Accuracy in 6.5x47L, neck turn chamber vs non neck turn chamber

[stubbicatt]

I am in the process of amassing information on the 6.5x47 Lapua. I see that I can get a "no neck turn" chamber or a "tight" neck chamber, which requires neck turning.

In your experiences, will the "tight" neck chamber shoot more accurately? If you have experienced an accuracy improvement is it significant and consistent?

Regards,
Stubb

 

[mikecr]

Couple things;
-It takes way more than neck clearances to affect accuracy, so there never will be a turn-vs-no-turn accuracy result (in itself).
-You don't turn necks because of a tight chamber, you get a tight chamber for your turned(or non-turned) necks.

As far as the attribute of clearance itself, you can shoot as well with high clearances as with low clearances, provided you manage all else to prevent any issues in it. I've run clearances loose(factory), tight, and fitted, and I see no advantage or detriment between them. My dies are all custom, so it's managed one way or another.

If you intend to turn necks to a target thickness, and you already know you can manage this well, then it makes sense to order chamber necks providing normal clearances(within your die's range).
If the thickness & thickness variance of your new brass suits you fine as it is, they're not ridiculously thick, you're using off the shelf dies, and there would be rational clearances in an available chamber, then it makes sense to go with that.

I set up a 6.5WSSM. New necks with this brass are 20thou thick!
Doesn't make sense to waste a lot of efforts in making necks 20thou thick shoot well. So I planned up front to turn them to 12thou thickness, and I set my chamber reamer appropriately. My dies were chambered with my barrels here.
I played with different clearances alone, and it made no difference on target paper.
It could have made a difference,, maybe. If my loaded ammo had a bunch of runout, and my shoulder bumps were inconsistent..
I don't know there. Difficult to prove

 

[mj2evans]

I am not a experineced BR guy but ended up with a 243 AI Shilen barrel for my Savage with a tight neck (turning brass required). Accuracy is 0.3" with just an average trigger puller. Now since I had to fire form and turn brass I did invest in Lapua brass - thought alone likely helps consistency of groups.

I turned (very lightly, just clean up) some HXP 30-06 brass for my factory Winchester M70. Guess what - better groups! The chamber neck its self is not tight by any means. I assume just getting consistent brass is the most helpful thing here (I do anneal and that seems to help too).

I wouldn't want to have to turn 500 cases for a varmint hunt but for a bench gun or big game rifle turning appears worth it. If you can match a no turn to your bullet/brass you might save some effort.

 

[gotcha]

All good info above. With Lapua brass you'll have very consistent neck wall thickness. I measured a blue box of 100 and had no more them .0005" R/O using a mandrel before firing. Agree w/ D. Moran that .003" neck clearance would be max. My F/F necks are .293", FL sized are .288", loaded round is .290". Make sure you know what actual "no turn" neck diameter is before proceeding. A chamber of .298" COULD be considered "no turn"

 

[stubbicatt]

So, If I understand, lets say I get a batch of Lapua brass and in order to get the necks to be of uniform thickness all the way around, I have to turn the necks. Once this is done, the necks are of course smaller. If I know this dimension (whatever it may be) before having a barrel chamber reamed, it would be good to specify this neck dimension of my turned, cleaned up and straight uniform necks, plus maybe .003" to allow growth upon firing?

The benefit is that the brass doesn't stretch so much at the neck each time its fired, and neck tension, concentricity, and seating/release force will tend to be more uniform throughout the batch of ammo by using the appropriate neck sizing bushing.

So it is a cart/horse sort of analysis. One gets no particular benefit from a tight necked barrel, per se, but one does get benefit from uniform brass, and the appropriate chamber neck diameter to allow this uniform brass to expand safely when fired, but not too much, might dictate a tighter necked chamber?

 

[mikecr]

I think you got it