Sizing die comparisons

[deepforks]

so i'm getting ready to start replacing my rcbs seating dies with some redding comp. seating dies. my question is, should i get just the comp. seating dies or the set? are the redding fl sizing dies better than rcbs, or, are the similar in quality? these are for my rifles (300 win mag/270/22-250/243).

[Bart B.]

I don't think there's any difference between either make's full length sizing dies. But their full length bushing dies are much better than the standard ones; Redding's type S full bushing or RCBS Gold Medal full bushing dies. They have interchangeable bushings so one that's the right size for your cases will size the fired case necks down just right without having to use an expander ball like the standard dies use. Case necks are much straighter on the case shoudlers and the cases can be resized more times.

Once fired cases are full length sized in a bushing die, most any seating die will put bullets in them very straight. If the sized case neck's not the right size and not straight, no seating die will straighten it up enough to matter. Therefore, I don't think you need to replace the seating dies. Bullet runout on a loaded round can be up to 1% of the bullet's diameter and excellent accuracy will prevail.

[Unclenick]

Actually, in German Salazar's review, the Redding Comp die did straighten things up a little. Targets are shown of the effect of the seating dies tested on precision. I have to point out, however, that while some gun/bullet/load technique combinations can have up to about an moa of group spread due to bullet tilt, others show almost none. You pretty much have to test to see if your particular gun and load setup is sensitive to it.

I think the best test is one Harold Vaughn devised. He intentionally tilted bullets a little over 0.2° (about 0.0035" tip tilt off-axis) in a 6 mm BR rail gun being fired in a tunnel range (no wind) and got about 3/8 moa spread by firing 8 shots, indexing the direction of the tilt 90° for each shot. The result was a 4 leaf clover, with each leaf being two holes (Rifle Accuracy Facts, H. Vaughn, Precision Shooting Pub, 2000, pp 133-134).

When A. A. Abbatiello did an 829 round test of M72 .30-06 NM ammunition in the 60's (probably an '03 based match rifle—he didn't say), he sorted with a runout gauge and got the bigger 1 moa dispersion (Handloading, NRA Books, 1981, pp 86-87). On the Shooter's Forum in the last week there's been a member posting about a .22-250 giving him 1 moa difference between straight and tilted as well.

Two factoids worth noting: Vaughn, former head aeroballistician for Sandia National Labs, used 6 DOF software with detailed information on his bullet to show the dispersion should have been 0.464 moa theoretically. It was was smaller, he surmised, because of a degree of self-straightening in the bore, calculating only two tenths of a thousandth of an inch of straightening was required to account for the difference. Indeed, he also found that if the bullets were seated closer to the lands than 0.030", the self-straightening increased, giving him a still smaller group. This likely explains why 0.020" off the lands or closer seating are frequently cited in accuracy loading, though that doesn't work all the time. Abbatiello also found a straightening effect wall, in that bullets tipped more than 0.004" off axis provided no additional group dispersion, as if any tilt beyond that degree were straightened.

The second is that both authors found you could reduce a group by orienting the tipping in the same direction for all your rounds. The double-holes in Vaughn's clover leaf are the example. Abbatiello was sorting rather than controlling tilt, but in the mix of degrees of tilt he found that if all were indexed to the same tilt (say, all tips pointing to 12:00 in the chamber) the dispersion due to tilt was halved as compared to allowing random orientation). This likely explains why you can tune self-loaders to half moa, despite the fact the forcefible feed is known to tip bullets. The bullets are tilted in the same direction.

Bottom line, test your load at your seating depth with some version of the way Vaughn did it. Drill a hole in a board or on your work bench about the size of your bullets. Stick loaded test cartridges into the hole nose-down and press the case with your thumb until you tilt the bullets enough to register about 0.008" TIR (Total Indicated Runout; the range of the indicator readings) or a little more on each bullet. (When you rotate a cartridge in the gage tool, the bullet tilt adds to the reading on one side and subtracts from it on the other, so TIR is twice the off-axis tilt; i.e., 0.008" TIR indicates 0.004" off-axis tilt). Use a Sharpie to mark the high side (or the low side, as long as it's always the same) of the tilt on the case head where you can see it looking into the back of your chamber. Collect about 20 of these. Now use the hole in the bench to straighten 20 more rounds to as close to zero runout as you can.

At the range, set up two targets. Target A for the tilted bullets, and Target B for the straight bullets. Fire up to four rounds from each box of ammo as fouling shots. Then fire alternating between targets. 16 will go into target A, indexing the bullet tilt direction 90° between shots. 16 of the straight rounds will go into Target B. Alternating causes shooting conditions variations like bore fouling, barrel temperature, lighting wind and other factors that affect the shooting to even out during the test. At the end, see if the group sizes are different on targets A and B. If A is bigger, you have tilt sensitivity in your gun and load combination. If not, you don't.

[Bart B.]

Unclenick, your info's good. Thanks for posting it. Salazar's article is one I'm familiar with. Most interesting to me is his cartridge spinner. It gauges bullet runout relative to the case body angle as it rests on the gauging rods. As virtually all cases are a bit out of round, that'll show up in his bullet runout numbers. And a rimless bottleneck case body's not what centers the case in the chamber up front where the bullet is. A .30-06 case shoulder is what centers the front of the case in the chamber when the round's fired; exactly like a NO-GO headspace gauge does in a GO chamber when the bolt closes. The body only touches the chamber at its back end where it could be anyplace but typically is somewhere against the chamber wall being pressed there by the extractor force. I think it's better if the front of the case is held by a round hole in something (I've used nylon washers) that represents the chamber shoulder and it's about mid point on the case shoulder. The back end of the case body at the pressure ring could rest between two points such as rollers or a V block.

'Twas either the US Army or Marine Corps Rifle Teams that tested M118 ammo in M14NM's machine rested at 600 yards some years ago. Ammo with more than .003" runout opened up their many-shot test goups. Less than that, there was nothing significant in accuracy. I've not seen any accuracy degradation in my .308's and 30 caliber magnums with runout max at .003". That has about the same bullet angle as .002" runout for 24 caliber bullets.

One thing I've noticed is crooked bullets seem to have less accuracy when seated so they're not into the lands when chambered. When bullets seat into the lands when chambered, that tends to improve accuracy over jumping into them.

[Unclenick]

Bart,

Your last point agrees exactly with what Vaughn found. Get the bullet in the lands and it's pretty much overcomes most of the bullet tilt. Some bullets are still not at their overall most accurate there, but when they are not it will be for other reasons, like Berger's experience with their secant ogive VLD's.

That reminds me that Walt Berger opined that the fact moly-coated bullets often showed a very slightly higher ballistic coefficient than their bare counterparts was due to the coating helping the bullet center as it entered the throat, so that it had minimal muzzle yaw and correspondingly slightly lower drag over the first couple hundred yards.

[rodfac]

Bart...are you saying that you slip a nylon washer over the neck and onto the shoulder of your rounds to be measured? If so, an accurate runout measurement would then depend on the concentricity of the washer, right? I'm testing runout on a Sinclair runout tester with two ball bearing 'roller points' for lack of a better word, with the runout mic riding on the bullet's surface, about half way between the case mouth and the bullet point. Is this your procedure as well as to the measurement point?

My questions are for both of you, Unclenick and Bart...here's my setup...am I doing this correctly? On good .222 Magnum rounds, bullets seated in a Redding Micrometer die, I get .000 to .004" runout as depicted in the pic below. Case runout, with factory new, unfired cases, before sizing or seating, measures .001-.003 at a point midway between the case mouth and the beginning of the shoulder; resizing in a new RCBS FL sizer plus bullet seating add's another .0015" or so to the overall measurement.

I also use a Lyman "M" die to expand case mouths prior to bullet seating. I do not tighten this die down in the press, having better luck allowing it to 'self-center' as the expander mandrel enters the case mouth. A somewhat limited amt of testing with this procedure seems to indicate that groups are improved, tho it's hard to tell if the runout is any better.

I should add that the rifle is capable of 3/4" gps at 100 yds for 7 to 10 continuous shot strings, with some cooling between shots...(depends too on how good my eyes are working on any given day!)

One last question/observation....is runout as we're discussing it, of material benefit in a light wt. bolt hunting rifle (less than 8 lbs with scope and sling) at 250 yds or less? I do know that my AR in 5.56 (Colt Match Tgt HB) does better at 600 with hand selected cases, Sierra 80 grainers, and minimal runout.

Best Regards, Rod

[Bart B.]

Rodfac, yes, a nylon washer. I put one on a bracket that clamped onto my two rods that have two ball bearings (less than .0004" runout on their outside diameter) at the back for the case pressure ring to ride on. That bracket had a 1" OD, .39" ID nylon washer positioned such that the .308 Win. round was fairly straight into it when the case shoulder fit into the hole. Then pressed the back of the case so its shoulder pressed firmly into the washer and turned the round holding its back end down on the roller bearings.

The hole in the nylon washer was uniform enough that a GO headspace gauge in it showed less than .0001" runout on its shoulder's small diameter; good enough for my use. And that nylon washer didn't scratch brass case shoulders like the Neil Jones neck-shoulder bushing die did I used to simulate the chamber's shoulder.

Unclenick, I don't understand why many of Berger's bullets need a jump-start into the lands. All the 30 caliber Sierra's I've shot shoot more accurate when seated into the lands. Even testing Sierra's with runout less than .001" with different distances back from the lands when chambered, the further back they were, the less accurate they shot. Not a whole lot of accuracy improvement when jump was more than about .010" but from zero to .010" it was more noticable.

And minimal muzzle yaw and correspondingly slightly lower drag over the entire bullet's flight increases the average BC the bullet has. Which is why perfectly balanced bullets fired straight into the rifling without being further unbalanced by swaging down to the bore profile will have the least coning and drag, which, with the smallest spread in muzzle velocty means the tiniest groups.

[stubbicatt]

Oh dear. The dreaded runout issue.

Years ago the guy who made NECO runout gages did some experimenting and determined that the brass which forms cartridge cases is not of uniform thickness all the way around the case. This lack of uniformity would cause the cases to perform differently when under the pressure of being fired. They would bend towards the thickest part of the case causing a "banana" shaped case, which would tilt the bullet in that direction as well.

IIRC, he had a device or tool which would assist folks by determining where the thickest part of the case was, so they could index the cases in their rifles causing the bullet tilt to be more consistent. As Unclenick so eruditely explained, if the bullets are all tilted in the same direction, they will group accordingly.

So, while I haven't followed up on this any further, whether the necks are straight or not, whether sized in a bushing die or FL die or not, one will achieve greater accuracy indexing his cases in his chamber towards the thicker side of the case. Perhaps file a notch in the case rim at the thick part, and orient the case towards 12 o'clock or something.

It makes sense to me that if, prior to firing, all the bullets are relatively straight in the unfired, loaded cases, then the inevitable tilt towards the thick side of the cartridge upon firing will also be more uniform. Ferinstance, if bullets have random runout all around the clock face from the thick side of the case, they may exhibit more random groupings on the target than if they were "straight" prior to firing.

YMMV.

[Unclenick]

Rodfac,

For the bullet I would have the indicator out nearer to the bullet tip. That will maximize the sensitivity of the reading. In the same way I think you'll find putting it just behind the case mouth will maximize case neck runout sensitivity.

The case neck runout is what is limiting how close to perfect your final result gets. I find the spherical carbide expanders available for Redding FL dies pull necks off less than conventional expanders shapes do (and use neck lube despite the carbide to have even less pull), but the least runout of the neck can be had by eliminating the expander altogether. To do that use either a bushing die with the right size bushing to bring the neck ID down to the same size the expander would have left it, or use a Lee Collet Die for the neck and separately set the shoulder back in a Redding body die (the body die doesn't touch the neck).


Bart,

Yep, no mystery why improved alignment results in lower drag. You still have the epicyclic motions (coning, nutation) introduced by the bullet tip realigning from coaxial with the bore over to the yaw of repose for flight down range, but once it settles into the new equilibrium, it's clear sailing to the target.

As to why the bullet in the throat trick didn't work consistently with Berger's VLD's, I don't really know any more than I know why it doesn't always work with some other bullets. Dan Hackett had an example in the Precision Shooting Reloading Guide of a 50 grain Nosler bullet in a 220 Swift that would not shoot five rounds into better than 3/8" at 100 yards when seated 0.020" off the lands (his presumed magic number). But when he accidentally turned his seater micrometer adjustment the wrong way and loaded 20 round to 0.050" off the lands, those 20 rounds gave him two 1/4" groups and two bugholes in the 1's. I do know Berger found tangent ogives (most Sierra MK's, though not the newer 125 grain .308 MK) self-center better than secant ogives (their VLD shapes), so Berger now has a "hybrid" ogive that starts out tangent then becomes secant after it's below bore diameter. That's to try to get the best of both worlds.

As to why different amounts of jump with the same bullet are best in different chambers, I don't know the reason for that, either, but my imagination has arrived at one theory for which I need to develop some hypotheses to test. It has to do with gas bypass around the bullet at firing. In the old Dr. Lloyd Brownell study from 1965 at the U. of Michigan, he suggests the reason pressure from a given powder charge drops as you increase bullet jump away from throat contact is that more gas then allowed to bypass the bullet before it obturates the bore, thus slowing pressure rise and lowering start pressure. Of course, if you seat the bullet too deeply and start to subtract too much from the powder space, pressure starts to rise again, despite the addition gas bypass opportunity.

Below is a plot from Brownell's data, with pressure adjusted down 20% based on Harold Vaughn's estimate of calibration error introduced by Brownell's use of the infinite cylinder Lamé Theorem for hoop stress on what was not an infinite cylinder, but the chamber end of an '03 barrel.



What my vivid imagination suggests is that bypassing gas in the right velocity and pressure range can form a gas cushion around the bullet, allowing it to self-center. Too much gas bypass and you have buffeting turbulence and a cocked bullet. Too little and you have no effective gas bearing. You can create both situations by propelling a bullet in a loose fitting tube with compressed air at different velocities, though it's easier to to with the bullet nose facing into the gas flow.

When you have the bullet in contact with the lands and no significant gas bypass occurs, you get good nose alignment but the back end of the bullet can still be tipped. Uneven radial expansion of the brass, as Stubbicat described, could throw the bullet base toward one side before the neck expands far enough forward to center it. The longer the bullet the more leverage any back end tilt has to pull the nose off-axis in the throat. Hence, part of the reason for greater problems in some rifles with VLD shapes. The other part is the greater angle of departure of a secant ogive from the bullet bearing surface won't resist being pulled off axis in the throat as firmly as a jammed tangent ogives does.

The end result, particularly for long VLD shapes, is a need to combine throat contact with tight control of the alignment of the back end of the bullet. One way is a custom chamber for tight-fitting outside-turned necks. I think that mechanical alignment likely accounts for the relative insensitivity of many custom benchrest and varmint guns to neck and bullet tilt and even to banana-shaped cases. For a looser commercial chamber neck, though, you need all the help you can get, whether from a gas cushion, perfect case wall uniformity, etc.

Of course, I may be completely full of beans with the gas cushion idea. I need to get a set of "if it's true, the observed effect will be" hypotheses to test. In the meanwhile, I know quite a number of folks have reported finding what seem to be two sweet spots in bullet seating depths in their guns. One close to or right in the lands, and the other with the bullet further back where my theoretical gas cushion may present itself optimally. The problem with these anecdotal reports is getting variables isolated. Changing seating depth changes pressure which changes velocity and barrel time, and so what people think is a corrected seating depth may actually be better adjusted coordination of barrel time and muzzle position "vibration". It's a great and complicated mess of possibilities, as always.

"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley