Lapua Answers Brass Sizing Q -- The Definitive Word from Finland

[Forum Boss]

We recently asked Lapua about observed dimensional changes in Lapua 6mm BR brass and loaded ammo. Here is the exchange. Bottom line, Lapua's spec is ".471" at the web, but current production is running smaller, about .468-.469". Note also they are listing a max neck diameter for loaded ammo of .272". So, if you ream for a .271" neck, it might be a little tight.

Janne Pohjoispaa, Nammo Lapua R & D Writes:

"Measurement D,diameter at web area) maximum diameter is 0.471", but actually it has slightly smaller diameter as you have noticed. We recommend using these maximum measurements as these will guarantee safe and reliable function in most situations. However you can set diameter 0.470" - .469" if you considered this an important feature.

We produce such tough case that I consider primer pocket expansion not a problem. Primers stays in place, yet most small rifle primers may be pierced if you go above of 4500 bars. This is, however, quite far from the 6 mm BR safe maximum pressures.

Best regards,

Janne Pohjoispaa

- - -
From 6mmBR.com

Janne,

Thank you for the information. Could you re-check Dimension D,web diam) from some actual loaded 105gr 6mm BR rounds from current production. We've checked samples from 4 different boxes of Lapua unloaded brass and the dimension D,at web) is running .468-469". This is causing some concern among our tool-makers as most reamers are spec'd for brass that is .471" at the web.

But if Lapua brass is now running .4685" on average at the web, then we may get primer pocket expansion at lower pressures than desired.

I have three important reamer makers,Pacific Tool, Henriksen, and Clymer) waiting for a definitive answer, and my readers want to know.

Lastly, is the dimension B,43.3 mm to ogive) the same for both moly and non-moly 105gr 6mmBR loaded ammo?

Thank you,

Webmaster, 6mmBR.com

On Tue, 21 Dec 2004 08:57:49 +0200, <janne.pohjoispaa@nammo.fi> wrote:

> Here is information you requested [re 105gr loaded ammo]:
>
> A: max. [cartridge] length is 60 mm,2.362 inches)
>
> B: Length from base to point where bearing surface ends and ogive begins is 43.3 mm,1.705 inches)
>
> C: Max. [shoulder] diameter is 11.68 mm ,0.460 inches)
>
> D: Max. [web] diameter is 11.96 mm ,0.471 inches)
>
> E: Max. [rim] diameter 12.01 mm ,0.473 inches)
>
> F: Max. [neck] diameter 6.90 mm ,0.272 inches) ,min. diameter is not available)
>
>
> Best regards.
>
> Janne Pohjoispaa
> Nammo Lapua R & D

 

[dreever]

Thanks for the info. Those of us using reamers for the .471 web really shouldn't see any problem using the new brass.
As a caution if one were to have a reamer made using the new specs, and then have future lots of brass from Lapua go back to the .471 measurement it's possible a problem could arise there, but perhaps not.
At any rate, it's just one more thing to worry about, but I wouldn't lose to much sleep. LOL!

 

[Riverine]

Nice to get the info straight from the factory--Thanks guys. I tend to agree that the couple thousandths variance shouldn't make a difference unless someone was trying to build a super-tight chamber. And then, yes it would be advisable to let the reamer-maker now about the changed web dimensions. I agree though that you wouldn't want to spec for a .468" web and then find the brass goes back to .471".

I contacted Pacific Tool and Gauge, and they did recommend going with a .272",as opposed to .271") neck if you want to be able to shoot the Lapua factory loaded ammo.