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Case length reduction upon fire forming Improved cartridges?

 
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Old 03-13-2003, 03:14 PM
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Case length reduction upon fire forming Improved cartridges?

I was wondering if you all could give me an idea of what to expect the case length to be if it started out at 2.720" on the virgin 338 Lapua case then necked it down to 30 cal and formed it with .018" total body taper using a 35 degree shoulder and a .270" long neck? At least I want a .270" neck left when I'm done. [img]images/icons/wink.gif[/img] I figured it would come down to 2.7" from 2.72" loosing .020" and was about to spec and order the reamer at 2.72" but Dave Kiff at Pacific Tool warned me that some guys have been telling him they had up to .070" - .100" reduction in length when improving the 338 Lapua case, leaving them with very little neck at all. This doesn't seem right comparing the 2.7" case length that some have posted for the Yogi, so I'm kind of holding off for a day or two to order the reamer until I get this worked out based on the input I get. It's a tight neck, .333", so I want enough length to the chamber to fire form and not have to worry about it being too long on the other hand.

Any experience or suggestions for a chamber OAL, brass length, etc would be very helpfull right now, thanks in advance. [img]images/icons/smile.gif[/img]
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Old 03-13-2003, 04:06 PM
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Re: Case length reduction upon fire forming Improved cartridges?

Brent,
With my 338 Yogi when blown out, using Norma or Lapua brass, it will shorten up between .020 and .030". I've done 140 of these cases over the last several years and haven't seen anything over .030". I usually end up with a case OAL of 1.690"/1.695".
I've never heard of anything like .070 -> .100". I keep my ear pretty close to the ground and have never heard of this.

Most of my med. capacity cases or Ackley improved will shorten up .005-.015 depending, just to give you something to reference to.

We have our very first match of the year this weekend. I'll ask my buddy that shoots a 300 Yogi what his OAL is after fire-forming. Just hung up the phone with Dave Tooley about 1/2hr ago and he'll be there this weekend. I'll run it by him also and see what his take on this is. Dave Kiff has been around this business to long to just say, "no, I don't think so". But I haven't seen or heard of this from anyone else to support his statement either.

Will let you know, if Sunday isn't too late.

P.S. - virgin factory brass has measured 2.715" +- right out of the box. I don't think I've ever had any go up to the 2.720" mark right out of the box.

Steve

[ 03-13-2003: Message edited by: Steve Shelp ]
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Old 03-13-2003, 05:00 PM
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Re: Case length reduction upon fire forming Improved cartridges?

Steve,

Thanks for the reply on this. I'm going to order the reamer tomarrow if I can.

Here's how the conversation with Dave went.
I asked him if he could figure the length to the base of the shoulder and the top of the shoulder if I just gave him the spec of a 35 degree shoulder, the diameter at the shoulder and neck length. He said no problem. When I told him I wanted .018" total taper and a .270" long neck is when he told me that I should know some guys have ended up with alot shorter necks than they had figured on. He said most were .050" - .070" shorter and one guy in particular was seeing a .100" reduction. I wonder if this was a recent problem, as he never mentioned it the last time we spoke of the improved Lapua case and he pointed out some specs for me of a 30/338 Lapua Imp in a recent issue of Precision Shooting magazing. He said one guy sent him the reamer back with some formed brass to inspect, but the reamer was fine and just the improvement made it shorter was all. He was just informing me so I wouldn't be shocked to find it left me a shoter neck than I counted on is all. He did make a firm recommendation that a 12-15 thou or so per inch taper, I can't remember exactly though, would reduce this considerably on this case. We discussed this issue at length also and I ended up telling him I'd talk to you guys about it before I ordered the reamer then.

I told him Neil Jones told me to go with .005" per inch taper and he just said this case with all the taper in it to begin with would shorten considerably verses other case designs that had less to begin with.

I figured the .008" per inch would give me a maximum blown out design and still stay away from tight extraction that some have said was an issue at only .005" per inch.

I checked the box of brass I have for length again. The vast majority measured (15-20 I checked) all measure 2.719-20" and a couple went to 2.722", and only a couple were as low as 2.717".

I wonder what Wayne's box is giving him for length, we both got them from Bruno's recently.

The way I see it so far, is that the chamber length needs to be at least as long as my trim to length of 2.72 so the necks aren't pinched at the end, and whatever jump it ends up being to the end is what it will just be?

So here I am, I have a .333 neck dia. and OAL of 2.72 chamber minus the .270" long neck leaves me with 2.450" from the base to the shoulder neck junction. The neck will be shorter than .270" initially but not any shorter than a 300WM is for sure. When I use this reamer for my 338 Lapua Imp this fall and the neck is opened up for the 338, the neck will be sightly longer on it than the 30 too, so it seems like this will work.

S1's Wolf he said, would loose .030" - .040" max upon fire forming, and also reminded me of the 300wm's super short neck too. My 300wm's don't have any longer than .240" long necks when they are sized all the way down too.

Thanks for all the guidance here. [img]images/icons/smile.gif[/img] All goes well, I'll have a reamer in 3-4 weeks.
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Old 03-13-2003, 09:01 PM
 
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Re: Case length reduction upon fire forming Improved cartridges?

Brent,
Just measured 20 cases, they came out just like your measurements exactly. Average length was 2.7185.
Wayne
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Old 03-14-2003, 12:44 AM
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Re: Case length reduction upon fire forming Improved cartridges?

Thanks Wayne, interesting. Close enough to figure what I need to right now. [img]images/icons/smile.gif[/img]
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Brent Moffitt
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Old 03-14-2003, 09:01 PM
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Re: Case length reduction upon fire forming Improved cartridges?

Steve,

I hunkered down around the house today and never ventured out in the wind today to get to my smiths in Wasilla. They abandon work and went out shooting. The wind was horrible once you get out of the valley headed that way, so I stayed home.

Whatever you find out at the range this weekend would be interesting still. Thanks alot and good luck with your high scores and tight groups! Enjoy! [img]images/icons/smile.gif[/img]
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Old 03-16-2003, 05:22 PM
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Re: Case length reduction upon fire forming Improved cartridges?

Brent,
I asked around and nobody is reporting anymore than .020" shortening on thier cases. There were (3) 338 Lapua Improved rifles and (1) 300 Lapua Improved fired yesterday. I asked each one of them and Dave also.
It was recommended to make the reamer length .005" over the max case length. So 2.720" + .005.
The gap created really doesn't cause any problems except when you don't keep it cleaned out and carbon builds up in there.
I did have this happen to me last year to where eventually I couldn't chamber a round. I explained this in a previous post months back to CrowMag, I beleive it was, if you want to reference back to it. So if you keep the throat area cleaned up from carbon buildup, it's not really a problem.
But you can't make the chamber shorter to begin with to compensate for the shorting upon firing, because the .020 or .030" shorting is going to happen no matter from where you start. If you trim brand new case back enough to fit into the shoter chamber they are simply going to shorten up .020 to .030 from that point instead of the 2.720 of a brand new case.
So me personally, I would keep the chamber as long as possible and keep the shorting of the case to the maximum possible length.
Remember if you don't end up with a .270" long neck with this 338, it isn't as critical as say a 22 or 24 caliber rifle. The surface area in contact with the bullet is much larger with the larger diameter bullets. So there is plenty of neck tension to hold the bullet if your neck ends up .010" or so shorter than what you originally planned for. My opinion only for what you think it's worth.
The only way around this would be to have a blow out chamber that is cut around 2.720+... fire the case to blow it out and shorten the case up.... OAL trim the case to the shorter dimension... let's say 2.695".... then have your actual rifle chamber cut with at 2.700" OAL reamer. But that would take 2 reamers and second barrel/chambering and a lot of work for no improvement in accuracy or functionality IMO.

We did ok yesterday. Didn't win anything, but I felt good with my results in the conditions that I shot in and compared to others in my relays. The 338 did a lot better than I expected.

Talk to you later!
Steve

[ 03-16-2003: Message edited by: Steve Shelp ]
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