300WM fire formed case length shorter than book trim length

[skerk]

Hey guys-

I zero'd my new savage this past weekend and measured headspace & length after fire form. Used Hornady factory 178gr ELD-X (if that matters). I'm showing case length to be anywhere from 2.602" to 2.605". The hornady book is showing for case trim length to be 2.610".

I'm curious as to why the cases would be short to begin with? And do I need to worry about it when reloading? Thanks!

 

[twister]

I wouldn't worry

 

[L.Sherm]

After about 3 firings I would trim them all to the shortest one and run with it.

 

[Mikecr]

Cases often pull necks back on fire forming, as the shoulder angles increase with this.
But the book could be wrong for your chamber and IMO there is nothing good in trimming too short.
You could sacrifice a bum case, trim ~1/8" away and use a Sinclair chamber length gage to find your real baseline from which to choose trim length. Then, trim as you reach it.

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[L.Sherm]

What is to short? If you believe saami 2.60 is minimum.

 

[MagnumManiac]

All belted mags shrink, some more than others. The shoulder on a 300WM will often lengthen by .018"-026" depending on brass manufacturer, 7RM, 264WM and 338WM often grow .012"-.018".
All of this pulls brass from the neck into the shoulder shortening the neck.
I do not trim to SAAMI trim length, my 300WM brass is kept at 2.635"-2.640" depending on chamber dimensions.
Shortest I run is 2.625" in ONE of my barrels, the reamer didn't have the usual .040" clearance at the end, only about half that.

Cheers.

 

[skerk]

good advice all, Thank you- will order Sinclair gauge because I'm curious now.

 

[Philward]

The factory new cases are shorter than the trim to length by a good margin. I'm sure loaded ammo falls into the same range as new empty cases.

 

[Rflshootr]

Are you measuring the case length before or after you resize? If before, resize and then check them.

 

[Philward]

After.

 

[shootallday]

You will be fine, if it really bothers you get these.

Peterson Cartridge introduces .300 Win Mag – Long

Peterson Cartridge Co. of Pittsburgh, PA announced it is producing .300 Win Mag casings in two versions. Peterson makes the traditional SAAMI spec’d version that headspaces off the belt. But it also makes a version that headspaces off the shoulder. Peterson calls that version the .300 Win Mag- Long.

www.petersoncartridge.com

 

[Rflshootr]

After a couple more firings, they'll be where you want them.

 

[BigPrince]

Had a similar concern in the past with 300wm and was concerned they were short and shoulder wasn't getting any bump in my Redding die even when touching the shell holder but just kept firing it and trimmed to shortest length of the brass for consistency.

 

[skerk]

I did get the Peterson long brass but overall length is still under 2.610, constant at 2.608.

after re-sizing majority are still at 2.604-2.606. Which is close enough, I was just puzzled when looking at the book and it was under. They're all Once fired, so I assume they'll grow. Mixture of Hornady, Barnes and the new Peterson brass.

Thanks for all the input!