How much neck length is too much? 7mm wildcat question

[smokinghole]

I'm working on a 7mm long action magnum that's wildcat-lite. I'm taking 28 nosler cases, pushing the shoulder back with a 30Nosler Redding Type S die and appropriate bushings. Then I push them back a tad more with a 33 Nosler die. I tried going straight from 28 to 33 but it crunched the shoulder on a good percentage of attempted cases, causing case shortening. Anyhow going from a 28 Nosler case to 33 Nosler with 30 Nosler die in between lengthens the neck from the 28 Nosler at ~0.276" to about 0.400". I think I might trim them all down to 0.385-0.390" as my trim to length.

Also to head a few off at the pass. I'm doing this to retain a standard bolt face, no belt, COAL of ~3.65 or less with berger 195s seated at the shoulder junction in standard AICS long action mags, off the shelf sizing dies/seater, and norma/Nosler brass. I know the 7 Dakota and 7LRM are close but both are Hornady brass, the Dakota needs a .545 bolt face, the LRM requires buying/borrowing a reamer from Gunwerks.

Anyway with that said, with a 7mm bullet is there too much neck length? I think the 7LRM has a neck ~0.360" .

My case before fireforming, of course, holds about 93.4gr H2O. If I stop at the 30 Nosler die my H2O is about 96.4gr it offers a 0.330" neck length and still allows for AICS mag length feeding.

The pictures show the coal, comparison with Federal 6.5 Creedmoor loaded with a 130gr AR OTM, and comparison with a 7SAUM loaded with a 175 Berger (excuse the ugly RP brass).

 

[Dave Karcher]

Interesting project. A 195 gr Berger out of a 28 Nosler case may produce enough velocity to blow up the thin jacketed 195 hunting round. Several people have had this problem. Have you considered the 197 gr Sierra SMK? Now to your question, a longer throat will help erosion and extend barrel life. But the real issue is, how much is too much? The Gunwerks guys selected 0.382", so your 0.330" result sounds about right. If you get this working, I'd like to know about it. I like the 7mm LRM all except the Hornaday Brass, you solve this problem.
Good luck!

 

[smokinghole]

Interesting project. A 195 gr Berger out of a 28 Nosler case may produce enough velocity to blow up the thin jacketed 195 hunting round. Several people have had this problem. Have you considered the 197 gr Sierra SMK? Now to your question, a longer throat will help erosion and extend barrel life. But the real issue is, how much is too much? The Gunwerks guys selected 0.382", so your 0.330" result sounds about right. If you get this working, I'd like to know about it. I like the 7mm LRM all except the Hornaday Brass, you solve this problem.
Good luck!

I've been eyeing the 197SMK too. I'm wondering what twist that bullet really needs. I've been reading the 155SMK in a 6.5-284 thread here and they seem to be thinking faster than recommended by Sierra is necessary. For 197s Sierra lists I'd need a 7.5" or maybe even just get a 7" in 5R from Kreiger to be certain.

I'm likely to order a reamer soon and I'll go to the 33nosler shoulder dimension. That puts my neck closer to 0.380"+.

A thing to note. I've heard the 7LRM might be migrating away from Hornady brass. I believe they're working with ADG brass which should give the LRM new life. With that said I am kind of waiting for that to happen or not happen. If adg starts making LRM. My project is likely to float off into never followed through land.

 

[Dave Karcher]

If you have not used Berger's twist rate calculator you should check it out. http://www.bergerbullets.com/twist-rate-calculator/
Here in Utah, 5,000 feet elevation an 8.5 twist is stable for the 197 gr SMK, the required twist is a function of lots of factors.
Thanks for the news about the 7mm brass.

 

[smokinghole]

http://forum.accurateshooter.com/threads/150-smks-and-the-6-5-x-284-update.3944073/page-2

I found this to be an interesting read. Sierra is recommending a 7.5" for the 6.5s but he now has a 7" on the way to see if it helps.

I'm in PA at sea level basically. A 7.5 should be fine for me, but that's a minimum recommendation by Sierra. I think a call to Sierra is in order when I'm ready to order my barrel.

 

[dkhunt14]

http://forum.accurateshooter.com/threads/150-smks-and-the-6-5-x-284-update.3944073/page-2

I found this to be an interesting read. Sierra is recommending a 7.5" for the 6.5s but he now has a 7" on the way to see if it helps.

I'm in PA at sea level basically. A 7.5 should be fine for me, but that's a minimum recommendation by Sierra. I think a call to Sierra is in order when I'm ready to order my barrel.

Sierra tends to over exaggerate twists. They recommend a 9 twist for their 240 Matchking. We shot them at 1000 yard BR with 10 twists and they were stable. Nice round holes in paper and great groups. I'm also in PA and this is not the only bullet we saw do this. I believe they tend to error on the side of stability. Matt

 

[Capt. Oblivious]

Thats certainly one good looking round there

 

[smokinghole]

Sierra tends to over exaggerate twists. They recommend a 9 twist for their 240 Matchking. We shot them at 1000 yard BR with 10 twists and they were stable. Nice round holes in paper and great groups. I'm also in PA and this is not the only bullet we saw do this. I believe they tend to error on the side of stability. Matt

Unless they'll blow up at 7.5tw and 3000-3100 fps, why not just go faster? I probably can't shoot 168s at that twist but I have a 7SAUM for them.

 

[ShaunBrady]

If you have not used Berger's twist rate calculator you should check it out. http://www.bergerbullets.com/twist-rate-calculator/
Here in Utah, 5,000 feet elevation an 8.5 twist is stable for the 197 gr SMK, the required twist is a function of lots of factors.
Thanks for the news about the 7mm brass.

I've had experiences similar to what Berger.Fan222 describes in that thread with 7s and agree with his conclusions. I have video of bullets bursting 150 - 200 yards out. I agree that it's likely more the lead softening from heat causing the failures than the jackets not being able to contain them. I'm sure it's not completely black and white. It looked to me like a long high temperature push down the barrel weakened the bullets and aerodynamic heating finished them off. That same long high temperature push produced barrel wear a lot different than you see in the typical target gun.

I also saw the bullets starting to fail later in long strings, then sooner as the barrel wore. I shot the 195 Bergers out of a 7/338 lapua improved at up to 3350 fps when the barrel was fresh, they wouldn't all stay together below 3200 fps after a few shots a couple hundred rounds later. A straight 7/338 lapua took about half again as many shots to start the same routine 150 fps slower. Both ate the Hornady 180 ELDm at the same point in their barrel lives.

The guns were built to test the costs and benefits of overbore for ELR shooting. The idea was to do the testing with a relatively small caliber before stepping up to more expensive equipment. We have a test range that is similar to day 1 of the KO2M and use a similar course of fire. At the time, we were using 5 shots per target on 4 targets from 1200 to 2200 yards.

My biggest take away from those cartridges was there isn't 1 number for how much speed a bullet will take. There is also the interaction between the case capacity required to produce that velocity and the barrel. That in turn is affected by the string lengths required by the COF. A hunting gun can get away with a lot more overbore. None of this is really big news, all of the shooting disciplines eventually settle into a fairly narrow set of cartridges that balance performance and barrel life. For a hunter, that number is probably around 3350 fps for the 195 berger out of a 1:8 twist. For the course of fire I'm shooting, 3100 fps is probably a better number. My impression is history has shown that's probably too high for a season of F class.

Twist rate can move that speed up and down, but at ELR distances BC effects similar to velocity spread start to be seen. The leaders in that game are generally running stability factors over 2.0. With some bullets, this helps reduce the shot to shot BC variations. I couldn't get the 195 Bergers to hold vertical as well as the 180 ELDm with a 1:8 barrel. The 195s also built pressure faster than the 180s and the 180s were able to reach higher speeds than the weight difference would suggest. Enough extra speed to overcome the 195s BC advantage out to 2200 yards. With the better vertical consistency, hit percentages were higher and more consistent with the 180s. We use rectangular targets, this effect will be even more pronounced with round ones.

The short version of that is if you're contemplating strings of 10-20, I think your case capacity target is a good one.

On the neck length, I suggest 1 caliber + 0.060, more or less. That'll give a long enough section to give consistent neck tension with a donut buffer long enough to reduce the number of times you'll need to turn the neck in the cases life. The only problem with too long that I see is they get easier to damage with the sizing die, especially with 35 degree or steeper shoulders.

On the LRM dimensions, GW published them in one of their emagazines. Not a reamer drawing, but enough to produce one. I agree that until they move on from Hornady brass, why bother.

Finally, and this is from a guy that has walked down both paths, is all this worth it to escape using the bulge buster die on the belted? The 7-300wm drops into that target case capacity and would be far easier to form. Another way to manipulate the overbore is with caliber. Instead of a 28 Nosler, maybe a 30 Nosler?

 

[smokinghole]

I've had experiences similar to what Berger.Fan222 describes in that thread with 7s and agree with his conclusions. I have video of bullets bursting 150 - 200 yards out. I agree that it's likely more the lead softening from heat causing the failures than the jackets not being able to contain them. I'm sure it's not completely black and white. It looked to me like a long high temperature push down the barrel weakened the bullets and aerodynamic heating finished them off. That same long high temperature push produced barrel wear a lot different than you see in the typical target gun.

I also saw the bullets starting to fail later in long strings, then sooner as the barrel wore. I shot the 195 Bergers out of a 7/338 lapua improved at up to 3350 fps when the barrel was fresh, they wouldn't all stay together below 3200 fps after a few shots a couple hundred rounds later. A straight 7/338 lapua took about half again as many shots to start the same routine 150 fps slower. Both ate the Hornady 180 ELDm at the same point in their barrel lives.

The guns were built to test the costs and benefits of overbore for ELR shooting. The idea was to do the testing with a relatively small caliber before stepping up to more expensive equipment. We have a test range that is similar to day 1 of the KO2M and use a similar course of fire. At the time, we were using 5 shots per target on 4 targets from 1200 to 2200 yards.

My biggest take away from those cartridges was there isn't 1 number for how much speed a bullet will take. There is also the interaction between the case capacity required to produce that velocity and the barrel. That in turn is affected by the string lengths required by the COF. A hunting gun can get away with a lot more overbore. None of this is really big news, all of the shooting disciplines eventually settle into a fairly narrow set of cartridges that balance performance and barrel life. For a hunter, that number is probably around 3350 fps for the 195 berger out of a 1:8 twist. For the course of fire I'm shooting, 3100 fps is probably a better number. My impression is history has shown that's probably too high for a season of F class.

Twist rate can move that speed up and down, but at ELR distances BC effects similar to velocity spread start to be seen. The leaders in that game are generally running stability factors over 2.0. With some bullets, this helps reduce the shot to shot BC variations. I couldn't get the 195 Bergers to hold vertical as well as the 180 ELDm with a 1:8 barrel. The 195s also built pressure faster than the 180s and the 180s were able to reach higher speeds than the weight difference would suggest. Enough extra speed to overcome the 195s BC advantage out to 2200 yards. With the better vertical consistency, hit percentages were higher and more consistent with the 180s. We use rectangular targets, this effect will be even more pronounced with round ones.

The short version of that is if you're contemplating strings of 10-20, I think your case capacity target is a good one.

On the neck length, I suggest 1 caliber + 0.060, more or less. That'll give a long enough section to give consistent neck tension with a donut buffer long enough to reduce the number of times you'll need to turn the neck in the cases life. The only problem with too long that I see is they get easier to damage with the sizing die, especially with 35 degree or steeper shoulders.

On the LRM dimensions, GW published them in one of their emagazines. Not a reamer drawing, but enough to produce one. I agree that until they move on from Hornady brass, why bother.

Finally, and this is from a guy that has walked down both paths, is all this worth it to escape using the bulge buster die on the belted? The 7-300wm drops into that target case capacity and would be far easier to form. Another way to manipulate the overbore is with caliber. Instead of a 28 Nosler, maybe a 30 Nosler?

Thanks for the detailed post. I've considered a straight 30 Nosler with a longer throat. For now I'll plan on a 33 Nosler shoulder with a long 7mm neck, or 7LRM if Atlas starts making their brass.