Sierra Blowups

[MGS]

On 3 occasions, there have been blowups noted with 7mm Sierra 180gn MK. Rifle is Kreiger on Barnard in 7WSM. MV 2975. How do we know they are blowing up? One instance a cloud of copper coloured smoke was noted about 50 - 60 yds down range & no impact on target. Others simply did not make it to the target & this was in the middle of a string of bulls, shots 1 - 4 bull, 5 no impact, 6 - 8 all over the target, but 9 & 10 bulls again. Next string 1 no impact , 2 - 6 bulls then no impact again. 3rd string 1 - 4 bulls,5 no impact 6 - 10 5 ring. Distances 800, 900, 1000yds
Sierra techs are making " must be a barrel problem", but 200 odd fired before this, & no problem.
Purpose of this is to ask if anyone has had similar problems.

Maurice

[Gadget]

Hi
Had this happen with a 6mm @ 970 rounds and unfortunately the tech's were right.
We re chambered tryed again no luck, so we stuck a bore scope up it's bum and the barrel had really bad pitting about 10" up the bore.
We pluged a new one in and no more problems.
So if you can beg borrow or steal a bore scope it might save you some time and Argo.
Graham

[bartman007]

Hello Maurice,

What twist rate are you using? 1:9 or 1:8.5 or ?

Also, what type of rifling is it? 5R?

I'm just wondering if at that speed, with a higher twist rate you may be exposing a shortcoming in the barrel.

Hopefully a bore scope can shed some light on your issue.

Bartman

[MGS]

Twist is measured 8.5in and it is 5R rifling. A borescope has been up & down this barrel so many times it almost has a permanenet home there. have checked chamber and every inch of barrel, but nothing obvious.

What we have now found is that right at the meplat there are some small"splits" which i will try to photograph and post here.

We are now wondering if the projectiles are starting to "unwind" as they are spinning at more than 250,000 rpm ??????????

[bartman007]

Interesting that Sierra state "Important Info", and it indicates that you must use at least a 1:8 twist barrel.

As the twist rate gets faster, should that require a slower speed? So that you don't tear the jacket off the projectile

On another note, why are the Bergers able to run 1:9 twist for the same weight projectile?

I think you should be OK with the 1:8.5 at that speed. So it will be interesting to see your analysis on the projectile tips.

[MGS]

The Sierra techs state that the reason for the 1:8 twist is that they expect these 180's will be shot out of a 7/08 which will not be able to reach the MV of a Std 284, so the faster twist is to avoid stability issues.

Berger apparently expect their 180's to be used at higher velocities.

Mhttp://imageshack.us/photo/my-images/8 ... eplat.jpg/

[DaveMc]

We have recently trialled the 180's in a 1 in 9.5 twist at 2800 fps with great results. Sea level, high humidity and warm (30 plus)

at 2975 I would have NO hesitation in going a 1 in 9 (or even 9.5). dropping an extra 14 -28000 RPM will make a hell of a difference. Don't forget a larger diameter bullet spinning at same speed has far greater gyroscopical forces (or acceleration) at play. The outside of the bullet is travelling much faster at same rpm. Berger have issued some statements on the 338 Lapua 300gr experiencing lead "slump" in the nose when overspun at high velocities. It would appear great bullet integrity is required as we push the boundaries on fast spinning high velocity large diameter rounds. A fine line between what is acheivable regarding stability vs holding together.

However - as with the Berger 6mm that were blowing up a while ago - it appears some barrels were much more susceptable than others and some barrel manufacturers even went to the trouble of producing shallower rifling depths to avoid overscoring the outside of the bullet.

[AlanF]

MGS,

One of the good things about 7mm nowadays is the large selection of projectiles for LR Target. There's always Hornady and Berger if the Sierras don't behave - I'd put them away till you get your next barrel and try something else in the 1:8.5 Krieger. Some Bergers had the same problem 2 or 3 years back, and they came up with the thicker jacketed "Target" version (currently in the yellow box), and I don't think there's been a problem since.

Alan