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Bullet length of the new 7mm Sierra M-K


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Just a quickie for you. Does anyone know the advertised bullet length of the new Sierra 7mm 183 grain Match King #1983. unless I'm being as stupid as my wife thinks I am, I can't seem to find it anywhere.

Cheers

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Henry Krank's online store showed it as 'In stock', the last time I looked. I've been considering giving it a go in my 284.

 

(At 1.591", 1 in 8.5 twist appears to be required to produce optimal stability at 284 velocities, the more common 1 in 9 produces an Sg value in the low 1.3s according to the Miller Rules spreadsheet.)

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Cheers for that guys. I'm just working out the S.G. with the Berger stability calculator. It turns out at 2,675 fps it should give an S.G. of 1.65 so, fully stabilised & still supersonic at 1200 yds through a 30in 1:8 barrel. It seems it would still be pretty stable down to 2550 fps. ;)

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  • 2 weeks later...

Hi Swaro. Yes, I know what you mean, but I'm just going by the recommended twist rate according to Sierra. I will probably run 180's through this barrel, but would like to have the option to try these bullets as, if they fly well, should be pretty impressive. All I'm doing with a 1:8 is having all my options open.

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Weight is only part of the equation. Bullet length is the key factor allied to 1) calibre and 2) twist rate.

 

The old 180gn SMK is 1.460-1.470" OAL and in a 9-inch twist at 2,800 fps MV has a stability factor (Sg) of 1.66

 

The new 183 is 1.591" OAL and at the same MV and in the same twist rate, the Sg value drops to 1.32

 

The 180gn Berger Hybrid which is the 'go-to' bullet currently for most long-range 7mm shooters is 1.53" OAL

 

2,800 fps (284 Win) in the 9-inch twist gives it 1.45 and 2,950 fps (short magnum) 1.48

 

 

Current recommended Sg values are a minimum of 1.5 not only to stabilise the bullet enough to group well, but also to allow the bullet to minimise drag, or put the other way to obtain its full BC value performance.

 

At the Hybrid's 1.45 level, Berger says it'll see its effective BC drop by 2%

 

At the Hybrid's 1.48 level the loss equates to 1%

 

The older 180 SMK gets its full BC whack as its Sg exceeds 1.5

 

The new 183gn model with a calculated 1.32 Sg is estimated to lose 5% of BC value which means it loses much or all of the improvement that the new shape provides over older models.

 

That's in 'standard ballistic conditions' of 59-deg F and 29.92 inches mercury atmospheric pressure. On a cold winter's morning at Bisley around freezing and maybe with a high pressure area over the region, the Sg would see a further drop to ~1.25 at which point bullet behaviour can start to become inconsistent.

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