Load development 7.62 x 39 check my math?

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aspenkarius

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So my latest idea involves hornady gmx sized 145gr bullets and an sks. (I have my reasons)

I began my journey in my hornady manual as it is the only one I have that lists 150gr loads for the 7.62x39 round.

I found lots of data but not for any powders I have so I want to be double sure before I fire them off.

My next step was hodgdon a site where I found data for a hornady 150gr SP and BL-c(2) which is my powder of choice for this project.

Now the bullet in question is shorter than the gmx and this lead to immediate problems as I couldn't even come close to the COAL of 2.200. I did notice that the hornady gmx 150gr is pretty much exactly the same size though and this is where it got intresting.

I deduced that the loads of 27-29gr BL-c(2) were out the window so I loaded down till I could hit coal of 2.220 which is max magazine fit and the gmx coal. This left me with 24gr powder (-12% from minimum load via hodgdon)

I found the CC/gr for accurate 2015 (max hornady load 27gr for the gmx) and figured that by volume it was 12% more per grain. (.0730cc vs .0645cc) I then did more math and found that the max recommended load for the gmx was equivalent volume wise to 23.8gr of BL-c(2) which was where I was at anyways.

I'm thinking that this proves a few things.

1. This round is good for a compressed load without exploding

2. I am 12% under min load for this powder and bullet weight.

Do do you think this is safety margin enough? I'm thinking of testing from behind cover with a string but I don't want to blow up my sks.

Let me know your thoughts. Thanks.
 
I think you are grossly over-thinking this.

COAL is determined by your rifle chamber/magazine. Period.

BLC2 is a safe powder to use. If the book shows a minimum, start there and work up to find what works well.
 
aspenkarius said:
My basic thought is that there is no load data for this bullet in this case with this powder.

I can find this case and this powder with a vastly shorter but same weight bullet.
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Don't worry about the length of the bullet if it's shorter. Just make sure to seat it deep enough for proper neck tension and then as Ganderite says, start low and work up.

Where did you get the idea that compressed loads can explode?
 
The bullet I'm using is close to .25" longer than the one I find data for.

As far as the compressed load I'm mostly worried about over pressure due to more bullet in the case than the data calls for. Now I'm hoping the lower powder charge will even that out, I'm just lookin for input from more experience loaders.
 
As the others have said if you have the same bullet weight (+/-10gr) with load data for the powder you want to use, then use what ever OAL that works for your gun/mag. Start at the minimum and work up.
 
I just gotta ask the OP what he is shooting these in? I was able to get 174gn RN bullets to fit under the COAL of my rifle and still get enough powder (WC735, pretty close to BL-C2) in it to get nearly 2000fps.
 
rookie wildcat said:
I just gotta ask the OP what he is shooting these in? I was able to get 174gn RN bullets to fit under the COAL of my rifle and still get enough powder (WC735, pretty close to BL-C2) in it to get nearly 2000fps.
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Are you sure about that??

It would be hard (as in IMPOSSIBLE) to get enough BL-C2 into a 7.62x39 case to get 2000 fps with a 174 gr bullet. I think you'd run out of powder space and top out at 1300-1400 fps.

Maybe you were thinking of the 7.62x54R, not the 7.62x39? 2000 fps would be possible in the 7.62x54R.
 
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A Textbook of Engineering Material and Metallurgy

...before tying a string to a trigger sit down this winter with a good materials textbook and work your way through not just the structure of metal, but of the nature of repeated stress on materials...

all the best
 
The sks is a very strong action. Coal doesn't mean much if the round will chamber.
I am curious as to what you want the gmx bullet for. At the lower velocity cup and core bullets actually preform better then most premiums

Also cubic volume is not a good way to compare powders. They have different burn rates and behave differently
 
rookie wildcat said:
I just gotta ask the OP what he is shooting these in? I was able to get 174gn RN bullets to fit under the COAL of my rifle and still get enough powder (WC735, pretty close to BL-C2) in it to get nearly 2000fps.
Click to expand...

With 174 gr FMJ (a bit longer than 174 gr RN) at an OAL of 2.220", I got 1800 fps over 24.0 grs of WC-735. WC-735 is faster than BLC-(2) by about 2.0 grs in that application, so in my gun, 2000 fps would require about 32.0 grs of BLC-(2) which would be over pressure if it could fit in the case (and it would not). I haven't tried it, but about 28.0 grs is about all that would fit IMO.
 
I wast comparing powders so much as looking to see if compressed loads existed for the 7.62x39. So far I gather that being a compressed load does not mean it will pressure spike. As my charge is 12% below min spec I'm thinking I should be fine. I know low volume charges may cause pressure spikes as well but my understanding is that relates to excess air volume which I do not have. To answer why I'm choosing to bang my head off a wall? Because I can. I tend to push the envelope when I get bored.
 
Loading below minimum can be as dangerous as above max loads. Causes weird pressures from the powder detonating vs burning. It's very decidedly unsafe.
None of the BL-C2 loads on Hodgdon's site are compressed. You're doing something incorrectly. And forget CC's altogether. They do not apply under any circumstances.
'Close' only counts with horse shoes, hand grenades and nuclear weapons.
 
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