6.5mm versus 7mm / .284"

[mactavish]

Thanks for the useful comments on the 28 Nosler, Laurie.

The method in considering this cartridge hinges around building a low round count ELR platform (given UK ELR opportunities being relatively few and far between) that is good for testing limits of precision repeatability at 2500+ metres. A secondary consideration is that it might still show a good account in the odd F-Open foray when the opportunity arises (given a hefty all-up weight and a premium recoil pad).

Final build details for this are still fluid, but 28 Nosler, with its (granted, mostly anecdotal) big new reputation for speed* and (perhaps gravy train inspired ) reports of stellar accuracy, have put it to the top of my list if I decide to go BIG on my 7mm idea.

(*QuickLoad indicates that with RS80, a 180gr ELDM with its huge .401 G7 BC has a max MV well over 3300fps out of a 30in barrel. Therefore, thinking goes something like; in terms of available MVs & BCs, this big 7 far out-classes even the Lapua Magnum in elevation and does so similar where wind is concerned out to around 3000m. I could even be tempted to consider this a [not so] poor man’s .338 Lapua Magnum LR project, but only evidence produced in practical application will confirm or deny this.)

As you pointed out however, that the brass currently costs over 30% more than that of even Lapua Magnum, and ran relatively hot will have high brass and very high barrel wear, are serious considerations. As is the veracity of the early adopter reports out of the US, of course. Though to counter, this rifle will only shoot around 300-400 rounds per year (my Creedmoor will do the donkey work in LR practice) and brass, once acquired, will render the 7 significantly cheaper to feed in terms of powder and premium LR bullets than would be for feeding a Lapua Mag. Also (albeit anecdotally again), there are murmurings of higher than anticipated adoption rates, “Creedmoor like” real-life cartridge efficiencies & accuracy, and short timescales until we have other, including Norma branded, cases (not to mention the ability to neck up/down 26 & 30 Nosler brass).

As I mentioned in the earlier post, this 7mm project will either be in the form of 7SAUM (perhaps as a stepping stone to a .338 LM) or it'll be one of the biggest 7s, which as just discussed, by measures comparable to the .338 will likely suffice in its own right in terms of ELR in the UK.

Decisions… decisions… In this game, the journey is often as much fun as the arriving.

Incidentally, time is on my side here, as none of this will happen until I’ve fully stretched the legs on my 6.5CM and now that I have 147 gr ELDMs running at 2825fps and shooting repeatable 6mm groups at 100 meters, it’s barely scratched the surface in terms of LR (at a little over 600m so far). This means I have plenty of time to further research and mull before I make that decision. However, if this spring goes to any kind of plan, the CM should be out to a mile (hopefully with some precision) by summer and then a hankering to go further than its realistic capabilities, will, I’m afraid, very likely arise…

Anyway, I’d be interested if you’d like to share your ideas on my mission brief and tell me how and why you might do anything differently?

 

Edited to add: Ooops, thread hijack alert!! Apology to meles meles for interrupting your convo...

 

[Catch-22]

@mactavish

I'm not so sure your assessment of saving anything over a .338 Lap or .338 NM is quite right.

the excellent 250g or 300g (0.368 G7) Scenar are pretty much an identical price as the 180g ELD per x100.

Also, Nosler suggest 88.0g max load of Rel33 (RS80) with 175g/180g bullets. Most people are loading their .338s to between 88-93g of powder.

You'll definitely get better speed but the heavier bullets will do very well in the wind despite having a slightly lower BC - whether it's better than the 180g ELD at extreme distance has yet to be seen as you say.

Thing is, our game is all about personal preference. If you wanna go build one massive 7mm, do so...enjoy the process. It's not cheap (it'll be a lot more expensive than you think!) but if you can afford it (and feed it) then good luck to you!

Also, worth reading the experiences of Nathan Foster who's played around with the 7 RUM, 7 STW and even created his own big 7mm boomer, a 7mm Practical. He notes seriously short barrel life in all but he still very much enjoyed shooting them at extended distances, and the success in hunting with them.

https://www.ballisticstudies.com/Knowledgebase/7mm+Practical.html

[mactavish]

Hi @Catch-22 and thanks for the considered response.

In regards to bullet price comparisons, like for like, .338 Hornady ELD-M (ELDM chosen as they happen to have highest [conventional bullet] BC in either calibre) are getting on for twice the price of the 7mm.

But yes, you're right, of course. A big 7 and a .338 Magnum are broadly comparable across a number of criteria and where they do differ, they're broadly and reasonably arguable in terms of the various compromises they need to accommodate.

Also, thanks for the reference to the 7mm Practical. I'm off to work shortly and I'll make a point of reading it during my break.

I drew up some fairly basic ELR Tables/Charts a few months ago (based on QL & Strelok Pro outputs) to help me visualise what I'm trying to achieve across this ELR project. I've added them here to show some of the data across the discussion.

Cheers.

Ballistics - Various ELR Tables.xlsx

[stephentri]

Hi mactavish 

What barrel length is your CM and what powder are you using for the 147 eldm

Thanks

[Laurie]

Re the 28 Nosler, ELR type use shows promise because of the very low string counts. How well it'd compete ballistically against the 375s and 40+ calibre numbers winning the Ko2M and similar, I've not a clue.

I'd doubt if it'd be much use in F-Class unless you dropped loads considerably. (In which case, it's much better to use the 7WSM!) You have two problems in F. First, I think you'd struggle to get through a  2 + 20 match especially on a warm summer day. Barrel temperatures will kill precision and consistency before the end of the match. Allied to that, you will kill the barrel very quickly.

Also recoil and torque will be heavy. In a 22lb rifle (F-Class ceiling) 88gn RS80 and a 180gn bullet at 3,300 are calculated to produce a free recoil speed of 7.2 fps and energy of 17.5 ft/lb. A few years ago, it looked like the .300WSM and 230gn Berger Hybrid might take over from the larger sevens as the external ballistics looked very good. They didn't, one reason being that users found that the recoil / torque made then hard to shoot consistently over 20 shots in a match. That cartridge's recoil values for a bit over 60gn powder charge and 230 at 2,800 fps works out at 6.1 fps free recoil velocity and 12.7 ft/lb energy, way lower than the 28 Nosler's even in such a heavy rifle.

That's not to say anything's impossible. IIRC Year 1 of the GB F-Class League was won by Pete Wilson shooting the 7mm BooBoo whose case has 90gn water capacity. As smaller sevens were developed though, this cartridge was very quickly sidelined.

Elwood and Desparado would be able to comment much better than me on the  feasibility of such a large seven in our F-Class as the Shehane is the largest case cartridge I've used in the 7mm calibre.

By coincidence, I was looking earlier today at what one or two recent loading manuals say on the 28 Nosler and 7mm RUM. Lyman says that groups / strings should be limited to three shots on the RUM and then allow complete cooling (not surprised with that vast case and huge powder charges). The authors laud the 28 as a long-range hunting cartridge, but warn that those considering it must be aware of the downsides, primarily recoil and short barrel life. It recommends checking barrel throat condition regularly after about 500 rounds, presumably based on experience with the test barrel used in load development. This would be on the basis of its use as a 'hunting round' and I've no doubt that the Lyman lab staff took that into account with very short strings and lengthy cooling-down periods between them. In range use where matches see 17 or more usually 22 shots fired, I think you can take that figure and halve it.

Defining over-bore capacity is a real length of string job, but to give an idea of where the Nosler stands, this case capacity to bore area table is a useful guide.

http://bulletin.accurateshooter.com/2016/03/defining-overbore-cartridges-the-great-debate/

The 28 Nosler exceeds anything in the table with an index value of 1,522 (95.9gn water capacity divided by 0.063 sq in, the area of a 0.284-in dia. circle).

As a baseline, many people fail to realise that the 243 Winchester is actually a very over-bore design as it is such a common and apparently non-exotic number. In this indexing method, it has a value of 1,164 and here's what a Bartlein Barrels engineer says about it:

 

First question that needs to be asked is.....What is your accuracy requirement? This will determine your barrel life more than anything else. A bench shooter or F class shooter will normally have a different accuracy requirement vs. a shooter using it for deer hunting etc...

How the barrel is made, type of powder, type of bullets, type of shooting, how it's being cleaned and how often, rate of fire etc... all effect barrel life.

If we do testing here in regards to steel and barrel life etc...we use .243win. because it is a barrel burner. Ammo makers are not fond of the round for the simple reason at approx. 1000 rounds they are replacing pressure test barrels. The wear is so heavy that it will start to get inconsistencies in pressures and velocities and if loading for Saami spec. ammo they have to hold a given pressure spec. and once they start to vary they pull the barrel.

Later, Frank
Bartlein Barrels

[Laurie]

2 hours ago, meles meles said:

For now, we'll stick with our 7mm Rem Mag, thus the question morphs:

"What bullet and powder do you recommend ?"

We have a 26" barrel and currently use:

• PPU Brass
• SMK 175 gn bullets (1975)
• 64.3 gn of Viht N560
• 3.267 COAL
• Federal primers

Can we do better ?

I doubt it. The 175gn SMK is a lovely bullet - very jump and throat tolerant which is a plus in a throat-eroding cartridge like the 7mmRM - and despite having been around for many years is still a very efficient design in external ballistics terms. N560 is very well suited to the cartridge characteristics. There is not a great powder choice thanks to 'Reach' - Norma MRP and Alliant Re22/25; Reload Swiss RS70 and 80. The new Viht N565 might suit, but I suspect is too slow burning given that 560 is nearly spot on in this respect.

Viht N165 will reduce barrel wear considerably, but likely lose you 100 fps MV compared to N560.

For long-range match shooting, you might do better with something other than PPU brass. My limited experience with this make (243 Win) is that appears to be very well made, but has a large weight spread (~7gn in the 243) suggesting that internal capacities will vary a lot too - no great issue for short-range, but it'll have an effect beyond 600 yards.

[meles meles]

Hmmmm,   as it appears there is no magic solution in terms of equipment it looks like we're going to have to up our game and learn to shoot...

[mactavish]

14 hours ago, stephentri said:

Hi mactavish 

What barrel length is your CM and what powder are you using for the 147 eldm

Thanks

Hi stephentri,

Long story short(ish:) … my CM is 24in (actually a Tikka TAC A1). I got the rifle last July and after initial successes with 140gr bullets (Berger Hybrid Target & ELDMs) using RS62 at around 2650 fps, I found stepping up speed using RS60 more of a challenge. Accuracy was relatively poor in OCW (compared to repeatable 5mm @ 100m groups with the 140s/RS62) and while ladder testing I started to see pressure signs (ejector ring) in the Lapua S/R brass earlier than I might have imagined at around 2850fps (Though tbf, these only showed in the Bergers and not in the ELDMs). When I finally got my hands on 147 ELDMs in October, I already had RS70 here waiting and have never looked back.

QL guided a start at 45gr and an overall excellent bout of OCW ended up giving 46.8gr with the above mentioned accuracy node at 2825fps. It's in bump sized brass. S/R magnum primers. COAL is 2.890 (=.045 jump). (Incidentally, there were no pressure signs right up to the 47.4gr max tested. Coincidently, compression is occasionally heard/felt on bullet seating at 46.8gr, indicating around 100% fill.)

Usual caveats apply. Obviously these figures work a treat in my rifle, but they are pretty hot CM loads. Work up your own loads carefully & observe for pressure.

Cheers.

[mactavish]

Many thanks for this, Laurie.

There is some very useful info and no doubt a great deal of experience here for me to ponder. Especially in regard to the 28 Nosler perhaps not lending itself so well to F-Open. While this won’t make or break choosing it,  it does bring bearings I need to consider. The barrel heating/cooling issue for one. I’ll certainly look at ways this might be controlled/mitigated, but those loading manual comments make for bleak reading in that regard. If the concept barrel can shoot careful strings and push up and over 800 rounds, then F-Open dabbles remain feasible. Significantly sub 500; not so much. Also, to be honest, I hadn’t considered the 28 Nosler would be quite that brutal without a brake… those figures are hefty!! Still, all this has made me now wonder if as new developments in cartridges, powders, bullets, barrels etc. come online, revisiting past concepts such as Pete Wilson’s once winning formula, with the odd modern tweak might once again find something viable in the class. Others’ ideas, as you suggest, might be very interesting to hear on this.

Anyway, I guess ultimately whether ELR and F-Open in the one platform is a viable proposition will depend most on how honest I am with myself over how serious I might want to take F-Open once dabbled.  I’m now beginning to fear such a proposition might prove itself compromised by the need for yet another project at some point.

In regard to taking on the likes of the.375+ in such as the Ko2M; that’s not where I see my goals with this. I’m just interested in pushing some way towards what they can achieve within a reasonable budget and within the natural constraints we find on ELR shooting here in the UK.

Your (and Bartlein Barrels’) comments re the overbore bearing on barrel life are very interesting and in conjunction with the interesting reading in Catch-22’s earlier link (showing a relatively successful attempt to mitigate big 7mm throat erosion) suggest a strong confirmation of what we’ve already started to establish. With an overbore index of 1522, the 28 Nosler’s significantly in excess of anything on that list. So, unless it’s blessed with something approaching alchemical qualities, then barrel life for anything approaching competitive shot strings, will be dire.

I can imagine looking out for lots more of those early adopter anecdotes across the pond will keep me quite busy in the next few months.

Thanks again.

[stephentri]

9 hours ago, mactavish said:

Hi stephentri,

Long story short(ish:) … my CM is 24in (actually a Tikka TAC A1). I got the rifle last July and after initial successes with 140gr bullets (Berger Hybrid Target & ELDMs) using RS62 at around 2650 fps, I found stepping up speed using RS60 more of a challenge. Accuracy was relatively poor in OCW (compared to repeatable 5mm @ 100m groups with the 140s/RS62) and while ladder testing I started to see pressure signs (ejector ring) in the Lapua S/R brass earlier than I might have imagined at around 2850fps (Though tbf, these only showed in the Bergers and not in the ELDMs). When I finally got my hands on 147 ELDMs in October, I already had RS70 here waiting and have never looked back.

QL guided a start at 45gr and an overall excellent bout of OCW ended up giving 46.8gr with the above mentioned accuracy node at 2825fps. It's in bump sized brass. S/R magnum primers. COAL is 2.890 (=.045 jump). (Incidentally, there were no pressure signs right up to the 47.4gr max tested. Coincidently, compression is occasionally heard/felt on bullet seating at 46.8gr, indicating around 100% fill.)

Usual caveats apply. Obviously these figures work a treat in my rifle, but they are pretty hot CM loads. Work up your own loads carefully & observe for pressure.

Cheers.

Thanks