Why are die bushings determined by your 'lot ' of Brass ??

[TheSilverFox]

My gun project is finally in its final stage.

I was talking to my smith today "Clay Spencer" and he was a wealth of knowledge for me.

Looking back to what he was saying regarding die bushings i got kinda confused.

Here it is, and i quote.

" for your 6mmbr you want a full legth die with 3 bushings that will be suited for most lapua brass. Wich is probably .267, 268 and 269, because your 6br will be a no turn neck chamber. Your bushing size will be determined by your lot of brass."

I really dont know what that means, Is it because brass is thicker or thinner sometimes?

I would appreciate a lesson on this subject.

 

[rstreich]

Like anything else, neck wall thickness can vary a bit from lot to lot. You'll want/need two bushings anyway,'cause unless you're religious about annealing your case necks, they're going to get work-hardened over time and require you to use a smaller bushing,you get more springback as the neck gets harder).

 

[TheSilverFox]

So, i'm lead to believe that as the brass gets older,it needs smaller bushings to acheive the same outside diameter as the 6mmbr no turn neck diameter, in this application.

While it should also not affect the inside diameter of the brass.

 

[rstreich]

Yep. Usually with Lapua brass, I'll see little or no springback when I size the neck on new brass. After a few firings, however, if I want to get it sized to .269 for example, I'll have to switch to the .268 bushing in order to get it to the .269 outside diameter. If it wants to spring back more than .001, it's probably time to anneal the necks.

As to the inside diameter, case necks can thicken over time, so you should always monitor what the finished diameter of your case neck is after you've seated a bullet.

 

[Forum Boss]

The other thing to keep in mind is that different bullet/powder combinations may work better with more or less tension.

We've seen differences in bullet diameters of up to .0008,eight ten thousandths) among major-maker bullets. Obviously if the cases are sized the same to start out with, you'll end up with a lot more neck tension if you use the fat bullets compared to the skinny bullets.

I experienced this myself recently. I thought my neck tension was maybe not enough for Scenar 105s,skinny), but then I loaded some 75 V-max,fat), and there was more than enough "grip". With each bullet combo, you may want to experiment with a couple different bushings.

Personally, when possible, I prefer a FL sizing die with the neck honed to give about .0015" tension. I've found this to give very good ammo with very straight necks,of course that requires a good machining job on the die). The downside of my system is that I have to use a separate neck-die if I want to impart more tension. Despite the lack of flexibility, if you get the neck diameter right, the no-bushing FL die can work remarkably well.

 

[TheSilverFox]

What Clay may have been saying about your no-turn neck is that it will like a certain amount of tension to shoot its best.

I assume your're talking about ID neck to bullet tension?
,Rookie question)

Another thing.
Clay did mention that to get a proper start to bullet seating i should get a small resistance when i closed the bolt with the firing mechanism removed. At first i thought he was talking about neck to chamber tension, but i believe i was rong.

Upon reading everybody's posts i can see that none of you consider neck to chamber tension. For example; a no turn neck chamber is .272 and a good bullet tension might leave the neck at .270 but thats ok?

 

[TheSilverFox]

Personally, when possible, I prefer a FL sizing die with the neck honed to give about .0015" tension. I've found this to give very good ammo with very straight necks,of course that requires a good machining job on the die). The downside of my system is that I have to use a separate neck-die if I want to impart more tension. Despite the lack of flexibility, if you get the neck diameter right, the no-bushing FL die can work remarkably well.

First - for the .0015" tension, you measure the O.D. of the neck before the bullet is seated and after the bullet is seated?

Second - I am geting redding full length type s dies with bushings so if i do notice too little tension, can i resize it with that full lenght die and use a smaller bushing?

 

[Forum Boss]

Personally, when possible, I prefer a FL sizing die with the neck honed to give about .0015" tension.

First - for the .0015" tension, you measure the O.D. of the neck before the bullet is seated and after the bullet is seated?

ANSWER: Correct, first measure OD after case is sized, then compare with OD of loaded round. Note, if you're shooting moly, the .0015 probably won't be enough. You may need .003"-.004" tension.

Second - I am geting redding full length type s dies with bushings so if i do notice too little tension, can i resize it with that full lenght die and use a smaller bushing?

ANSWER: Absolutely. That's the advantage of a bushing die. You can also play with the amount of the neck that you size. With a non-bushing FL Die, it sizes the whole neck every time,unless the die was deliberately made with some taper).

Upon reading everybody's posts i can see that none of you consider neck to chamber tension. For example; a no turn neck chamber is .272 and a good bullet tension might leave the neck at .270 but thats ok?

Short-range benchresters tend to like pretty tight clearances--maybe less than one thousandth over a loaded round. Some guys have experimented with a "fitted" chamber neck. This is so tight that you don't need to size the neck after firing. The brass expands maybe half a thousandth, releases the bullet, and then the normal springback gives you enough tension. For most of us, something between .002 and .004 clearance,in the chamber) over a loaded round is good. On a 6BR shooting Lapua brass, a .272 neck will give you slightly more than .003 clearance, or about .0015 per side. But this dimension isn't critical. Some guys are running .274s or .275s with excellent results. I know many of the Palma guys want at least .004 clearance,.002 per side).

 

[TheSilverFox]

Hey Moderator.

Could you please read my previous post replying to LYNN

And make shure my commenst are accurate.

That would be great.