets max out at 2000 FPS. I don’t have a chronograph, so I loaded to the nearest Hornady X load data for 2000 fps and AA1680 powder. Loading more 1680 powder should result in more muzzle flash. Powder availabilities are limited, so trying to use what I have.

 

[44 AMP]

moved to Handloading forum.

 

[jetinteriorguy]

Accurate 2495
Accurate 2015
IMR 4198
Alliant Reloder-7

Any of these give ~2,000fps in a 16" barrel 7.62x39/123gr for 30,000 psi with decent case fill/burn completion ( > 85% in both cases)

But you need an internal ballistics program (like QuickLoad) and a chronograph to predict/check what you're doing.

Thanks, I wish I could lay my hands on some RE7, it’s produced the best accuracy of any powders I’ve tried so far. I recently replenished my 1680 somewhat and have plenty of jacketed bullets on hand, but I’d really like to get some plated to mess around with while retaining some sense of economy.

 

[armoredman]

I loved AA1680 for 7.62x39mm, but 21.5 grains was my CAST load, so you are loading a wee bit light for jacketed bullets. For the Hornaday 123 gr, the lowest loads was 24gr 1680, up to 27grains, with excellent results from my vz-58, CZ 527M and BREN MS 2.

 

[FrankenMauser]

1680 is 1st cousin to H110/w296.
It doesn't like low-volume/reduced loads and is therefore
unsuited for what your're limited to w/ plated bullets.

1680 and its NitroChemie derivatives and close relatives are some of the most popular powders for subsonic loads in x39 and .300 Blk; and for super and subsonic loads in .30-30, .22 Hornet, and .458 SOCOM.

If it was as bad as you suggest, I don't think the powders would be as popular as they are.

 

[Unclenick]

Loading more 1680 powder should result in more muzzle flash.

It may seem counterintuitive, but the percentage of powder burnt in the barrel goes up with the powder charge. This is because the peak pressure is higher with more powder, and the powder burns faster at higher pressure. How much the burn rate increases with pressure exceeds the corresponding increase in bullet velocity in the barrel, and thus, the powder gets ahead on completeness of the burn with an increase in charge weight.

That said, depending on the powder, increased charge weight also increases the number of under-oxidized combustible molecules, such as carbon, nitrogen, and, in particular, hydrogen, that are in the propellant gases in addition to still-burning powder. Hot hydrogen will burn when it encounters oxygen in the air as it exits. This has a significant effect on the fireball, though a good flash hider, like the Vortex units by Smith Enterprises, can do a lot to cut down on it.

So, whether or not you get a larger or a smaller fireball is going to depend on the balance of those factors. I haven't tried it with that particular powder, so trying it to see what happens will be key.

Also, give Berry's a call and ask what barrel length the 2,000 fps number comes from. It is usually a not-to-exceed peak pressure that actually determines whether you are over-driving the bullet or not. Also, obviously, a nice, smooth, toolmark-free bore will let you drive them harder before copper fouling starts to accumulate. Accuracy falling apart is another indication of driving the bullets too hard.

 

[reddog81]

It may seem counterintuitive, but the percent burnt in the barrel goes up with powder charge. This is because the peak pressure is getting higher, and the powder burns faster at higher pressure.

Yep, more powder = higher pressure = more powder burnt inside the barrel. Some times these things can be counter intuitive but once you understand what is happening inside the chamber/barrel it makes sense. The powder only has a tiny fraction of a second to do it's job. If the pressure isn't high enough it doesn't function properly.

It reminds me of buying a Harley and my Dad suggesting to alter the air fuel ratio so that it runs richer in order to cool the engine down a bit. You see from the factory the bikes are set to use the absolute minimum fuel possible even if it means the bike runs lean/hot. Putting more 3% more fuel into the engine with each stroke means a small amount of fuel isn't instantly vaporized and that tiny amount of extra liquid actually makes the engine run cooler and last longer. Your mileage goes down a bit, your fuel bill goes up 3%, but the bike is happier. I never would have guessed adding more fuel would actually make the engine run cooler and without a detailed explanation would not have believed the comment.

 

[44 AMP]

You can see the same general principles at work boiling water. The temperature water boils at is dependent on the pressure of the vessel.

open pot, pressure is air pressure. At sea level pressure water boils at 212F. But go up several thousand feet and water boils at a much lower temp. Go the other way, increase the air pressure and you can reach much higher temps without reaching boiling. (pressure cooker)

Modern smokeless powder is engineered to burn at a certain rate, and keep burning at the desired rate as the pressure goes up. I can't begin to explain how they do it, or how they figure it out, but they have.

the higher the pressure the "harder" the powder has to burn, to be able to burn and that burn in turn adds to the pressure. The reaction has to become more energetic, just to be able to keep going, and it keeping going adds to the pressure and, and, and... its a loop, that keep building on itself until the limits are reached and no more pressure gets added (full powder burn) OR the system vents (bullet exiting barrel).

The guys who figured it out (AND got it to work) were some "pretty smart cookies" as we used to say.

Lots of other things involved, as well, but in general that's my understanding of the principles at work here.

 

[ciwsguy]

Thanks for the replies. Learn something new every day.

 

[Unclenick]

Modern smokeless powder is engineered to burn at a certain rate, and keep burning at the desired rate as the pressure goes up.

Not clear exactly what you meant by "desired", but, for clarity, I should state that a powder's burn rate is determined under standard measuring conditions. It does not stay the same as pressure and temperature change. If it did, powder burned in the open on the ground would burn as fast as it does inside the gun, and putting cartridges in a hot chamber would not make them shoot at a higher pressure than those in a cool chamber. But what is amazing about modern powder chemistry and engineering is that it can affect the amounts by which pressure and temperature influence burn rate, mitigating both to varying degrees, and that is, indeed. impressive.

 

[44 AMP]

What I meant by "desired" is the rate desired by the design chemists and it does change at different points on the pressure curve.

Being able to create something like that, where the actual burn speed changes to suit the changes in pressure as the powder burns (and the volume of the gas are changes as the bullet moves down the bore) THAT is quite impressive in my book.

 

[Unclenick]

Here's an example of a solid propellant burn rate vs. pressure plot from MIL-STD-26C. It's for a solid rocket propellant, but the same principle applies to powder materials. The horizontal axis is operating pressure in megapascals, and the vertical axis is the first derivative of pressure with respect to time in megapascals per millisecond, which, in a closed bomb, is a number proportional to burn rate. Both axes also have the voltage output that corresponds to the measurement value for their particular transducers.

 

[RKG]

You can see the same general principles at work boiling water. The temperature water boils at is dependent on the pressure of the vessel.

open pot, pressure is air pressure. At sea level pressure water boils at 212F. But go up several thousand feet and water boils at a much lower temp. Go the other way, increase the air pressure and you can reach much higher temps without reaching boiling. (pressure cooker)

Modern smokeless powder is engineered to burn at a certain rate, and keep burning at the desired rate as the pressure goes up. I can't begin to explain how they do it, or how they figure it out, but they have.

the higher the pressure the "harder" the powder has to burn, to be able to burn and that burn in turn adds to the pressure. The reaction has to become more energetic, just to be able to keep going, and it keeping going adds to the pressure and, and, and... its a loop, that keep building on itself until the limits are reached and no more pressure gets added (full powder burn) OR the system vents (bullet exiting barrel).

The guys who figured it out (AND got it to work) were some "pretty smart cookies" as we used to say.

Lots of other things involved, as well, but in general that's my understanding of the principles at work here.

1) I am not sure that the phase change (e.g., water into steam) observation is relevant here. As a general matter, jumping a phase change requires significantly greater energy than simply temperature changes within a given phase.

2) If (and I am not sure it is so) that this post assumes that combustion chamber increases as the bullet travels down the bore, I do not believe that is accurate for most cartridges. Combustion ends when peak pressure occurs, and I think that modern piezo equipment has shown that, depending on cartridge, peak chamber pressure occurs somewhere between zero bullet movement from the case to maybe an inch or so. After that, combustion chamber will decay as the bullet travels down the bore, simply because combustion chamber volume is increasing. (Remember "Deflategate" and the Ideal Gas Law?) However, so long as post combustion chamber pressure exceeds bore friction, the velocity of the bullet will increase. Up to the point where the bullet exits the muzzle and combustion chamber vents to the atmosphere.