.762x39 for 100 Yard Group

[AlNyhus]

A comparison of the water capacity (filled to the case mouth) of your 30PPC, Mike's 30 Major and the 30BR will be important when deciding on a case capacity to hit.

Based on the work I've done with shortened .30s and 'bigger' 30BRs, there's some guidelines I use. When looking at changing case capacity to make a different burning rate powder 'work', expansion ratio becomes an important piece of the puzzle.
If you start with a combo that already has a good expansion ratio, it takes quite a bit more change to make that work with a different burn rate than going the other way.
Comparing what are considered to be 'perfect' cases for precision is worth doing when you're thinking about doing this.
My two cents on a Sunday morning -Al

 

[LaMan]

It's been a while since I checked. I don't remember...Pretty much right between a ppc and a br, though. I may get a chance to check a few later on today.

Like maybe the 6ARC necked up to 30

 

[gunsandgunsmithing]

Like maybe the 6ARC necked up to 30

The Grendel is bigger than the ARC., by like .030. Frankly, I'm not sure why or how there was a place for the ARC, being between a ppc and a grendel. I suppose it was likely more to do with proprietariness or anything other than a significant performance reason. Being SAAMI recognized is the only reason I'd consider an ARC. The PPC and 6 Grendel are both easily formed with Lapua brass, be it 220R or 6.5 Grendel being used. Blowing out or shortening is just too much trouble for the difference, IMO. Using something that is easily formed from premium quality brass is more important to me than the saami recognition of something that takes more effort and expense to form. You might be able to neck down and push the grendel shoulder back to ARC length, but WHY? You might be able to fire form 220R brass to ARC length too, but WHY? The ARC is a shortened 6 Grendel or a lengthened PPC. Not much room in between there. That's what I don't understand. It could be viewed as a saami version of either, but without the best brass available.

 

[gunsandgunsmithing]

A comparison of the water capacity (filled to the case mouth) of your 30PPC, Mike's 30 Major and the 30BR will be important when deciding on a case capacity to hit.

Based on the work I've done with shortened .30s and 'bigger' 30BRs, there's some guidelines I use. When looking at changing case capacity to make a different burning rate powder 'work', expansion ratio becomes an important piece of the puzzle.
If you start with a combo that already has a good expansion ratio, it takes quite a bit more change to make that work with a different burn rate than going the other way.
Comparing what are considered to be 'perfect' cases for precision is worth doing when you're thinking about doing this.
My two cents on a Sunday morning -Al

I checked a few cases. Best I can tell, call it 38.0 grains of h2o, Al...with the water barely crowned on top. My usual method is to add a couple of drops of dish soap to the water when doing this. I'm sure you understand why without me getting into it. I like to use alcohol for comparisons but it kinda skews your request for h2o capacity.
I will add that when I first started testing a 30 Major, I had my pressure trace hooked to it. I sent several messages back and forth and spent some time on the phone with the software developer. At first, he thought I had something hooked up wrong or something. After conversing a fair amount, he said it was the most "efficient' case he'd ever seen. That and a dollar might get ya cheap cup of coffee somewhere but it has been a great shooting cartridge for me. I've shot lots of small groups with h4198, rl7 and other powders that were a little on the slow side for the case. N120 was always what I came back to, though. It was the magic pixie dust. Regardless of what the burn rate charts say, in this cartridge and light bullets, those lots were always a good 1.5-2 full grains faster than h4198, by both pressure and velocities.
H4198 and rl7 both shot very well but it was a pain getting enough in the case and handling them while loading. With n120, my go to base charge of 30.8gr, dumped from a measure, comes up to a little over half way up the neck. This is what I call very good as it left the bullet just barely compressing the powder column.

 

[AlNyhus]

Mike, my fired 1.500 long 30BR cases average 40.8 of water, filled to a slight meniscus....a bit of dishwashing soap as the surfactant.

On a brief mini tangent, I've never understood why cases capacities are listed "....with a bullet seated normally" or holding a "....certain amount of a certain powder" . Neither way gives any real information!

It will be interesting to see what Jackie's 30PPC case holds.

 

[Spook]

I was messing with a .338 X 39 for a while and was amazed at how well it grouped with factory bullets and Lapua brass w/large primers. Now if someone would just make a light , custom .338 bullet.....

 

[jackieschmidt]

A comparison of the water capacity (filled to the case mouth) of your 30PPC, Mike's 30 Major and the 30BR will be important when deciding on a case capacity to hit.

Based on the work I've done with shortened .30s and 'bigger' 30BRs, there's some guidelines I use. When looking at changing case capacity to make a different burning rate powder 'work', expansion ratio becomes an important piece of the puzzle.
If you start with a combo that already has a good expansion ratio, it takes quite a bit more change to make that work with a different burn rate than going the other way.
Comparing what are considered to be 'perfect' cases for precision is worth doing when you're thinking about doing this.
My two cents on a Sunday morning -Al

Al, that is kind of my thinking on going all the way down to the capacity of the factory tapered .762x39. There might, (big might), be a sweet spot there with the current 4227.

I still think it will need a 100 grain bullet Built on about a .875 jacket. I’m pretty sure my dies will handle that with no problem. From my experience with the 30PPC in the past, I think 2800 FPS is well within the realm.

The big question would be agging capability at 100.

 

[jackieschmidt]

Pinch trim die makes quick work ....

George, I see Corbin makes a pinch die. What would you recommend.?

 

[geo.ulrich]

Sent you a p.m.

 

[jackieschmidt]

I picked up a few old fired steel AK rounds at the range, and sized one down, then turned the neck to .0095 thick for a .330 neck. This bullet is seated with the base just shy of the neck shoulder junction.

A zero freebore might be too tight, as that base needs to be at least 1/16 above the neck shoulder junction.

I’m going to make up a couple of cases and send them to PTG.

Heck, it almost looks like a real Benchrest round.

I checked the capacity of this case against my own 30BR with a fine ball powder, and it is 5 grains less.

 

[AlNyhus]

Jackie, did you ever get your hands on any Lapua cases?

 

[jackieschmidt]

Jackie, did you ever get your hands on any Lapua cases?

Not yet. I have been busy getting ready for the Tackdriver 2, and just never got around to ordering any.I might do that this morning.

 

[jackieschmidt]

Not yet. I have been busy getting ready for the Tackdriver 2, and just never got around to ordering any. Nobody seems to have any.

 

[AlNyhus]

I don't have a new Lapua 7.62X39 case here anymore. But I jotted down the dimensions of a new one years ago and kept it. It measured .443-ish @ .200 ahead of the extractor and .395-.396 at the body/shoulder junction. I normally measure the water volume but didn't on that one for some reason.

Not sure if those dimensions hold true for the current stuff........ -Al

 

[AlNyhus]

Precision Reloading in Mitchell, S.D. is about an hour from me....they show it in stock. -Al

Shop Rifle & Shotgun Reloading Supplies | Dies | Bullets | Hulls | Wads | Powder - Precision Reloading

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[jackieschmidt]

Precision Reloading in Mitchell, S.D. is about an hour from me....they show it in stock. -Al

Shop Rifle & Shotgun Reloading Supplies | Dies | Bullets | Hulls | Wads | Powder - Precision Reloading

Shop online for shotshell and metallic reloading supplies, shooting supplies, metallic presses, rifle reloading components, shotshell reloading components, shooting accessories, rifle reloading brass and Bullets.

www.precisionreloading.com

AL, just ordered 100. Thanks.

 

[gunsandgunsmithing]

I don't have a new Lapua 7.62X39 case here anymore. But I jotted down the dimensions of a new one years ago and kept it. It measured .443-ish @ .200 ahead of the extractor and .395-.396 at the body/shoulder junction. I normally measure the water volume but didn't on that one for some reason.

Not sure if those dimensions hold true for the current stuff........ -Al

Al, fwiw I found a couple of posts on other forums that might be of some interest to both you and Jackie. I'll just do a copy and paste here. The first post, the guy is using different brass to form into his 6.5 Grendel chamber but it compares some different makes and capacities, as well as his findings. I agree with what he says about the Lapua 7.62x39 brass being heavy and soft.

I want to collect a list of all the case water capacity and maybe of older brass and new Lapua?
Or is there maybe a difference in Lapua 6.5 Grendel Brass in Europe and the US?

What's my actual problem, the fireformed brass have lot less case water capacity then QuickLoad calculates and you can't use the loads in the reloading manuals.
Ok that's no real surprise, that's why I bought new 6.5 Grendel Lapua brass and fireformed my old 220 Russian I used in my 6PPC before.
Both have almost the same water capacity, but lot less then Quickload calculates and I still can't use the loads written in the reloading manuals, not enough case capacity.

Now the surprising thing is that new S&B 7.62x39 brass fireformed to Grendel only have 0.7 grain less water capacity then my fired Lapua 6.5 Grendel.
And they have more capacity than the original Hornady 6.5 Grendel brass!

So here my avg. water capacity results after firing I would like to share and hope you could share your results to compare:


6.5 Grendel factory brass:

- Lapua 6.5 Grendel: 36.35 grain H2O / 1.524"
- Hornady 6.5 Grendel: 35 grain H2O / 1.515"


Fireformed brass:

- Lapua 7.62x39: 33.73 grain H2O / 1.501" (Easy to form, poor capacity and they are extremely soft. You will have very fast overpressure signs -> 27g Tac with 128g Scenar!)
- S&B 7.62x39: 35.58 grain H2O / 1.502" (They are quite tricky to form, doesn't work with a standard FL Die!)
- used .220 Russian brass, served before in my 6PPC: 35.42 grain H2O / 1.487"

Below is another post with h2o capacities listed for different 7.62x39 brass.


I posted this before, but it relates to your question. It's fair to say that the heavier the case, the less volume. This is because the case measures the same externally, so any weight difference will mean the internal volume has to change. Heavier brass - less volume. Lighter brass - more volume.

Sometimes this difference is inconsequential. But it is not always so. As well, smaller cases will see faster, more aggressive swings in pressure.

Smaller cases like the 7.62x39mm or 223, are more sensitive to changes. These differences can take a safe load to unsafe, just by switching the brass you use. You will get a large change in pressure for not much of a velocity increase.

In 2008, I tested six different 7.62x39mm cases. Here are some of the results. There have always been differences in weight and volume with different lots made by the same manufacturer.
---

Empty Case Weight by Manufacturer - without primer

Igman 105.8 grains
Prvi Partizan 111.9 grains
Federal 118.0 grains
Winchester 122.1 grains
PMC 122.4 grains
Lapua 126.6 grains

Case Capacity – water weight in grains

Lapua -- 33.6 grains
Prvi Partizan -- 35.2 grains
Igman -- 36.0 grains
Federal -- 37.6 grains
PMC -- 37.6 grains
Winchester -- 37.9 grains

Pressure Differences Caused by Changing Case Brands

The powder charge, bullet and primer remained the same, but look what happened to the pressure when the case capacity was reduced. The 'Pressure by Case' goes from the most to the least internal volume. As case volume decreases, pressure increases.

These volume differences show why you should always start at the minimum load listed!!

The Load
Bullet - Lapua FMJ
Primer - Winchester Large Rifle
Powder - 28.5 grains of AA1680
Cartridge over all length of 2.200 inches

Maximum Safe Pressure - 51,488 PSI

Pressure by Case

Brand -- Pressure (PSI) -- Percentage of case filled
1. Winchester -- 50,264 -- 86
2. PMC -- 51,150 -- 87
3. Federal -- 51,150 -- 87
4. Igman -- 56,522 -- 91
5. Prvi Partizan -- 59,701 -- 93
6. Lapua -- 67,921-- 99

 

[AlNyhus]

Great info, Mike! The Lapua case capacity looks like it's where it should be. As to whether it's weight translates into long life....who knows? Jackie isn't afraid to go into uncharted waters and that's how we learn stuff, right?

Being a pretty simple sort, I'd push the Easy button, shove the shoulders back on Lapua 6.5 Grendel cases and make it a .30 'Mini Major'.

Good shootin'. -Al

 

[gunsandgunsmithing]

Great info, Mike! The Lapua case capacity looks like it's where it should be. As to whether it's weight translates into long life....who knows? Jackie isn't afraid to go into uncharted waters and that's how we learn stuff, right?

Being a pretty simple sort, I'd push the Easy button, shove the shoulders back on Lapua 6.5 Grendel cases and make it a .30 'Mini Major'.

Good shootin'. -Al

I love easy!
It's been too long for me to remember all the details from testing 7.62 brass to form the Major out of but I do recall nothing came close to the Lapua Grendel brass for pressure. I remember trying some Remington small primer 7.62 brass in it, as they did use to make it, but they no longer do. My standard match load (Lapua Grendel brass)wrecked it on the very first firing. That didn't take long! Lol!

 

[jackieschmidt]

It does surprise me that the Lapua case has that much less internal volume than the others. The only reasoning for this, I would assume, is Lapua was going for strength, especially for someone reloading fo a gas gun.