.308 lap to 7mm-08 neck length

[johnboy]

I'm looking for some advice please. I recently bought myself a 7mm-08 tikka sv.

 

For brass i got a 100 once fired lapua .308 ran them through a 7mm-08 fl die twice weight sorted a nice batch of 50 or so and loaded up a pressure ladder and a few choice loads for testing.

 

Results the 10 pressure test ladder rounds went into an inch (cleaning between every shot) and the second three shot group went .3 c/c.

 

So as you can imagine i'm very happy. However now that the cases are effectively fireformed i intend to neck size as required. The only problem is the necks are still showing about 15-20 thou short, they are also a mess (not flat and even as i like).

 

My question is do i fl size a couple of times to hope the cases stretch out in length or do i just set the trimmer 20 thou short, tidy /uniform them all up and neck size?

 

Also will a 20 thou shorter neck in any way affect accuracy?

nb this is predominately a stalking/culling rifle but i would like to start some target/ longer range stuff just to improve my shooting.

 

thanks

 

John

[kent]

Check the thickness of those necks, I used std 7-08 rem brass from Winchester in mine for this reason

[Laurie]

Necking members of the .308 Win 'family' up or down frequently produces minor problems or issues, one being too short case-length. In practice, it produces an undesirable rather than disastrous situation - no safety issue in itself, but over time you'll likely erode the chamber neck-section ahead of the case-mouth, and it'll probably also acquire a hard fouling coating that won't be touched by routine barrel cleaning. In theory at any rate - I don't know how much it really applies in practice - that could create chambering and pressure problems when you fire a (longer) standard 7-08 case as the fouling will reduce clearances. As you're using a factory Tikka, the odds are that the original clearances are such that this issue will never arise.

 

Nevertheless, this a messy (literally and figuratively) set-up, and it's not one I'd be happy to live with.

 

There are two other issues in the neck-up / down situation between .243, .260. 7-08, and 308 cases - the resulting neck thickness as Kent rightly points out. Reduce the neck diameter by 20 thou and the same amount of metal is partly forced into a smaller circumference, so becomes thicker. Again, the odds are that in this case and with a factory rifle's chamber, any slight thickening isn't an issue. the simple check is to see is fired case necks are still tight on a bullet - if not, you've got plenty of expansion and safety isn't an issue. If a bullet is a tight fit in fired cases from full-pressure loads, that may or may not point to inadequate clearances, but at the very least you want to check this out. The other potential issue is that the practice changes the position of the neck to shoulder junction on the case and by doing so may form a 'doughnut', a thick ring at the bottom of the new neck section. On sizing, it's shoved to the inside neck wall surface and is death to accuracy if the bullet seating position is such that the bullet shank is pushed through the doughnut - you can feel the increase in pressure needed on the press handle in bullet seating. Without looking at the two case drawings I can't say whether it's an issue in 308 to 7-08, but it was a notorious problem with the commonly used practice of expanding Lapua and Norma .243 Win brass up to .260 Rem in the days before Lapua started making the latter.

 

Generally speaking, Winchester 7mm-08 brass has a good reputation and before Lapua 260 was produced was often necked down to the 6.5mm calibre as it was so superior to Remington's efforts in its own cartridge. It needs some measuring, sorting and batching plus flash-hole / primer pocket uniforming, but I'd say that's maybe a better option.

 

This sort of practice has to be done and checked out VERY carefully in custom or rebarrelled rifles with minimum neck-clearance no-turn chamber neck dimensions. Small changes in the brass through re-forming without doughnut reaming / neck-turning may reduce clearances enough to cause pressure problems.

[jungle_re]

Some really interesting information there Laurie. I've recently acquired 100 lapua .308 cases precisely to do this with and am now having second thoughts.

Please bear with me as i'm very new to reloading. I understand that the formed cases will be slightly under the sammi specified length and that overtime they will stretch out slightly. If you neck turn after the resizing and after each firing will this mitigate against the donut effect or at least minimise it? Is it best to re trim all the cases after firing to the lowest neck so that they are all equally under and keep doing this till they meet the length after firing your aiming for? This will be going in a brand new rifle so i really dont want to damage the throat or chamber at all by sloppy practices (lets face it it will be new and shiney and for a good while i'll be overly precious with the thing).

 

I'm now thinking i'll be better off selling off the lapua 308 and hang out to see if i can source some of the nosler brass or swap the above work for case prep on winchester brass.

[kent]

Some really interesting information there Laurie. I've recently acquired 100 lapua .308 cases precisely to do this with and am now having second thoughts.

Please bear with me as i'm very new to reloading. I understand that the formed cases will be slightly under the sammi specified length and that overtime they will stretch out slightly. If you neck turn after the resizing and after each firing will this mitigate against the donut effect or at least minimise it? Is it best to re trim all the cases after firing to the lowest neck so that they are all equally under and keep doing this till they meet the length after firing your aiming for? This will be going in a brand new rifle so i really dont want to damage the throat or chamber at all by sloppy practices (lets face it it will be new and shiney and for a good while i'll be overly precious with the thing).

 

 

 

I'm now thinking i'll be better off selling off the lapua 308 and hang out to see if i can source some of the nosler brass or swap the above work for case prep on winchester brass.

If this is a hunting rifle I should use the Winchester and see how you go, you don't want to get to anal about brass that 1. does its job 2. is likely to be lost in the grass or mud

[jcampbellsmith]

There's a wide variety of brass available for the 7mm-08 http://www.midwayuk.com/epages/Midway_UK.sf/en_GB/?ObjectPath=/Shops/Midway_UK/Categories/9315/652/670/9013/7378

 

I would be looking at Norma brass myself if it was one I needed. Regards JCS

[mutley]

There's a wide variety of brass available for the 7mm-08 http://www.midwayuk.com/epages/Midway_UK.sf/en_GB/?ObjectPath=/Shops/Midway_UK/Categories/9315/652/670/9013/7378

 

I would be looking at Norma brass myself if it was one I needed. Regards JCS

could also try nosler brass aswell

 

atb Matt

[jungle_re]

I think i'll sell the laupa then once i can get some suitable replacements.

[gbal]

There's a wide variety of brass available for the 7mm-08 http://www.midwayuk.com/epages/Midway_UK.sf/en_GB/?ObjectPath=/Shops/Midway_UK/Categories/9315/652/670/9013/7378

 

I would be looking at Norma brass myself if it was one I needed. Regards JCS

 

Jcs is good advice_norma brass in the proper cartridge is excellent-gives nothinG oaway to Lapua,and lasts a very long time in a hunting rifle,and you avoid all the hassles reforming brings,as per Lauriie's reply.Why go this road for an inferior end product.Get Norma 7-08 brass and have the best with no hassles.

Lapua is good,but not necessarily to mess about with.It's reputation begins with out of the box no prep needed.

Norma is just as good for most purposes.Get the stuff made for the chamber in the first place,unless you really have to wildcat,!

 

Gbal

 

 

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