Wasn't expecting this....

[Hook]

I have been loading for and shooting a Grendel upper for about a year. It has been a blast, but not without difficulties. When I first started, brass was unobtanium, so I reformed a batch of small primer 7.62X39 I had set aside and used it. It averaged about 15-20 grains heavier than the 4 pieces of Grendel brass I'd found, so I reduced the powder charge accordingly. Eventually I bought some Starline Grendel brass but am still shooting the reformed brass.

Got to thinking the other day about the overabundance of LR primer 7.62X39 brass I have available and wondered if it would perform as well as the SR primed brass. If it did, I could hoard the Starline brass I have for 'important' stuff and use the reformed for fun. My basic frugal nature would be tickled not to have to buy more expensive Grendel stuff. I just didn't know what the difference would be using the LR primers.

To try to get an idea, I loaded a stiff load using Starline brass and another load with 2 grains less using SR primer reformed brass. The velocities averaged within about 25 fps of each other. Then I tried the same reduced load in the LR primer reformed brass (BTW, it weighed the same as the SR primed reformed brass). It averaged almost 100 fps SLOWER. I was worried the LR primer would jack up pressures and I'd have to drop the powder charge more but then this happened....

Now, I only tested 3 of each of these loads but assume they are representative. How in the world can large primers produce less velocity than small primers? Am I missing something?

[LR1955]

I have been loading for and shooting a Grendel upper for about a year. It has been a blast, but not without difficulties. When I first started, brass was unobtanium, so I reformed a batch of small primer 7.62X39 I had set aside and used it. It averaged about 15-20 grains heavier than the 4 pieces of Grendel brass I'd found, so I reduced the powder charge accordingly. Eventually I bought some Starline Grendel brass but am still shooting the reformed brass.

Got to thinking the other day about the overabundance of LR primer 7.62X39 brass I have available and wondered if it would perform as well as the SR primed brass. If it did, I could hoard the Starline brass I have for 'important' stuff and use the reformed for fun. My basic frugal nature would be tickled not to have to buy more expensive Grendel stuff. I just didn't know what the difference would be using the LR primers.

To try to get an idea, I loaded a stiff load using Starline brass and another load with 2 grains less using SR primer reformed brass. The velocities averaged within about 25 fps of each other. Then I tried the same reduced load in the LR primer reformed brass (BTW, it weighed the same as the SR primed reformed brass). It averaged almost 100 fps SLOWER. I was worried the LR primer would jack up pressures and I'd have to drop the powder charge more but then this happened....

Now, I only tested 3 of each of these loads but assume they are representative. How in the world can large primers produce less velocity than small primers? Am I missing something?

Hook.

Having loaded a bunch of 7.62 X 39 stuff when the only place making Grendel brass was Lapua and none of it was in the USA, all I can say is -- I don't know why.

I do know that with the same charge as real Grendel brass, the 7.62 X 39 normally got higher velocities but I could tell the pressure spike was severe. Also, the large primer pockets opened up in two or three shots. This is pretty much the result of the 7.62 X 39 brass being thicker so there was less powder space which no doubt spiked the pressures.

There is enough real Grendel brass about today that forming 7.62 X 39 is pretty much a waste of time. You get lower velocities and shorter brass life. Probably OK for throw away stuff but otherwise not as efficient as real Grendel brass. If I wanted some throw away brass I would buy some once fired, shoot it to make sure it functioned in my rifle, then load it up and keep it for those times when losing brass is expected.

LR-55

[Lemonaid]

My experience is similar to LR1995's. It may depend on what brand of 7.62x39 brass is used. Also, tested results with CCI 400 vs. CCI 450 (magnum) primers I got slightly higher velocities with the the CCI 400. It may be a function of how the primer effects the burn of the powder.
The other minus for using 7.62x39 brass is the cost of a separate load development. A plus is that if you have lots of LR primers you can still shoot if SR primers are hard to get.

[Hook]

The other minus for using 7.62x39 brass is the cost of a separate load development. A plus is that if you have lots of LR primers you can still shoot if SR primers are hard to get.

My pluses are:

1. I have a large supply of 7.62X39 brass and only one rifle left in that caliber
2. Retired, so have the time to reform
3. Basic chintziness keeps me from buying new brass when I can reform from a large stockpile

I have a good supply of LR and SR primers so that isn't the issue.

Went out this morning and tried a different, lighter load. I use it for plinking and fun. It consists of a 120 gr bullet (Barnes MBs in this case) and 22 gr of DP2200, all in reformed brass. This time the velocities were roughly the same, around 2050 fps. Shot 5 3 shot groups at a moderate pace without any bbl cool down. The first 3 shots were using SR brass and the next four groups used LR brass. The groups measured 1.34", 0.225", 0.94", 0.90", and 1.14".

Don't have a clue what the almost 100 fps swing was caused by in the other loads....maybe my chrono had a hangover. It is encouraging that the primer size doesn't seem (at least a first blush) to make a difference in lower velocity rounds because that is what I intend to use the reformed brass for anyway.

Happy shooting y'all!

[tdbru]

Hook,
with my LR reformed 7.62x39, I get way high pressures if I use the same load as SR 6.5Grendel brass. at least that's been my experience. I still have a few reformed cases. Once Grendel brass became more available I quit doing all the work to reform 7.62x39 brass.

remember to consider neck turning as most cartridges' neck wall thickness increases as you reduce the diameter. if the neck wall get's too thick it can't release the bullet and pressures go up quickly.

also, I've had good luck extending all my brass reloading life via neck annealing regularly.

best of luck,
tdbru

[Hook]

Thanks for the comments everyone.

tdbru, as stated in the original post, reformed cases averaged 15 or so grains heavier in both SR and LR 7.62 cases. I reduced loads from the get-go to achieve the same velocity as with Grendel cases. None of my loads, including those using Grendel cases are maximum. Neck turning has not been necessary so far as a bullet will easily slip into the necks of fired cases indicating proper dimensions. I've also been annealing before reforming and plan to reanneal after each firing if needed.