338BR/338 Whisper noob questions

[love2shoot]

hmmm.......looks like I gotta sell some things....seeing a couple potential future projects.......

for a 338 sub....pardon my stupidity as I am just starting to educate myself on this caliber potential.......wouldn't a 308 necked up and shoulder moved back work? and yes, I am thinking possible mag fed in a semi auto......thinking on a 308 ar10 style rifle. just not overly impressed with the durability of 6.8 bolts....both from personal experience and lots of friends who use the caliber also.....only reason I might consider it in a ar15 is the fact that it would be at lower pressures then a standard 6.8.

other potential project I am considering is a semi auto variation of of richards 50 thumper. using a 300 wsm but cut back a little further then he did. It does not look like the really long 750grn projectiles would fit in a magazine, but some of the lessor ones I believe could work.........

[Grimjaw]

Originally Posted by love2shoot

for a 338 sub....pardon my stupidity as I am just starting to educate myself on this caliber potential.......wouldn't a 308 necked up and shoulder moved back work? and yes, I am thinking possible mag fed in a semi auto......thinking on a 308 ar10 style rifle......

I think that would be 338 Federal, loaded subsonic.

[Romeyo]

Hi folks!

I'm just curious, has anybody ever had problems shooting 338 300smks/scenars, or similar bullets, with "slow" twists?

I'm asking because there's a formula (millers twist rule) you can use to pretty reliable predict bullet stability.

According to that formula even a 1 in 10 twist would be sufficient to stabilize a .338 300gr smk.
That formula gives you a gyroscopic stability factor (Sg) that needs to be 1.0 or higher to proper stabilize a bullet. In general you're looking for that Sg to be greater than 1.4 because of imperfections of the bullet, load and because of changes in atmospheric conditions.
Military uses a Sg of 1.5 to 2.0 because of the extreme conditions you might find on a battlefield.

Overstabilisation (Sg of 2.0 and higher) might (and will) cause precision problems because twist magnifies the symmetrical imperfectness of a bullet causing shot groupings to open up.
However, I have to agree with amafrank: understabilisation will likely cause much more problems than overstabilisation, especially if you're worrying about baffle strikes.
Some bullet/twist combos however tend to tumble at the first one to two hundred yards before flying stable causing bad precision or maybe even keyholing at short distance but very good results at long ranges.
Thats because bullet stability INCREASES during flight because of the bullets ability to maintain rotation is greater than to maintain speed.
However this adds drag to the tumbling distance (decreasing BC) and might cause baffle strikes, but can be corrected by using a faster twist.

Using this formula (there are downloadable excel sheets aviable) the Sg of a 300grain .338 SMK (1,7in bullet lenght) @ 1050fps in standard atmospheric conditions using a 1 in 10 barrel is 1.45.
1 in 8 twist: Sg 2.27
1 in 7 twist: Sg 2.97
1 in 6.5 twist: Sg 3.44

To me 1 in 8 seems to be perfectly fine, thats why I wanted to know if anybody experienced bad precision, keyholing of baffle strikes with that combo?

However, if you plan to use that extra long VLD bullets turned from solid copper alloy bars you better have a fast twist.
For calculation I chose a .338 295grain SP bullet from GS Custom Bullets (South Africa)(I chose this particular one because it's the heaviest/longest .338 solid i found) and did the same calculation:
1 in 10 : Sg 0.62
1 in 8 : Sg 0.97
1 in 7 : Sg 1.26
1 in 6.5 : Sg 1.47
(standard atmospheric conditions)

On their homepage you'll find a datesheet that contains smililar information, however beeing more "optimistic": http://www.gsgroup.co.za/338295SP142.html so seems that my calculations are not that far off, and after all on the safe side.

What's also confused often is the fact that muzzle velocity doesn't influence bullet stability that much. Bullet stability is mostly influenced by the bullets shape (lengh and "meat" at the outer diameter) and barrel twist.

I got this knowledge by reading books like applied ballistics for long range shooting from bryan litz (berger bullets chief ballistician), which I would highly recommend to anybody interested in the subject. Its a VERY interesting and easy read (you're not overkilled by euqations, formulas and thelike).

I know that there's always a difference between theory and praxis, however I often got the feeling that theres much mystic and many misinterpretations about whats going on when shooting a gun just because people are afraid of a tad science an math ^^


Btw: I would also be interested in how hard it would be shortening .308 brass and necking the case down to .338 instead of necking up 7mmBR brass?

[amafrank]

one thing the books don't teach is that books are great for theory but don't always work in real life. Once you get below the speed of sound the stability rules change. This is the reason why everyone is suggesting twist rates that seem a bit faster than recommended by millers formula. Also realize that the spin rate of the bullet is directly affected by the velocity and that will affect stability. So now in the real world we use the formula to get in the ball park and testing to dial it in. 1 turn in 10 will not work with the subs and 300gr bullets. Also note that once a bullet tumbles its gone. It won't tumble for the first hundred or two hundred yards and then suddenly be stable. It may wobble or nutate slightly but no tumbling. Many publications have covered the yaw or nutation of a bullet as it clears the muzzle and this behavior is one of the reasons that .223 suppressors are not recommended on barrels less than 10" in length for full auto use.
Overstabilizing is bad for artillery and bad for long distance shooters because the bullet (or shell) nose doesn't tip and follow the trajectory of the projectile. In long range shooting that causes a flow across the projectile which will create drift due to the Magnus effect. It can also cause the fuses on artillery shells to fail due to impacts not being on the nose....newer fuses seem to have made up for a lot of this but it can still happen.
For us in the shorter range world of subsonics none of those overstabilizing issues will really be a problem. Our big worry is overspinning a weak jacket. For the most part I've not seen any issues there either. I've fired 110gr plinkers through my 30-221 with an 8" twist at 2500fps and not blown up the bullet. All of these are the reasons I suggest that over is better than understabilizing.

Frank

[Romeyo]

Alright thanks! I know that there's always a difference between practise and theory. But since I'm quite interested in ballistics and whats going on I wanted to know why people are leaning toward those 6.5twist barrels and where/why theory is failing. I guessed that those formulars get inaccurate when coming to the trans- and subsonic range. Thanks for clarifying!

Any comment on shortening and necking down .308 brass in stead of starting with 7mmBR brass?

[amafrank]

One of the main reasons everyone is using BR brass...either 7mm, 6mm or 30 cal, is because the benchrest brass is more consistant in thickness and also because it uses small rifle primers where the .308 uses a large rifle primer. The smaller primer is giving more consistant pressures in the benchrest round....not sure how it fares in the subsonic range. You can use .308 brass for making 338BR but you will have to neck turn the brass for it to work. The case is much thicker in the body than the neck needs to be. When you use the BR brass its already cut to the shorter length. Normal .308 brass is 2.005 long and the BR brass is 1.55" + or -. So you can see that the .308 cases will need a bit of trimming and they will be cut down into the thick part.

Hope that answers the question

Frank

[Romeyo]

Lapua makes .308 palma brass that uses small rifle primer pockets, a smaller priming hole and "lapua quality" tolerances, they're cheaper than 7mm Rem BR brass and also readily aviable. So why not giving it a chance? I don't mind neck turning since It won't have a bad effect on accuracy I guess

Only thing I'm worried about is case forming. Is it as simple as running a proper shortened (and lubed) .308 palma case through a .338BR (full lenght) sizing die and then neck turn and trim it?

Trimming and shorting would be done on a leathe, so no real issue.

Martin

[amafrank]

Originally Posted by Romeyo

Lapua makes .308 palma brass that uses small rifle primer pockets, a smaller priming hole and "lapua quality" tolerances, they're cheaper than 7mm Rem BR brass and also readily aviable. So why not giving it a chance? I don't mind neck turning since It won't have a bad effect on accuracy I guess

Only thing I'm worried about is case forming. Is it as simple as running a proper shortened (and lubed) .308 palma case through a .338BR (full lenght) sizing die and then neck turn and trim it?

Trimming and shorting would be done on a leathe, so no real issue.

Martin

you pretty well got it. cut to near proper length, run through the sizing die(lubed), trim and neck turn. Should be good.
The Lapua Palma brass will also be thicker most likely, since you're cutting it down from the .308 length. That cuts down the case capacity so you probably won't get as much powder in the case as you could with the BR brass. You'll have to work up loads for that particular brass too since it will give different velocity than the BR brass but thats pretty much required anyway. I also don't know how important it is to have the small rifle primers so maybe cutting down 308 brass would be fine...


Frank

[Romeyo]

Alright, sounds good. I'm not really worried bout the reduced case capacity since those subsonic loads doesn't require all of the aviable space inside the case even with deap seated bullets, do they?
And since a friend of mine got quickload for some years now I guess creating starting loads won't be too hard, although I guess that most "7mmBRcase" loads would work too (at least subsonic loads) just creating slightly more pressure and speed.

Martin

[d-mon]

High Romeyo,

For making your 338 br cases from 308 cases, try the following method:

Full lenght size the 308 cases in a 338 br dies with out the expender.
Then trim down (with a lathe or dremel first then case trimer, maybe with a .300 pilot at first because of the neck thickness).
Then you can use a K&m expender back to 338 that will make the neck snug fit on the K&M 338 pilot of their neck turning device.

Once that is done, you can load straight away.
I use that method for my 300 whisper brass with great results.
Good luck
ND