performance differences: fireformed 7.62 vs genuine Grendel brass

[NordicG3K]

I've been shooting my two Grendels for a few years now, and 99% of my shooting has been with handloads using fireformed IMI 7.62x39 brass. I've heard a number of times that the case capacity of fireformed 7.62x39 brass is slightly less than genuine 6.5 Grendel cases.

Can anyone give me an approximate fps difference between two similar loads using the different brass? I've got 1000 rounds of the IMI brass and I'm trying to decide if there's going to be a big enough difference in performance to justify investing in the more expensive Grendel brass.

Thanks in advance.

[Bwild97]

I think you may find your answer here, in the early pages of this thread More-Chrony-reports!!-((WARNING!))-lots-of-Pics Using Grendel brass will keep velocities respectable, as well as chamber pressures.

[pinzgauer]

Been shooting IMI 7.62x39 pretty much since 2008/2009. LR1995 did quite a bit of testing, and my experience was very similar to his. Here's the high points:

- Case capacity is lower (5-10%) requiring less powder to avoid excessive pressure. I measured mine and found LR's posts to be dead on

- Lower case capacity does not automatically translate to lower velocity. Depending on the load, velocities were very close to published loads for Grendel brass. IE: don't assume 10% lower velocity due to 10% lower case capacity. This will vary with powder & bullet selection

- the lower capacity does impact usability of certain powders & bullets. You run into compressed loads before you reach optimal load. This is less of an issue with the newer bullets (123g AMAX, etc) or the 90-100g options. Stay away from the long bullets (120g AMAX, etc) and it's not a disadvantage

- Fireform loads are as accurate or more than full power loads. Several of us have seen this, and I found a fireform load using 100g bullets that shoots to same POI as my full power 120g loads. So most of us do not consider the fireform process "wasteful".

- IMI 7.62x39 brass lasts longer than Wolf, but less than Lapua Grendel. I see 8-9 loadings (including the fireform) from my brass before primer pockets get loose and I retire them. I'm not having primer fall out, but notice the reduced seating pressure while reloading. Now I use a Lee case trimmer pilot to try to push out the primer by hand. If it pops out with handheld pressure I retire it. If not, it goes back into the loading pile. My conclusion the slightly lower life before loose primers is more a function of using large vs small primers. It's not brass quality as the IMI is very good brass, and quite possibly was made by Lapua as well.

Ultimately, fireforming made sense in '08-09 with the great IMI deal combined with $1-1.5/case Lapua cost. But with 7.62x39 brass now much more expensive, and Lapua/Hornady Grendel brass widely available at reasonable proces, I would not recommend fireforming unless you had brass in hand. I still shoot IMI with 2520 & 123g AMAX's. Works great, I'll keep shooting it until the brass is worn out.

If starting from scratch it just makes sense now to use factory Grendel.

Have 7.62x39 brass on hand? If you are just looking to shoot a solid load, it can work well. Hard to beat some of the published IMI loads for the Sierra 120g and 123g AMAX's. AA 2520, 8208, etc. ( I use 2520)

If you want to play with many different loads, are looking to get the last 50fps of performance, or use published loads? Then the answer is clear, get Lapua or Hornady brass.

[bwaites]

10% difference in case capacity really equates to more like 2-3% difference in velocity. Velocity is a function of pressure, and a smaller case can generate pressure easily enough with modern powders. You will be somewhat limited in powders, because the bulkier powders that some use to great effect might not allow a good fill. BUT...CFE 223 might be a perfect fit for those using 7.62 brass, since it is denser and finer and allows more powder into a smaller case.

If you have lots of IMI, and don't mind fireforming, its a reasonable option. I would not buy 7.62 brass because it was few dollars cheaper, though. I would just buy Hornady or Lapua.

[NordicG3K]

Sounds like good advice guys. I purchased the IMI brass when it was cheap, and at that point the only other real option was Lapua which I simply couldn't afford in quantity. My 18" Grendel loves the Nosler 120gn BT out of once-fired IMI cases. I'll stick with my IMI until it wears out and then upgrade.

[LRRPF52]

I asked one of the head engineers at Lapua this year if IMI brass is made by Lapua, and he said it is not. IMI brass is great brass, though. I prefer small rifle primer Lapua brass in the Grendel. I have tried fire-formed 7.62x39 Federal, and I even have some Lapua 7.62x39 that I could use, but why bother. The PPU brass is very short-live in my experience. Primer pockets are done after the 2nd load.

[txgunner00]

I'm with the others on the slight reduction in velocity. I horse traded for 1000 pieces of IMI brass so that's the only reason I have it. I use it for subsonic loads, which fireforms it, then load full power hunting/ blasting loads with it. That way if I can't recover the brass in the field I'm not out much.

[pinzgauer]

I'll stick with my IMI until it wears out and then upgrade.

That's my plan.... but I'll declare my IMI experience a raging success:

• I was able to get brass & shoot when there was no Grendel brass available at any price
• The Large Rifle primers aspect worked to our advantage when all the Small Rifle primers were sucked dry post 2008 election
• Performance is very good for 120g and smaller bullets
• Case life has been reasonable

So in the same circumstance I'd go for IMI again. And if I could find virgin IMI 7.62x39 at the same price, I'd buy some more.

Being able to use common brass in a jam is also one of the Grendel advantages. Much easier to find russian brass than the parent case for the 6.8.

As to who made the IMI 7.62x39 brass, back in the 80's much IMI 5.56 was made by PMC. While the IMI 7.62x39 brass does not look like PMC (color, annealing, etc) that would be another candidate.