Sizing new Lapua brass, need datum measurement.

[GentleBen]

Being cautious here and need some guidance

I'm sizing up some new Lapua 6mmBR brass for the first time and wondering what the datum line measurement should be when using a Hornady headspace gauge attached to my calipers.

New Lapua brass out of the box measured on average 1.1480 to 1.495 and they barely fit into a Wilson case gauge. I played with 10 pieces and seemed to have to size them quite a bit for them to drop into the case gauge flush to where they are supposed to be. I had to size a few pieces down to 1.475 to make them fit flush.,how low can you go?)

Puzzled however ... a PTG no-go gauge measured 1.550,did not fit, or course) and a PTG go-gauge measured 1.500,sometimes 1.495) and dropped in nicely,I can tell it is undersized however). Also, some new Lapua 90gr Scenar bullets had trouble falling into the gauge but I chalked that up to the slightly expanded necks caused by the presence of the bullets. A little tap and they seated nicely.

My main question is what is the optimum datum line measurement when first-time full length sizing new Lapua brass? I've read where the trim length is 1.555 but I can't find the datum line data. I would hate to over size the shoulders on the new brass.,I am using a Redding full length die, non-bushing.)

Yes, I plan to order a Harrell's FL bushing style sizer but first I have to size some brass and load a few bullets to shoot to send Harrell's some fire-formed brass.

Or, should I dispense with the sizing and just trim, prime, load and shoot? Or, should I just shoot the commercial ammo to get by fire-formed cases and let Harrell's take care of my measurements for me. Still I'd like to know how you all size your brass.

 

[mason]

have you looked at the 6BR info page on this site..

 

[GentleBen]

yes, I could not find info on the datum line. please give me a hint if it is there. or, the handling of new brass.

 

[alf]

Why are you concerned with sizing now? Do they not fit your chamber? If they do, just load 'em up and shoot 'em. If they don't, size them down til they do fit with slight resistance on the bolt. Trim later.

There is no optimum datum line measurement. That measurement is only relevant to brass fired in your chamber to use as a reference point to set up your die to bump the shoulder back a set distance.

 

[GentleBen]

Thanks, alf, I am accustom to reloading .223 and you deal with all sorts of brass quality...and the datum line was a good way to find an optimum case length and shoulder bump. Come to think of it, I used a good bit of new .223 Lapua and more often than not found little to no difference in their brass right out of the box and after sizing it.

I did not know if the same consistency applied to 6mmBR brass. Its 'odd' shape throws me off a bit and keeps wanting me to have to do something to it.,ha)

So I'll just trim, chamfer and debur with my Giraud trimmer. Then prime, load and shoot. I read on this site where 1.555 is the accepted trim length.

thanks again

 

[Ackman]

Lapua brass is very uniform in length. Mine is .223, 6BR, and .243. I've measured a whole ton of 6BR cases and they were all 1.555'- 1.557'. With .243 brass I've measured variation in casehead to neck/shoulder on several headstamps including Lapua. There's nothing you can do about it. Don't worry about the datum line on your brass. It doesn't need trimming, just a good inside chamfer then load them and shoot.

 

[Bob3700]

Ben,

With new brass, you should be able to just load and shoot. Once the brass is fired in your chamber, that is the time to measure several fired pieces and establish the 'Datum' length you are referring to.

The 'Datum' length you are referring to, the gunsmith calls headspace. That dimension has a tolerance, it is the dimensional difference between the go and no-go gauge. The gunsmith can set the headspace very close to zero or a couple of thousandths larger.

So if you don't know the exact headspace the gunsmith set the chamber to, you just have to wait, fire some brass, measure the datum to base, and call that dimension ZERO. Then you can size the brass slightly shorter than your chamber, usually .001' - .0015'.

HTHs

Bob

 

[GentleBen]

Well, Bob, thanks all too much. That's about as a good an answer as I could ask for. Explained very well. Now I can go about loading some test rounds.

What a great forum this is. It is every bit as good as the reloading forum on AR-15.com ... and it is a good one as well.

 

[Mikem]

On one batch of my new Lapua 6BR brass, using the same tool as you. My reading was 1.1425, But this will change depending on your chambers head space once fire formed. I then like to keep my brass about .001 less than after firing, but this requires a slight shoulder bump. This only applies to neck sizing. But if you find the bolt hard to close, your cases head space may be to long, and this should not be a problem when FL resizing.
Mike.