Neck sizing - calculating bushing size confusion

[kumaichi]

Hi All,

Just getting into this art of reloading and had a question about selecting the correct size of the bushing for neck sizing.

My understanding is, there are two ways to calculate the bushing you need:

1. Take a loaded round, use a micrometer to measure the O.D. of the neck, subtract .001"-.003" and that's the size you need. Using this method, I have a Lapua 250gr .338 and it's O.D. is .369" which means I should get a .368"-.366" sized bushing.

2. Take an empty piece of brass, measure the wall thickness, multiply times 2 and add the diameter of the bullet. This gives me the following calculation:
.338 + (.014" x 2) = .366 - .001"-.003" which is .365"-.363"

Here is where I'm confused, the two different methods gave me slightly different measurements. And, if I measure the new Lapua brass, it's currently sized to .366". What am I doing wrong?

Any guidance is greatly appreciated.

 

[oldgrayone]

Re: Neck sizing - calculating bushing size confusion

I use method 2. You will need a .364 bushing for your brass, and remember you will get a little "spring back" in your neck after you size. Some brass springs more than others and some manufactures are thinning down their brass so keep a eye on that also. I usually buy the calculated size bushing and also get the next size small just incase.

 

[ChrisGarrett]

Re: Neck sizing - calculating bushing size confusion

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: craig_m</div><div class="ubbcode-body">Hi All,

Just getting into this art of reloading and had a question about selecting the correct size of the bushing for neck sizing.

My understanding is, there are two ways to calculate the bushing you need:

1. Take a loaded round, use a micrometer to measure the O.D. of the neck, subtract .001"-.003" and that's the size you need. Using this method, I have a Lapua 250gr .338 and it's O.D. is .369" which means I should get a .368"-.366" sized bushing.

2. Take an empty piece of brass, measure the wall thickness, multiply times 2 and add the diameter of the bullet. This gives me the following calculation:
.338 + (.014" x 2) = .366 - .001"-.003" which is .365"-.363"

Here is where I'm confused, the two different methods gave me slightly different measurements. And, if I measure the new Lapua brass, it's currently sized to .366". What am I doing wrong?

Any guidance is greatly appreciated. </div></div>

I use the Redding Neck and FL S bushing dies for my Sako 338LM and have both the .365 and .364 bushings. I've only used the .365 bushing.

Like the fellow above states, neck wall thicknesses vary and in the case of 308 brands, can vary substantially.

I usually measure a neck at 3 spots around the circumference and then kind of average out what I'm getting. For 308 cases, I have .332, .333, .334, .335, .336 and .338 bushings (and a .339 and .340 bushing for step down duties) ususally using the .336, .334 and .332 sizes, for about .002" of neck tension, for the various brands of brass that I reload.

As to your methods, you can just measure loaded rounds for each brand of brass and subtract .002", or you can carefully measure the neck walls and double that, backing out the bullet diameter and decide on how much neck tension you want to apply.

Neck OD is controlled by a bushing size, say .336". With new, pliable brass, you'll generally get .001" of springback, making the final OD somewhere around .336-.337". If your neck walls measure at .015", that's a total of .030", subtrast this from a OD of .336" and you're left with .002" of neck tension.

If you don't get springback because your brass is work hardened, you'll have a tighter grip, for maybe .003" of tension. Conversely, if your walls average only .0145" for a total of .029", then you'll less grip on the bullet, as the OD will stay the same, but the necks, being thinner, aren't contacting the bullet quite as much, so you need to understand what's affecting what.

It goes something like this:

Bolt guns for target: .001"-.002" is sought.

Gas guns: .002"-.004" of tension is the target.

Heavy magnum bolt guns: .002"-.004"

Lever guns: .003"-.004"

These numbers are just guides. Things can ultimately vary in the real world, due to the variance in neck wall thickness from brand to brand and even within brands.

I have some Winchester hunting ammo brass (50 pieces) that has unusually thin walls and even a .332" bushing leaves little tension on the bullet, whereas my component Winchester brass seems to work well with the .334" bushing.

I use Norma and Lapua brand 338LM brass and the .365" bushings works well for both. I can't speak to the other brands floating around, out there.

Chris

 

[kumaichi]

Re: Neck sizing - calculating bushing size confusion

I had a feeling it wasn't going to be as easy as just taking some measurements. Thanks for the replies. So I figure I'll get the .365", .364 and .363" bushing.

One last question, how do you know you've got the correct neck tension?

Thanks again.

 

[oldgrayone]

Re: Neck sizing - calculating bushing size confusion

Craig,
You want the tension enough to hold the bullet in place and not allow it to move while in your mag, but you do not want a large amount of force on the bullet. Less is better in this case. There is a good book called Handloading for Competition by Zediker that makes for some good reading. Or have fun and load up a few with differnt bushings and see how they shoot

 

[ChrisGarrett]

Re: Neck sizing - calculating bushing size confusion

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: craig_m</div><div class="ubbcode-body">I had a feeling it wasn't going to be as easy as just taking some measurements. Thanks for the replies. So I figure I'll get the .365", .364 and .363" bushing.

One last question, how do you know you've got the correct neck tension?

Thanks again.
</div></div>

I wouldn't bother with the .363" bushing. Save the money and buy a torch to anneal your case necks down the road. This will pay dividends with your expensive brass investment.

Even though we like to talk about seating exactly .0025" into the lands, or weighing our charges down to .005 grains, or getting exactly .0015" of neck tension, at that level, there's a lot of variation because this stuff is all man-made and there are imperfections in bullets, neck wall thicknesses, scales, etc..

Don't over analyze things.

There are little chores we can do to speed things up, to make multiple shots more consistent and more accurate, don't get me wrong, it's just that you're probably not going to see a lot of discrepencies in your groups, if you have .0015" of neck tension at the 12:00 position, .0020" at the 4:00 position and finally, .0025" at the 8:00 position, on any given loaded round.

As for proper neck tension, there is no definitive amount, but it is something that you measure. This is why I gave you a range, that many of us here, adhere to. You could always load up some rounds, mark and measure each, for COAL, place them in a magazine and fire a round to see if you get any bullt setback, or shifting within the case neck.

I no longer crimp my 223 rounds for ARs because my sizing die puts enough tension on the bullets with the brass I often shoot, to prevent any significant bullet setback even after chambering the same round 3 times. I can measure before and after chambering a round and see if the bullet shits in the mouth and how much. No, or little shift, equals enough tension in that gun.

My 3# HK-91 bolt carrier group, slamming foward and stripping off a round is probably going to jar things more than cycling a bolt action rifle, so it's not an apples to apples thing.

Hell, a lot of benchrest guys like to soft seat their rounds, meaning they'll use the least amout of tension to just touch the bullet walls, barely insert the bullet so it's very long and then insert them into the chamber and close the bolt. The bullet, being seated quite long, will bump up against the lands and self seat itself every time, no guessing, no fussing. I've done this once and I could easily twist the bullets out of the case, with only light pressure from my thumb and index finger, as an example.

One only has to hope that they don't have to 'safe' their weapon before firing off that round because that bullet's not coming back out with the case and exposed powder charge, lol. One big mess.

Chris

 

[kumaichi]

Re: Neck sizing - calculating bushing size confusion

Wow, you guys are great. All the information I've found on the internet makes my head want to explode. It's a lot easier and makes more sense getting the info from the experience you guys have. You're right though, I do get stuck in all the details, I'm a perfectionist by nature so it happens, have to learn to let go some, heh. I don't know why I get so wound up, I'm not a bench rest shooter and as long as the rounds go where I'm aiming, that's all I'm really looking for.

Also, thanks for the book referrence, got it coming from Midway

. I like to read, hope it doesn't cause more confusion.

Thanks again for taking time out of your day to help a newb...