RCBS Form / Trim dies: Design feature question

[Brians356]

I am waiting for availability of a technical person at RCBS (not as simple a process as one might hope) but meanwhile I thought I'd toss this out here, in case someone who knows trim dies can offer an opinion:

I have several RCBS "trim" dies (actually Form / Trim dies) and there is a peculiar trait common to them all: The mouth end of the neck portion is flared .001" - .002", i.e. the neck sizing portion of the die "opens up" slightly (verified using pin gages.) When I withdraw a case from the die, the end of the neck looks like a trumpet bell, flared open slightly, after the brass springs back. (One might expect pulling the neck back though the narrow portion of the die neck would remove the flare, but it doesn't.)

Note: This is NOT caused by the neck protruding past the end of the die. In all instances the case necks are short enough to remain within the neck of the die, so there is nothing to trim off.

Anyone know why these form / trim dies are designed like this?
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[ireload2]

I think your brass might be springing open due to work hardening. Try annealing your brass and it should stop.

Most RCBS trim dies that I have worked with have a straight neck diameter although I have a few with a little reverse taper as determined with .001 increment gauge pins. The case neck still has to pull back through the smaller part of the die neck so it should not make much difference. I think this reverse taper comes from the hand polishing process after the dies are heat treated. It is easier to reverse the die to polish the neck since there is no top thread on the die to get in the way.

Most RCBS trim dies are about .010 larger in the neck than a FL die so it does not matter much because you still have to size the neck with a FL die to get the neck small enough to retain a bullet.

 

[Brians356]

I think your brass might be springing open due to work hardening. Try annealing your brass and it should stop.

I will try that. The cases I sized were once-fired factory ammo, but it's still worth seeing what annealing will result in.

I think this reverse taper comes from the hand polishing process after the dies are heat treated. It is easier to reverse the die to polish the neck since there is no top thread on the die to get in the way.

Then why don't their FL dies come out this way? In fact all the FL dies I own (RCBS and Redding) taper slightly in the opposite direction - they are tighter at the mouth than near the shoulder. (This makes sense, since most if not all SAAMI chambers' necks are similarly tapered.)

Most RCBS trim dies are about .010 larger in the neck than a FL die so it does not matter much because you still have to size the neck with a FL die to get the neck small enough to retain a bullet.

That's true, and I was aware of that but did not mention it because I was only curious about the odd flared neck portion, not because I was concerned about any practical effect on subsequent loading.

Thanks, good comments!
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[ireload2]

I have noticed it with FL dies but it is a little harder to detect. I have FL dies tapered both ways but it is not much more than .001 to .002 taper. Some are tapered about .0005 if I take time to check with a small hole gauge. I worked a long time in manufacturing and expect to see things that vary a few tenths to a few thousandths if you measure a lot of them. I guess I don't pay much attention to it since so many production dies are .006 to .008 smaller than they need to be. If anything they need more material honed out of the die neck.
I have a large spreadsheet with the dimensions of many of my dies recorded. It is interesting to measure 4 or 5 sets of dies for the same round. I think they are more consistent than the chambers of ordinary production rifles. Measure about 100 FL dies and you will find a full range of small taper variations. For the most part the manufacturers do a pretty good job except for the excessively tight necks. Some dies are so tight they would work with .004 thick necks.
Many SAAMI chamber drawings have tapered necks so that if the worst case chamber results the neck is just straight. That is to avoid a reverse taper in the neck that would lock a case in place. The other SAAMI drawing is for the ammo. There is no SAAMI drawing for a die so you have to look at the case which is the result of the FL die.

Then why don't their FL dies come out this way? In fact all the FL dies I own (RCBS and Redding) taper slightly in the opposite direction - they are tighter at the mouth than near the shoulder. (This makes sense, since most if not all SAAMI chambers' necks are similarly tapered.)
Thanks, good comments!
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[Brians356]

ireload2,

I did anneal the cases (checked my notes) but I ran another case through the 270 Win trim die after annealing it 50% longer. I ran it all the way in, then immediately withdrew it far enough to pull the case mouth back to the minimal diameter portion of the neck, and let it sit there for a few minutes. The neck was still flared at the mouth, but it was a bit less pronounced.

I do not have any FL dies which are reverse tapered like these trim dies. If I had, I wouldn't have been surprised enough to start this thread on the subject.
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[Brians356]

Update:

Re. my 270 Win trim die:

I carefully probed the neck portion from the mouth end, using Class ZZ "minus" pin gages. It tapers from .309+ at the mouth down to ~.3055 at the base of the neck. That's a full .0035+ if my arithmetic serves.

But it is not a linear taper. The pin gages enter the neck end the following distances:

.309: .010"
.308: .025"
.307: .125"
.306: .230"
.305: pass through, loose

So my "trumpet bell" analogy is apt.
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[Shaggy357]

The die is designed to swage the neck burrs back after trimming. Any burrs left on the outside of the case should swage back down to proper diameter. Old school trim was hacksaw and final touch up with a file.

 

[ireload2]

If you look at enough Handloader Magazines you will eventually see a few photos of RCBS's polishing bench back in the 1970s.
The worker sits in a chair so his shoulder is close to the level of the polishing "lathe". The lathe is just an electric motor with a chuck to spin the dies.
They used some sort of dowel and abrasives to free hand polish the dies. It would have been easy to put some taper in the neck if your shoulder is much higher or lower than the die while polishing. .0035 is a lot of taper It is possible a dull drill or reamer contributed to that.
I bought a 25-06 Rem 700 about a year after they standardized that round. Unlike my other 700s it did not want to shoot. I tried 4 different bullets from 90 to 120 grains and they all shot about 1 1/2 groups. Not very good. One day I rolled a round across a table with a smooth flat top and the 120 Hornady wobbled about .030. Having spent a long time on a lathe I knew that I could see .003 run out and what I was seeing was terrible. I took the dies to work and chucked them in a new LeBlond Regal and engaged the clutch. The sizer looked ok but the seater looked like it was bent about .030 and the seater plug had a lot of run out.
I called RCBS and they said send samples of the brass and bullets back with the die set. About a week later I got a new set of dies and my groups immediately went down to between 5/8" and 3/4". So bad dies do happen. RCBS replaced that set and I learned to look out for the unexpected.
There is no excuse for the sloppy assembly of your ammo.

Update:

Re. my 270 Win trim die:

I carefully probed the neck portion from the mouth end, using Class ZZ "minus" pin gages. It tapers from .309+ at the mouth down to ~.3055 at the base of the neck. That's a full .0035+ if my arithmetic serves.

But it is not a linear taper. The pin gages enter the neck end the following distances:

.309: .010"
.308: .025"
.307: .125"
.306: .230"
.305: pass through, loose

So my "trumpet bell" analogy is apt.
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[Brians356]

The die is designed to swage the neck burrs back after trimming. Any burrs left on the outside of the case should swage back down to proper diameter. Old school trim was hacksaw and final touch up with a file.

Fair enough. But it doesn't take a trumpet-shaped bore to swage off any neck burrs. If it was a pure cylinder it would still do that - and probably even more effectively. As it is, it leaves the case neck flared at the end. I'm struggling to grasp how that is advantageous. Sure, it still needs running through a FL die before loading, but having the case neck flared that dramatically can't lend itself to proper forming in the FL die (brass spring-back being what it is).
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[ireload2]

If you are interested you might consider lapping your die unless you want to return it to RCBS.

Here is a site where the guy does a pretty good job of explaining the die lapping process with a common barrel lap.

https://rickaverill.com/projects-past-and-present/lapping-reloading-dies/

 

[Brians356]

If you are interested you might consider lapping your die unless you want to return it to RCBS.

Here is a site where the guy does a pretty good job of explaining the die lapping process with a common barrel lap.

https://rickaverill.com/projects-past-and-present/lapping-reloading-dies/

Thanks, yes, I have read that some years ago.

Of interest to me are these comments (bold emphasis mine):

The neck in most cases will have a taper; it is difficult to make both ends of the neck area the same. This is not a major problem, just keep the base slightly smaller…….

When I lap the neck I make sure that the inside or the base of the neck is smaller than the open end, this insures that it is uniform for its entire length when sized.

I believe it is a fallacy to assume the case neck will emerge with a cylindrical OD just because the base of the die's neck has a smaller ID than the rest of the neck. The well-annealed cases I have formed prove that a flared neck can emerge from a flared die neck. Apparently the neck takes a "set" inside the flared neck which is not ironed out entirely when pulled back out through the smaller ID. Brass spring-back, once again, trumps the best-laid plans.

And for case forming, I don't believe it is advantageous to have a reverse-tapered neck (not unlike the trim dies under discussion here). In fact I feel a form die should taper smaller towards the mouth, just like all my FL dies do, since SAAMI chambers are so tapered, and unturned case neck walls taper as well (thinner towards the case mouth.) And I believe a "normal" taper stresses the neck material less that pulling the neck out of a reverse taper. This is somewhat analogous to why expanding a neck with a conventional pull-through die button is inferior to using a proper tapered mandrel.
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[ireload2]

SAAMI chamber drawing are only tapered the amount of the tolerance.
This insures that if the mouth is maximum and the base of the neck is minimum you will at worst case have a straight neck.
So it is more of insurance against the possibility of hard or no extraction due to a reverse taper in the chamber neck.
If you want to draw more solid conclusions use your brass with a different die and use different brass in the existing die.
You should be able to isolate the source of the problem.

Wilson bushings are tapered about .0005 and always have been. I have never heard anyone complain about the trumpet shape using those bushings.
When installed to give the smaller sizing the bushing taper is reversed in the die.
I don't think the small taper amounts to much either way compared to the expansion of the brass when fired and the force applied during the sizing cycle.
I think I have seen the trumpet shape when forming 7.65 Mauser brass from SL-54 .30-06 brass. I still have some of that brass so I can check it out. Since that Mauser is an all original iron sight rifle I did not pay much attention to the trumpet at the time.

Thanks, yes, I have read that some years ago.

Of interest to me are these comments (bold emphasis mine):

The neck in most cases will have a taper; it is difficult to make both ends of the neck area the same. This is not a major problem, just keep the base slightly smaller…….

When I lap the neck I make sure that the inside or the base of the neck is smaller than the open end, this insures that it is uniform for its entire length when sized.

I believe it is a fallacy to assume the case neck will emerge with a cylindrical OD just because the base of the die's neck has a smaller ID than the rest of the neck. The well-annealed cases I have formed prove that a flared neck can emerge from a flared die neck. Apparently the neck takes a "set" inside the flared neck which is not ironed out entirely when pulled back out through the smaller ID. Brass spring-back, once again, trumps the best-laid plans.

And for case forming, I don't believe it is advantageous to have a reverse-tapered neck (not unlike the trim dies under discussion here). In fact I feel a form die should taper smaller towards the mouth, just like all my FL dies do, since SAAMI chambers are so tapered, and unturned case neck walls taper as well (thinner towards the case mouth.) And I believe a "normal" taper stresses the neck material less that pulling the neck out of a reverse taper. This is somewhat analogous to why expanding a neck with a conventional pull-through die button is inferior to using a proper tapered mandrel.
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[Brians356]

SAAMI chamber drawing are only tapered the amount of the tolerance.

Not strictly true. There are some SAAMI chambers with considerable neck taper called out. A good example is 250 Savage - .2895 at the base of the neck, .286 at the end of the neck! But some do indicate a cylindrical neck, so your tolerance range would apply there. Many call out a .001" taper (e.g. 257 Roberts, 6.5 Creedmore.)

Edit: The 270 Win (which die I have been discussing in detail) has a .002" neck taper called out by the SAAMI Minimum Chamber.

Wilson bushings are tapered about .0005 and always have been. I have never heard anyone complain about the trumpet shape using those bushings.

Who would even notice a .0005 flare in a neck? Few could even measure it. What I'm seeing is impossible not to notice at a glance. One of those "What the @&%$?!" moments. I'll post some pictures.

When installed to give the smaller sizing the bushing taper is reversed in the die. I don't think the small taper amounts to much either way compared to the expansion of the brass when fired and the force applied during the sizing cycle.

In the case of only a .0005 taper, that seems reasonable. I would find deliberately tapered bushings useful. I've noticed random Redding bushings are tapered, but alas I fear not by design.
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[ireload2]

I think you are decieved by the .250 Savage chamber drawing. The .2895 dim is theoretical at the intersection of the shoulder and the neck.
However that intersection has a fillet radius of .100 +.030. So physically the chamber is not going to measure .2895. If you want to draw it on a CAD program you can measure it and find out exactly what it would be. However it has +.002 tolerance on the .286 which allows it to go as large as .288. The .2895 does not really exist because that material has been removed in the process or creating the .100 radius. As that radius sweeps up the neck the neck OD may never exceed .288. You would have to draw and measure or do the trig to find the diameter at the tangent of the neck and the fillet radius.

Who would find .0005 taper? Ask around I bet some of these folks have had this same discussion. After all, they do need another forum just to discuss neck donuts. There are people obsessing over a .001 donut when the chamber may have .005 clearance over the neck. Even so the neck expander in a reloading die is intended to pull the neck ID of the case out to a uniform diameter. But I would bet you a wooden nickel that the junction of the shoulder and neck are always small by a few ten-thousandths.

I am remembering more about the 7.65 Mauser cases that had the trumpet shape. I cut the excess material off with a tubing cutter and you always get a deep groove rolled into the case neck before it separates. The case mouths had a lot of extra metal turned in like the crimp used on a .30-06 blank.

Not strictly true. There are some SAAMI chambers with considerable neck taper called out. A good example is 250 Savage - .2895 at the base of the neck, .286 at the end of the neck! But some do indicate a cylindrical neck, so your tolerance range would apply there. Many call out a .001" taper (e.g. 257 Roberts, 6.5 Creedmore.)

Edit: The 270 Win (which die I have been discussing in detail) has a .002" neck taper called out by the SAAMI Minimum Chamber.



Who would even notice a .0005 flare in a neck? Few could even measure it. What I'm seeing is impossible not to notice at a glance. One of those "What the @&%$?!" moments. I'll post some pictures.



In the case of only a .0005 taper, that seems reasonable. I would find deliberately tapered bushings useful. I've noticed random Redding bushings are tapered, but alas I fear not by design.
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[Brians356]

I think you are decieved by the .250 Savage chamber drawing. The .2895 dim is theoretical at the intersection of the shoulder and the neck.
However that intersection has a fillet radius of .100 +.030. So physically the chamber is not going to measure .2895.

The .2895 is indeed to the imaginary intersection of lines. However the radius there will make the actual diameter there larger than .2895, not smaller.

Suppose you measured just enough closer to the end of the neck to clear the radius, and extrapolated linearly. It's still going to be on the order of .2890, and the taper on the order of .003" across a fairly short neck, if the chamber minima are heeded. With the +.002 tolerance, the least taper allowable is still .001" (base of neck +0, end of neck +.002) and this may be how typical chambers are cut.

I have yet to measure any chamber which does not have a "normally" (smaller at the mouth) tapered neck. Nor have I measured any FL die which does not. All was right in my world until I encountered these trim dies.

FWIW my RCBS 6mm Rem trim die has these neck IDs, measured from the mouth end with pin gages:

.277: .010
.276: .065
.275: .235
.274: pass through, loose

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[ireload2]

The .2895 is indeed to the imaginary intersection of lines. However the radius there will make the actual diameter there larger than .2895, not smaller.
I suspect the radius is to accommodate variations in ammo.

Suppose you measured just enough closer to the end of the neck to clear the radius, and extrapolated linearly. It's still going to be on the order of .2890, and the taper on the order of .003" across a fairly short neck, if the chamber minima are heeded. With the +.002 tolerance, the least taper allowable is still .001" (base of neck +0, end of neck +.002) and this may be how typical chambers are cut.
Normal production tooling usually cuts a slight taper even if the reamer is straight.

I have yet to measure any chamber which does not have a "normally" (smaller at the mouth) tapered neck. Nor have I measured any FL die which does not. All was right in my world until I encountered these trim dies.

I have a lot of dies that are straight to within in .001
I also have a 1965 date code set of RCBS 40-65 dies. They were made for originals with .406 groove diameters back then and not the .408s we have now.
The start of the neck is .430 and the area of the case mouth is .418.
By contrast modern RCBS Cowboy dies and modern Lyman dies are perfectly straight and measure .421.
Recent Redding dies are straight and measure .416 (ugh).


FWIW my RCBS 6mm Rem trim die has these neck IDs, measured from the mouth end with pin gages:

.277: .010
.276: .065
.275: .235
.274: pass through, loose

My RCBS 6mm Trim die is pretty much .277. A .278 will almost start (maybe .010) at each end. the .277 will go through with some drag from either end but it does feel a little larger in the middle - maybe .0005 bigger.

I remember being in a meeting where people were debating the chamfer on a pyrolytic carbon heart valves. Should it be .001 or .0005 was the question.

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[ireload2]

I just got a used RCBS trim die in the mail for the 40-60 Win. I already had one but the second one was really cheap and I thought I might be able to lap it out if it was a little smaller than the first one. The first one checks at .433 in the neck. That die appears to have been made in 2002. The second one today was made in 1974 and measures .422. Totally amazing.

 

[Brians356]

Aren't you aware they gradually shrink over time?

PS has it a .422 cylindrical neck, or a taper?
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[ireload2]

The neck of the second 40-60 trim die is cylindrical mostly, with a little bit of reverse funnel - big at the mouth about .001. That is surprising because in 1974 there were no replica 1876 Winchesters in 40-60 around.
My 1965 40-65 Dies as mentioned above have taper at the base of the neck from .430 to .418 at the case mouth. However if you subtract the case wall thickness of .010X2 = .020 from the .418 diameter you get a .398 ID at the mouth. If you expand it with a .404 or .405 expander for a ,406 bullet you will find that a lot of the taper disappears. If you use a modern barrel at .408 and expand to .409 for a .410 cast bullet even more of the taper is gone.

I am currently working on an accuracy project for a Japanese Type I rifle. This is a Carcano with a Mauser type magazine made for the Japanese in the 6.5X50 Japanese caliber. Chambers are about .454 at the head end. Both Norma and PPU brass measure about .446 at the head. This pemits the base of the case to offset and produce a huge bulge when fired. While the bore is in mint condition groups only run about 2" to 3" at 100 meters. And this rifle has a 31" barrel so it has a decent sight radius.
The project is to convert .303 Brtish brass into 6.5 Jap brass. My first case formed in a Pacific trim die had a huge funnel burr at the case mouth. Re running it the die did not get rid of it so it was just an artifact of the case mouth and brass temper. After trimming I had a case that was close to a line to line fit at the head of the chamber. The shoulder of this rifle produces fired cases with a .427 shoulder diameter. The Lee FL die produces a .414 shoulder and the trim die produced a .412 shoulder. I may have to cut off a 6.5 Carcano FL die to get a FL sizer that does not create so much working of the brass.

Anyway I think the funnel at the end of some cases ran through a die shorter than the case is often a normal occurrence. I have seen it before on all those 7.65 Argentine Mauser cases that I have made from US military 30-06 brass formed in both RCBS and CH trim dies.
I don't know what is happening with your .270.

Aren't you aware they gradually shrink over time?

PS has it a .422 cylindrical neck, or a taper?
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[Brians356]

My first case formed in a Pacific trim die had a huge funnel burr at the case mouth. Re running it the die did not get rid of it so it was just an artifact of the case mouth and brass temper.
...
Anyway I think the funnel at the end of some cases ran through a die shorter than the case is often a normal occurrence. I have seen it before on all those 7.65 Argentine Mauser cases that I have made from US military 30-06 brass formed in both RCBS and CH trim dies.
I don't know what is happening with your .270.

I have another RCBS trim die, the 6mm Rem described earlier, with a similar trumpet-shaped neck flare as the 270. I ran an annealed R-P case through it, and the case did not emerge with the flare near the case mouth. Earlier I described having annealed a 270 case (actually 30-06, sorry) 50% longer than normal, and partially withdrawing it from the die to allow the neck to "set" in the narrow portion of the die neck, yet still had a flare at the mouth.

After re-reading your comments, it occurred to me the 30-06 cases I have been testing in the 270 form/trim die have all been from old Frontier brand fired factory ammo. The brass is pristine, not corroded at all, but it has a funny look to it - yellowish color, and a different sheen. I am now convinced this phenomenon is attributable to the composition of the brass alloy. My next step is obvious - run some non-Frontier 30-06 cases through the 270 trim die.

I appreciate your patience and persistence.
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