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7mm Practical twist rate

20 Jun 2018
@ 07:37 am (GMT)

Nathan Foster

From my morning mail:

Good Evening,

I`m writing you after`purchasing the Lee die set and chambering my rifle in 7mm Practical. I read the info you provided and gave the instructions to my gunsmith in Norway. The barrel is a Lilja stainless 3 groove 1-8" twist. Bullets used is Hornady 180gr ELD-M. Powder used is VV N570
primer 215 fed. I have got good velicities, probably because of barrel lenght of 31".

My problem is that 1 out of 3 bullets is desinigrating (blows up in midflight)
Speed is around 3100 fps.

I have also tried Lapua 180gr Scenar-L bullet at 3150 fps without failiure.

Do you have any info/experience on this matter? I have tried to get in touch With Hornady directly, but so far no Luck. I thought that since they post BC for this bullet With 1-7.5" twist and speed of mach 2.5 that the bullet would tolerate this without failure...

Did I get a bad batch of bullets? Or are my expectations too high regarding speed?

BTW thank you f or providing the New bullet seater for the ELD-M :-) I love it!

Best Regards

Hi ...., I have written about this extensively in the books and made notes about this on the website. I blame these stupid twist rate calculators for this because they neglect other vital factors which include velocity versus bore grit finish versus bore dimensions (see my Accurizing and maintenance book).

As stated on the site, the optimum twist rate for the 7mm Practical is 1:9. I have stated this over and over again in various publications, I cannot make it clearer. The velocity of the Practical is high enough to generate stability in a 9 twist. By adding further twist, it simply strains the bullet, more so when other factors such as bore finish come into play.

The Lapua will not fail because it has a tough jacket. But it is also for this reason that it will not expand at long ranges, making it entirely useless for long range killing.

Certainly a very harsh lesson for you. Every day, I teach readers to try and see through the hype of this industry, to look beyond marketing, to treat expert advice as hearsay until you have tested all factors for yourself. You will now recognize this to be true for yourself.

I really feel for you. The only way to rectify this is via a change in twist. But then again- perhaps I am wrong? Perhaps an 'expert' will come along and tell you to use a 195gr bullet because it kills like lightning? So you have to ask yourself, are you prepared to be duped twice?

Again, my heart goes out to you. We have Norwegians living here in Taranaki and they are very dear to me.

Replies

20 Jun 2018
@ 08:14 am (GMT)

Nathan Foster

Re: 7mm Practical twist rate
Guys, please learn to trust your own gut and good judgement with regards to whether you believe I have performed serious research on matters of the rifle and ballistics, or whether I am just another voice making noise.

For my own part, I can tell you that my wife and I have poured our heart and souls into the research you see on this site and within the books. Nothing came easily. This research is based on many years of hard work, sacrifice and due diligence. For example, when I say "take your damned mufflers off your guns during testing" its not because 'Nathan doesn't like mufflers'. Get real, its because Nathan has tested more rifles than most of you have had friggen hot dinners. I have seen what can and does go wrong - over and over again. Opinion and experience are two vastly different things.

So the next time some idiot offers you offhand advice and says, "no Nathan is wrong" and that such and such is best- take time to really think things through before you act.

I am passionate about helping you- for your sake. I want the very best for each of you.
20 Jun 2018
@ 10:11 am (GMT)

bryan long

Re: 7mm Practical twist rate
A lesson bought, is a lesson taught.

Books are cheaper than a barrel.
20 Jun 2018
@ 11:24 am (GMT)

Paul Leverman

Re: 7mm Practical twist rate
Well, I for one among thousands I am sure, trust your experience and voice of reason. Not too many people can actually say "I wrote the book on that". My pat response to the others is usually a slight smile and "Really? I did not know that." and just carry on with what I've learned here.

All of us here appreciate your help and guidance, and because of your and Steph's dedication to us, we are advancing our knowledge and performance.

Thank you very much.
21 Jun 2018
@ 07:11 pm (GMT)

Jonathan Kitterman

Re: 7mm Practical twist rate
Has the twist rate been verified in the rifle? It is a little confusing as to what the twist rate is in the original email. Has bore condition been checked for a burr?

I just looked at Lilja's website and they are showing 7mm barrel blanks over 30 inches are either 7 or 9 twist.

http://riflebarrels.com/calibers-and-twist-rates/
22 Jun 2018
@ 09:18 am (GMT)

Nathan Foster

Re: 7mm Practical twist rate
Hi Jonathan, its the 8 3G as shown in the Lilja list.

It won't be a burr because as stated, the Lapua is accurate.

This is not my first 8 twist rodeo.
24 Jun 2018
@ 07:09 am (GMT)

Ales Grznar

Re: 7mm Practical twist rate
Hi Nathan,

Same problem. Last week I finished for my friend my first 7mm Practical.It is on Tikka,Benchmark barrel 33" long, 8 "twist. 8 twist was his wish because other possibility was 9,5". This rifle is only for target shooting. First rounds with ELD looks very good (from 100m to 900m). I know it is stupid question. Have You any experience with heavier bullets?

Ales Grznar
Czech republic

25 Jun 2018
@ 12:43 am (GMT)

jason

Re: 7mm Practical twist rate
I asked a guy on the internet the other day, selling barrels. All faster that 9. I said what, no 9 twist, he says oh nobody wants them any more.
I have no doubt of your dedication and advice. Other local forums have many people falling into common traps, and nobody can even think for themselves. Just look at work place health and safety a mate in the forest was saying just the other day, the logs get marked and cut on the mark, there's guys that if you marked there boot they would cut it.
02 Jul 2018
@ 11:14 am (GMT)

Iain

Re: 7mm Practical twist rate
Some years ago I recall some discussion about thin jacketed bullets fired at high velocity from quicker than standard twist 3 groove barrels coming apart. One suggestion was that the reduced number of lands, creates corespondingly higher increase on the loaded side of each land, and thus greater stress on the jacket.

ELDMs have a very thin jacket and the short bearing surface would also increase stress from the lands.

So - thin jacket - short bearing surface - increased stress from reduced land contact area - fast twist - heavy bullet (still with thin jacket) - high velocity.

Maybe it's simply a combination of things. 8" isn't superfast in todays world. 9" has been used for yonks.

Perhaps it would be helpful to email Hornady and Lilja and ask them if they have any thoughts on the problem. Lilja are enthusiastic about the 3 groove and have been making a lot of them in various cals and twists for a long time. Both companies are responsive and helpful, in my experience.

You could also maybe try TMKs which have a heavier jacket than the ELDM but which should still work well at LR. Unfortunately the TMk in 7mm is offered only in 160gn
02 Jul 2018
@ 02:57 pm (GMT)

Mike Davis

Re: 7mm Practical twist rate
8'' isnt superfast.........
ok I will so some simple maths for you and use round figures to make things simple....
if we set the speed to 3000 fps and we have a 1 in 12" twist rate we get 3000 rotations persecond x 60 seconds =3x6=18 add all the zeros 180,000 rpm

now we are talking 1 twist in 8'' so its half as fast again.....3 rotations in 24'' as 3x8=24 so its rotating that poor wee piece of metal at 270,000 rpm thats a heck of alot of G Forces trying to rip the guts of the projectile outwards (the reason projectiles mushroom when they hit and dont just bend/deform like they do in hammer test) ANY TIME you increase the twist rate/RPM you are increasing that "rip its guts apart" force just ask the guys who waaaaaay back in 1900 and nuts n bolts decided to make a wee number called the swift..ever seen a swift with a 8'' barrel Lian???? ever tried to use the likes of a hornady SPSX in one??? or indeed pushed same pill at much over 3,000 fps????


you dont NEED TO because the wheel has already been invented and the results are KNOWN the projectiles WONT REACH TARGET intact


Nathan and Steph have done the hard yards and tested that sucker to the limits BEFORE putting it out there so why would you try to "fix what aint broke"?????

I will bet my left testicle if Nathan says " if you do this to this projectile/barrel/cartridge this will happen" and if you do it well it WILL happen....
on a brighter note if you load the said rifle you have just made up with undersized ball and push them out at just over subsonic speed with lots of lube infront of projectile they will be good for plinking small game at up to 50 yards according to Finnish research OVER A CENTURY AGO.... you see most anything you care to try has been done before usually more than once and if we are #A lucky enough someone will have written down results
#B clever enough to read them ...finally #C wise enough to take note of findings you can save yourself a whole lot of hard earned coin and headaches.


chances are a FMJ may preform admirably in that twist (in that it will reach target duee to stout construction)but it will be pisspoor for much more than punching paper/gongs


thats enough from this grumpy old man......
02 Jul 2018
@ 07:27 pm (GMT)

Iain

Re: 7mm Practical twist rate
Relax Mike. I understand all that.

Perhaps I would have been clearer had I said 8" isn't that much faster than 9" ? 9" (Nathan's recommendation for the Practical) was the twist figure I referenced as being common, not 12".

As Nathan's correspondent pointed out, Hornady quote tested BCs at various velocities from a 7.5" twist for the 7mm 180 ELDM, which does beg the question as to how did the tested bullets hold together for Hornady if the twist rate is the ONLY issue. The tests don't publish muzzle velocities.

According to Hornady, their testing methodology is based on practical radar measurements of the relevant projectile's velocity in flight, and not the slipstick formulas of yore.

My point remains that there may well be more than issue at play.

02 Jul 2018
@ 07:48 pm (GMT)

Mike Davis

Re: 7mm Practical twist rate
its a 12.5% increase......1/8th as fast again
you can be sure projectile will handle that twist at say 7mm08 speeds

the point is Nathan and Steph have done the hard yard and KNOW what works.... trying to 2nd guess that is insulting to put it mildly

the twist rate AND velocity go hand in hand as the 2 combined add up to extreme stress on projectiles..... you can only drive your car so fast around a corner before it slides off road....put better tyres on it and you can push it harder but there comes a point when the Gforces/centrifical force at play will overcome the ability to hold road surface the forces at play INSIDE a projectile are the same....youve either got to use a harder jacket , a slower twist or slow it down
the big mover in fast twists is the subsonic crowd with teenee peenee wee cases eg .300blackout they get away with it and use it to good purpose as speeds are slow...even thier full power supersonic loads arent any faster than a .308 winchester so projectiles are ok.

03 Jul 2018
@ 08:41 am (GMT)

Nathan Foster

Re: 7mm Practical twist rate
Hi Iain, its not just the Lilja barrels, I have seen this in regular 6G U.S match barrels. As Mike said, I have been observing this for some years now, noted the way it effected Berger bullet production in 2011, spoken with them about it though the answers given to customers are not the same as the talk we had.

You are correct, it can be more than one factor as I wrote in the Accurizing book but without the deeper explanation is just as easily summarized with Mikes's tire example.

Mikes communication may have the gruff tone of our deep south but he is correct in his interpretations. Mike asked me to delete his comments because he felt his posts were too harsh and did not want to offend you with his generalized tone. But in some ways he is only echoing my own frustrations. I have been speaking about this issue since 2011, it is not new, it is not unique. It could be addressed by Hornady and others via education. It would be unwise for bullet makers or barrel makers to try and address this within the physical designs (other than gain twists). We have already been through this with Berger to their detriment.

At some point, a guy has to ask himself why he wants an 8 twist magnum. If he wants to shoot to and beyond 2km, then yes, he may need an 8 twist. At these ranges, the magnum is no good for clean killing so there is no point adopting a killing bullet. One can instead use a long and tough match bullet. But if a guy wants to hunt and kill, then he needs to understand both his own range limitations, bullet killing limitations and the issues of twist rates. Unfortunately, I appear to be the only researcher going against the hype. But that's fairly normal for me, a result of staying away from graphs and theory and instead, shooting the rifles while also having the benefit of a wide readership with ongoing input and feedback. But this does not help the average guy who followed the recommended twist rate on the bullet makers website only to find that his gun won't shoot, then found my work afterwards. It would be more helpful if industry members put in equal efforts, testing the bullets across the velocity spectrum, not just through a 7mm-08 test rifle (or more likely a 32"barreled .284).

The only advice I can offer, is to be wary of the industry and trends. Just because there is a lot of noise, graphs, computer models and hype around any subject, this does not mean to say that the trend or its accompanying data are correct. There are many situations where the industry (both producers and customers) jump on certain band wagons but neglect important variables when chasing a concept.

Under the conditions here, the G1 BC's for the 180 ELD-M at sea level at 59f from a 9 twist Practical are as follows:

3000-2500fps .798
2500-1600fps .777
1600-1100fps .748


From a 9 twist Rem Mag at a mild 2800fps, the BC's are:

2800-2500fps .777
2500-2230fps .748
2230-1900fps .730
1900-1100fps .700

From memory, I believe Hornady rate the G1 BC for the ELD-M as .796 from their recommended 8 twist. But in the above field data, you can see the BC was achieved with the 9 twist via its muzzle velocity.

Hope that helps a bit.




03 Jul 2018
@ 09:02 am (GMT)

Nathan Foster

Re: 7mm Practical twist rate
Ales, regarding your friends rifle. If it is shooting the 180gr with good accuracy, it should hopefully stay this way. Just monitor it over the first 40 to 60 shots. If it is going to cause problems, these will show after the barrel is broken in and prior to 100 rounds. You will know for sure by the time you have done three good shooting and cleaning sessions.

If the rifle was not broken in when you tested it, the first shots over the new throat could have created a medium to heavy layer of copper fouling throughout the full extent of bore. This can sometimes help stability (until it goes the other way and the fouling is too heavy). So what you have to watch for, is how the rifle behaves after the barrel is cleaned and when it no longer produces this level of copper fouling.

If a heavier bullet is needed, the 195gr Berger goes well in the match Practical 8 and 8.5 twist rifles.
03 Jul 2018
@ 11:27 am (GMT)

Thomas Kitchen

Re: 7mm Practical twist rate
hornady expanded bc as follow
Mach 2.25 (2512 fps* and above) - Velocities above 2,500 fps and distances out to 300 yards.
Mach 2.0 (2232 fps*) - Mid-range shooting where the bullet spends time in flight after slowing down.
Mach 1.75 (1953 fps*) - Long-range shooting where much of the bullet flight occurs after velocity loss.
7mm 180 gr. ELD Match
1 in 8.75" Twist 0.777 G1
0.391 G7 0.748 G1
0.376 G7 0.731 G1
0.368 G7

im not sure how much above 2512fps hornady test their projectiles.

lot of people read this and that and think we need more twist,
most of people that do long range target/ELR shooting are running mono's now that don't have the same problems as jacketed bullets and they want extra stability for getting through transonic.
we are hunters being humane is the is the main thing so mono's and that slow of speed is out of the equations anyway.

a guy i know brought a rifle mainly for the action and scope but was saying he couldn't get it to shoot.
turns out its a 30/338 lapua improved with a 1-8 twist, his only option is mono's there's just nothing else that would handle that stupidity.
punched info into calculator my 1-10 at 3000fps is still less stabile then his 1-8 at subsonic speed, which just brings it home.


05 Jul 2018
@ 10:40 am (GMT)

Iain

Re: 7mm Practical twist rate
Thanks Nathan, Thomas and Mike. My curiosity regarding the Hornady testing remains. That bullet also has tested figures for a 7.5" twist which would be even more stressful than 8".

The only thing that I can think of is that testing was done with something like a standard 7RM and a 24/26" barrel, which would generate velocities way below that 3100plus from your correspondent's Practical in a 31" barrell. Nathan alluded to 2800fps, which makes sense to me, and would still be very effective at long range.

The ELDM isn't marketed as a hunting bullet, and no F Class shooter would punish themselves with a 180 7mm at 3100fps - not for long anyway.
So perhaps Hornady test at the lower velocities likely to be used in a competition oriented launch pad. OTH I'm sure there are plenty of Big 7 users around the world who potentially could have the same problem.

Hornady are very upfront with there testing and their data, so it would be nice if they shared that missing piece of info with us.

 

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