Neck Clearance

[Trev]

Hi Guys

I have a 308 win with a 338 neck. Now I got 100 rounds of Lapua brass for it and measured them up and they come with a neck OD of 0.336. Now when I press in a projectile the od goes to 0.338.

My question is, when yu fire the cartridge, the pressure builds up and expands the case and the neck as too release the projectile. How much expansion should you allow for.

I will neck turn the brass down, but just wondering how much clearance you guys use, with a neck of 0.338 in the barrel and a od neck of the loaded round being say 0.336, thats really only one thou clearance the case has for expansion. To me that doesnt sound enough. How much clearance do you guys give your case.

Thanks Trev

[Curtley78]

G'day Trev,

I almost asked the same question as I have a .308 that I am developing a load for.

I am using Federal cases and the OD on the neck measures .338 Once fired the median measurement is .345.

You will nead to fire at leats five rounds and then take the median value.

I had opted for a bushing of .340 however I was unable to acquire one and was only able to obtain one in .333.

I was talking to Keith Highfield's about this on Wednesday apparently this is working the case too much and will lead to problems later on such as splitting etc.

He informed me that if you can turn the projectile in the case then theres not enough tension.

I also think that by increasing neck tension too far that this may also increase pressure and velocity.

As an example, I developed a load for my .308 using Vihta Vuori N 150, they list a maximum load as 46.6 grns delivering a muzzle velocity of approximately 2760 fps.

Now I zeroed the rifle at 100 and at 200 I received a 2 and a quater inch drop, now I didn't have access to a chronograph but I ran some numbers through 'Load from a Disc' and according to the program I was running about 3150 fps and I had loaded a grain under the maximum load.

Then again this powder may be a little more sensitive to our warmer climate.

[seantheaussie]

Once fired the median measurement is .445.

You will nead to fire at leats five rounds and then take the median value.

I had opted for a bushing of .440

No offence but I hope an edit is required

[malcolm]

Yoo Sean-------- you had better remeasure that once fired case again.
That's the trouble with dislexia, it makes you press the worgn buttons on the kyebaodr
Cheers Malcolm

[Ackley Improved]

Mate

0.338 neck, for a BR rifle you look for about 0.002" clearance.

With that rifle however, I would go for a little more, around the 0.003".

So, with a projectile seated in the case, the neck should measure 0.335".

You buy a neck turner, are you going to use me to turn?

Also, I have a sling here for the rifle if you are interested? It is TIS sling and has the proper clips that attach it to the AICS.

The 308W Shane is putting together for me now will have a 0.340".

Sean... as for your bushing size... load a case with a projectile, measure the neck and then subtract 0.002-0.003" from that to get a correct bushing size. Federal brass??????

Cheers
AI

[Curtley78]

Yea, lucky I haven't done any reloading tonight (I've had a few beers)

Trev .345 was the median of a fired case and the bushing I had opted for was .340.

Federal Brass? I know, but I just couldn't wait to get the rifle on paper and besides it managed an inch group at 200 and I do intend to get some fine grade brass.

Don't worry I don't intend to insult anyone, I won't show up to the next Fly shoot in Canberra with Federal brass.

[Ackley Improved]

Yea, lucky I haven't done any reloading tonight (I've had a few beers)

Trev .345 was the median of a fired case and the bushing I had opted for was .340.

Federal Brass? I know, but I just couldn't wait to get the rifle on paper and besides it managed an inch group at 200 and I do intend to get some fine grade brass.

Don't worry I don't intend to insult anyone, I won't show up to the next Fly shoot in Canberra with Federal brass.

Sean, run this by me again.. I dont know if I am catching this right. What 308 barrel is it? factory or a custom cut chamber? With the Federal brass, when you seat a projectile, what is the outside neck measuring?

I do not know why you are measuring the outside diameter of fired cases... Obviously you only want to resize the case a minimum to reduce over wokring the brass, but you need to size it so you are getting this 0.002" grip/tension on the bullet.

Hence, measureing a case with a bullet seated, and then get a bushing size 0.002" under that. The idea of the bushing is to manage and choose how much grip you want. You will most likely find going to a new brand of brass will result in needing a new bushing.

If you were using a 0.340" bushing, I assume the loaded case is measuring 0.342".

Cheers
AI

[Trev]

Thanks Guys

Trev, PM me the cost for the sling. My mate had a eck turner and he never uses it so he gave it to me. Sat down last night and turn down 150 cases too 0.3325 and 0.333. So that should put me right on the moneyt for 2-3 thou clearance.

Thanks Trevor

[chris.tyne]

G'dat Sean,
Mate just to clear things up are you talking about neck clearance and/or tension as they are two totally different thing's.

Regards Chris.

[Curtley78]

Chris,

I am talking about neck tension.

Trevor,

My .308 is an ex big bore rifle with a Maddco barrel, it was not chambered with a tight neck.

The OD of a neck on a loaded round measures .338.

I fired off five rounds and then measured the OD, the median was .345.

I had opted for a bushing of .340 so as not to work the brass too much however 'Tobler' did not have one in stock, all I could get was the one in .333 which from what I was told is what they use for Lapua brass, it does not bother me as I will eventually be using Lapua brass.

I am pretty certain that standard dies such as Lee & Simplex etc reduce the OD of a neck to .330 (which would destroy my theory in the previous post on excessive neck tension causing increased pressure and velocity unless Vihta Vuori were using bushings of a lesser tolerance when recording loads).

I recently spoke to another bench rester at Siverdale who is revered as one of the holy men of the fraternity about this and he said that reducing the neck more then .338 would be working the brass too much and that minimal resizing is all that is needed, he also said that if you can turn the projectile easily in hand once loaded then there is not enough tension.

Would I better off using one sized .335 and would using a bushing with a greater tolerance result in increased pressure and velocities?


Regards

Sean

[Ackley Improved]

Sean

I am using a 0.334" bushing with Lapua brass. That is around or very close to 0.002" tension.

Lapua loaded brass was measuring 0.336" to 0.3365". That is why I had the chamber cut to 0.338" on the gun mentioned in this thread. It is too tight however I feel for a tactical rifle. That is why my new one will be 0.340. Plenty of room there.

I think the 0.340" bushing you were wanting is pointless. Its chamber is large on your rifle, and you will need to work the brass from the 0.345" to 0.334" using a bushing. This is because the idea of a bushing die is to control the neck tension to a desired amount. Ultimately around the 0.002" mark.

Sorry to say, you'll be working that brass to get the desired effect on neck tension.

Cheers
AI

[Rinso]

The problem here is very simple and Sean has already alluded to it ..

My .308 is an ex big bore rifle with a Maddco barrel, it was not chambered with a tight neck.

As an NRAA TR rifle the chamber will be basically factory spec which is simply far to large for anyones liking. Factory chambers do not lend themselves to BR type loading techniques such as neck turning and neck sizing for tension.
generally I would not bother with a bushing neck sizer with a factory chamber anyway.