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Bushings Mandrels Spring-back and Bullet fit ("Tension")

Herb Stoner

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Jan 13, 2019
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I am moving some case prep from a Dillon 750 to a zero press and am not "feeling" the mandrel expansion much at all. I didn't notice it on the Dillon as I was simultaneously sizing cases. I've been using a mandrel (Porters Precision Die and gage pins) sized .002 under the bullet diameter to achieve the recommended seating fit or "tension". (I know tension may not be the concise term so please pardon my use of the term here for expediency.) I've also been interested in experimenting with more tension based on some reading.

With SAC dies for 6.5mm I'm using a .288 bushing so with neck wall thickness of .013 I should be getting an inside neck diameter of .262 prior to the mandrel which is what I want for .002 tension but there isn't much, if anything, to expand with the mandrel. I don't feel much, if any, resistance with the .262 pin. Maybe I'm just throwing a hot dog down a hallway? Before I buy a bunch of bushings and pins I don't need I wanted to see if some of you have experience with this.

Should I get a .287 or .286 bushing to allow for springback to ensure the pins actually expands the neck a bit or will I just overwork the brass? Brass is annealed with my AMP prior to sizing.

Also, thoughts on .0025 or .003 tension?

Thanks in Advance!
 
In terms of feel, you are not going to feel a lot of resistance if you are only expanding the brass .002 or .003. When I size mine there is very little resistance and I have all my calibers set to .002 expansion when I send the mandrel through and usually get around .001-.002 neck tension.
 
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In terms of feel, you are not going to feel a lot of resistance if you are only expanding the brass .002 or .003. When I size mine there is very little resistance and I have all my calibers set to .002 expansion when I send the mandrel through and usually get around .001-.002 neck tension.
I don't expect to feel anything like I would with sizing or seating but I thought I should feel something. My concern is there may be some spring-back from the sizing bushing and I'm putting a .262 pins into what may be a .2625 or .263 neck. If that's the case the consistency of my neck tension would relay on the consistency of the spring-back from the bushing rather than the pin for ensuring consistent neck tension.
 
I don't expect to feel anything like I would with sizing or seating but I thought I should feel something. My concern is there may be some spring-back from the sizing bushing and I'm putting a .262 pins into what may be a .2625 or .263 neck. If that's the case the consistency of my neck tension would relay on the consistency of the spring-back from the bushing rather than the pin for ensuring consistent neck tension.

Did you check the inside diameter of your necks after they came out of the bushing/sizing die?

...
 
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Did you check the inside diameter of your necks after they came out of the bushing/sizing die?

...
No - but I will. I don't have a full set of pin gages but I could at least know if the inside diameter is less than .262 (my smallest relevant pin gage currently).

Thanks!
 
If you’re using an AMP every firing, squash away, no need to worry about work hardening, use the smaller bushing.

I hit an AMP prior to sizing every time too, and while I was making great ammo in 6mm using a .268” bushing before a .241” mandrel… I wasn’t sure the mandrel was getting enough engagement.

I’ve since gone to a tighter .266” bushing (in order to get more engagement from the mandrel) and my ammo is even better now (more consistent).
 
If you’re using an AMP every firing, squash away, no need to worry about work hardening, use the smaller bushing.

I hit an AMP prior to sizing every time too, and while I was making great ammo in 6mm using a .268” bushing before a .241” mandrel… I wasn’t sure the mandrel was getting enough engagement.

I’ve since gone to a tighter .266” bushing (in order to get more engagement from the mandrel) and my ammo is even better now (more consistent).
100% my experience as well.
 
Thanks! I’ll get a bushing .002 smaller and A couple of pins to play with a little more neck tension.

Anyone going with more neck tension than .002?

I've been fine sticking with a mandrel that's .002" under, I think that's about right... but I'm not ruling out trying an even tighter bushing at some point...

There's a strange interaction that happens between the bushing and the mandrel. I don't want to say "spring back" because I don't think that really explains it (and not sure how much those of us using an AMP and/or annealing every firing should even think about that), besides, I've never been sure spring-back is reliable enough to count on it working out the same way every time anyway.

I think the interaction between the too-small bushing (squashing it down) and the mandrel (opening it back up) gives the case necks a particular sort of "set".

I was trying to describe it to another shooter the other day, and it's kind of like if someone asked me to make a trumpet, and I don't know shit about making a trumpet or working with brass lol (other than ammo cases), but if I wanted it to hold a certain shape, then I'd squash it down more than I needed to before opening it back up, just makes more sense to me as far as getting the brass to hold a consistent/repeatable shape...
 
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I've been fine sticking with a mandrel that's .002" under, I think that's about right... but I'm not ruling out trying an even tighter bushing at some point...

There's a strange interaction that happens between the bushing and the mandrel. I don't want to say "spring back" because I don't think that really explains it (and not sure how much those of us using an AMP and/or annealing every firing should even think about that), besides, I've never been sure spring-back is reliable enough to count on it working out the same way every time anyway.

I think the interaction between the too-small bushing (squashing it down) and the mandrel (opening it back up) gives the case necks a particular sort of "set".

I was trying to describe it to another shooter the other day, and it's kind of like if someone asked me to make a trumpet, and I don't know shit about making a trumpet or working with brass lol (other than ammo cases), but if I wanted it to hold a certain shape, then I'd squash it down more than I needed to before opening it back up, just makes more sense to me as far as getting thr brass to hold a consistent/repeatable shape...
Yeah - that more or less how I thought it should work.

Thanks!
 
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If you’re using an AMP every firing, squash away, no need to worry about work hardening, use the smaller bushing.

I hit an AMP prior to sizing every time too, and while I was making great ammo in 6mm using a .268” bushing before a .241” mandrel… I wasn’t sure the mandrel was getting enough engagement.

I’ve since gone to a tighter .266” bushing (in order to get more engagement from the mandrel) and my ammo is even better now (more consistent).
Just deciding on what bushings to get and had the same thought that maybe a .268 bushing wouldn't give me much action on the .2410 mandrel I have. So I'll go with a .266 to start. My fired rounds are .277" with Peterson brass, probably be .279" on the Starline I just got since it's a bit thicker. I saw Redding recommended doing it in stages if you are sizing .008" or more from fired to sized. Do you start off with the .266, or use something like a .273 first to do it more gradually?
 
Just deciding on what bushings to get and had the same thought that maybe a .268 bushing wouldn't give me much action on the .2410 mandrel I have. So I'll go with a .266 to start. My fired rounds are .277" with Peterson brass, probably be .279" on the Starline I just got since it's a bit thicker. I saw Redding recommended doing it in stages if you are sizing .008" or more from fired to sized. Do you start off with the .266, or use something like a .273 first to do it more gradually?

For me it's one pass through the sizing die with the .266" bushing, then into the corn cob for a little while to get the lube off, then they see the mandrel.

I wouldn't worry about the difference of a few thou on the brass thickness (especially if you plan to anneal every firing), the mandrel is what really sets the necks.

I really do think I've seen a slight improvement since waiting until after I tumble the lube off before hitting the mandrel. For a long time I used to go: sizing > mandrel > tumble, and while great, I do think it's worth waiting to mandrel after tumbling off the lube. It's the difference between "almost perfect necks that just spent a little while crashing into other cases" vs "perfect necks that haven't crashed into anything".
 
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