The Importance of neck tension?

M

murphy

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Maybe I have the wrong forum but feel that i need to reach a wider audience than is avalable on the Savage forum. Recently I purchased a new Savage model 16 in caliber 7mm/08 and started reloading for it. Bought a new Lee die set, the one with full length size mandrill type die as opposed to Lee collet die. Assembled some reloads in the usual manner, but being the curious creature that I am did something a bit different. Now I know that die makers hate people like me and lawyers make a good living out of them but I did it anyway. Got some fired cases and neck sized only with Lee factory crimp die, loaded some rounds with Sierra 100g varminter pills then Factory crimped again. Loads assembled in this manner with all else being equal show a considerable accuracy gain over loads assembled "properly". The unconventional loads shot 5shot 100yd groups of around .6" while loads assembled in the normal manner were around 1.2" Obviously this is an unusual situation but it did bring home to me the importance of neck tension. To me it is quite profound that the tiny amount of force exerted by a case neck can so alter the harmonics of a rifle barrel when it is compared to the tremendous force of the exploding powder charge. Is there a magic neck tension or as I suspect, all rifles are different? Thought very light neck tension may be the answer but with this particular load group size went out to 2".:D
 
Sizing and neck tension

If I'm reading your post correctly, you neck sized your cases with the crimp die, seated your bullets and then crimped the bullets in place. Hadn't heard of anyone doing that before. You probably would have gotten the same result if you'd removed the expander mandrel from the full length die and only partially resized the neck by raising the die up in the press. How much were you moving your shoulder back when you full length re-sized? How did you determine your seating depth? Do you trim your cases for uniform length?

The way you sized your neck and crimped the case acts as a fulcrum at a point on the bullet and that allows the bullet to self-align in the free bore. Since you didn't move the shoulder back the case used the full headspace and the shoulder of the case seated in the shoulder of the chamber and the fire-formed case body filled the chamber better. What you basically did was improve alignment. Crimping (which is a dirty word in the accuracy world) uniformed the tension. Crimping can compensate for excessive jump.

If you just sized with a collet die you probably would get the same results. Try varying seating depth a few thousandths both ways and see what happens. - nhk
 
To nhkuehl and alinwa, For some folks the road to accuracy can be full of pitfalls. And yes I do trim my cases, even new ones and am very conscious of the importance of seating depth. My biggest problem is that none of the rifles that I have owned in past and I have owned quite a few, had necks or chambers with tolerances close enough to demonstrate the value of concentricity in measureable group size. For a factory rifle the Savage seems well put together and careful reloading practices are necessary to get the best out of it. Since my experience with the Savage, went out and bought a Lee collet die set for both the 7mm/08 and a comercial Mauser in 25/06. Group size for the 98 are down by about 50%, so even with a rifle with fairly loose tolerences something good is happening. I used to think that concentricity was bunkum and seating depth was everything but I guess there comes a time when we need to swallow our pride.
 
Murphy,

Seems to me that the cases you sized "correctly" have in fact been sized to whatever dimension Lee considers is appropriate for all chambers (or maybe the average chamber) & not necessarily the dimensions of your rifle's chamber. When you fiddled with the crimp die, you didn't change the body dimensions (or didn't change them to the extent that you did with the Lee full body die), so they were a better fit in the chamber & despite you using the crimp die to neck size, the rounds aligned better in the chamber & shot better.

I suspect that you would have got equal or better performance if you had used just a Lee neck collet die to start with.

Full length sizing dies (preferably body only dies) are the tool to use to get your cases j-u-s-t fitting the chamber without resistance & that is achieved by applying more & more size to case bodies until the bolt just closes without drag.

John
 
To John Kielley, Now that I have a Lee collet die for the 7mm/08, am doing a series of experiments with that die. Had no luck at first because I was using old work hardned cases and the collet die was having trouble giving proper neck tension. The bullets did not have enough grip on the case and accuracy was poor. Today I have annealed some cases and if that does not work will need to buy some new brass. Am pretty fortunate to be retired and have my own rifle range so I just go and have a few shots when ever my wife lets me.
 
I think you will find you need to anneal, cut to OAL and FL size every 5 cycles or less.

Lupa brass makes a difference. You can neck up 243 or neck down 308. If you neck down 308 double check the Neck od on a loaded bullet for clearance. You may need to turn the neck slightly.

The Redding three die competition die set with the micro tops is very good. With the bushing die you can set the neck tension as you wish.
 
Octopus, my 7mm/08 is being fed 308 lapua brass necked down and they are a good fit in the neck of my savage rifle. A fired case gives a strong enough grip that they don't fall into the powder space but gentle enough ,not to need neck turning. The Redding competition die is one that I did not know about, The seating depth adjuster on the top sounds like a particularly good idea.
 
If your fired cases won't accept a bullet freely before sizing, as in slip right on into the case, you could be treading on thin ice. If necks thicken ever so slightly, and the bullets aren't freely released you could find out how strong your action is in the most unpleasant fashion.

Redding bushing sizing dies are top quality and allow neck tension to be adjusted in 0.001" increments. I'd recommend a light skim turning of those case necks though so that they're 0.002" smaller loaded than your chamber neck (which is likely the OD of the fired case plus 0.001 or 0.002", but could be less in your case).
 
Thanks Larry, Your warning is most appreciated, have only tested the strength of one front locking bolt action rifle in my life time but that was enough.
 
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