Tight Neck

[superx10]

Hi I have two 308 barrels one has a .342 neck the other a .340 neck....... a loaded Lapua case measures .338 both barrels have had over 1000 rounds with no problems, the .342 barrel shoots marginally better( very small sample).

My question is what is considered the most effective neck measurement for accuracy and practicality and at what point is it to large and at what point is it to small?
Thanks in advance.

[tachyon]

I have barrels with .340 necks and .343 necks. Both are smaller than the 0.3433 SAAMI spec for 308.

500px-Cartridge_308.png

That 0.3433 is the maximum dimension for the neck. You do need some clearance in your chamber around the neck to allow the bullet to release cleanly.

On the practical side of things, my 0.340 neck chambers require turning necks and will not, for example, chamber ADI factory target ammo loaded with the 155 grain Sierra projectiles. The 0.343 neck chambers will chamber this ammo without any issue.

Also on the practical side of things, the shoulder can centre a case (if there is zero headspace) or the bullet can centre things (if jammed), or the case body can do it. What I'm trying to say is that the last thing that might vaguely be useful in centralising the case is the neck, remembering we genuinely need 1-2 thou clearance to release the bullet cleanly so would need the case to "flop" down that much under gravity and it might reasonably be expected to bottom out on other things before we even get close.

In a larger necked chamber the neck will expand more. In my 0.340 chamber, annealing every time, turned to 0.014, post firing, my Lapua cases can still (just) hold a projectile. In the larger neck chamber, with identical cases, I can drop a projectile in post firing. There is, as you might expect, a couple of thou difference in the case neck diameter. This means the neck is stretched more in the 0.343 chamber, and sized more to get the same neck tension.

Given we are well north of 10 firings per case with no split necks this seems entirely irrelevant.

I can not pick any difference in accuracy between 0.340 and 0.343 barrels. I do have 2 x 0.343 neck barrels, from the same manufacturer, chambered at the same time by the same gunsmith. One is a hummer. One is not.

So, what does all that mean.

I turn my necks because, when I outside size, if the neck is of a uniform thickness the bullet should be in the middle. That sounds like a good idea to me.

With turned necks, a smaller neck chamber should give slightly superior brass life, but, quite possibly, you may only see this if you are not annealing every time.

[GSells]

Hi I have two 308 barrels one has a .342 neck the other a .340 neck....... a loaded Lapua case measures .338 both barrels have had over 1000 rounds with no problems, the .342 barrel shoots marginally better( very small sample).

My question is what is considered the most effective neck measurement for accuracy and practicality and at what point is it to large and at what point is it to small?
Thanks in advance.

Hi Scott , I’ve tried 1-2 thou before and had wild SD’s and extreme spreads ! Cut the necks to 5-6 thou and had consistent velocities. Had the same problem with the blue meenie barrel chamber cut by Wilddog. It’s a very tight no turn neck and chamber and I had a random spike in the velocities now and then ! I took a couple of thou around the necks of all of the cases and consistent 4.5 to 5 fps SD’s .

I guess the answer to your question is ! Make your neck tolerance as tight as you can without compromising velocities? For me about 5-7 thou .
Regards Sellsy

[superx10]

Thanks for the information guys appreciate it. It Looks like i will be neck turning.