Tolerance in COL and Crimp

This is the third of a five part series.

In this part we look at Cartridge Overall Length (COL) and crimp to discover what tolerance is recommended and whether or not we can get accurate measurements with the available instruments.

For the COL, or Cartridge Overall Length, and crimp measurements, digital calipers commonly read to 0.5 thousandths of an inch (0.0005″). The COL tolerance (±0.005″) is ten times that resolution so the equipment we have available for this measurement is common and relatively inexpensive.

And because we see three or four significant digits for these two measurements — the “0.469” image here is showing a crimp measurement with three significant digits — we have a very accurate measurement.

In addition to more accurate measurements, notice in the table below that the COL and crimp tolerances are much tighter when that tolerance is translated to a percentage of the example measurement.

Parameter Example Tolerance Allowed Variation
Powder Weight 4.0 gr. ±0.1 gr. 5%
COL 1.200″ ±0.005″ 0.833%
Crimp 0.469″ ±0.002″ 0.853%
Bullet Wt 200 gr. ±0.05 to ±0.5 .05% to 0.5%

These tolerances need to be much tighter to achieve reliable and accurate operation. But some “tolerance” must still be allowed because of variations during the reloading process.

By “variations during the reloading process,” I mean of course that things happen during reloading and that not every round is going to be exactly the same.

One source of variation, as we’ve already noted, is the allowable variation in powder drop. With that parameter, we’re pretty much at the limit of what our tools can measure and while we trust (!) that the reloading equipment’s powder drop is close to the same for each round, I’m sure we all realize that the number of physical particles of Clays or WST or whatever you might be using isn’t going to be exactly the same each time.

But for powder weight, it doesn’t appear that the allowed 5% variation is a problem. We might say that the tolerance on the powder drop is comparatively loose.

For the remaining parameters, however, that’s won’t be the case.

For COL and crimp and bullet weight, we need much tighter control.

And while our measuring instruments give us the ability to measure things more closely — for bullet weight we will have 3-4 significant digits instead of just 2 which will improve our precision 10-100x — we don’t measure every round.

There will always be some round-to-round variations.

One source of variation is the occasional boo-boo that, while caught and removed, may still contribute some error to the remaining rounds on the reloading machine’s head.

Consider, for example, what happens in the machine — I have a Dillon 650 — when there are five rounds in the reloading ring versus only one.

When the ring comes up and the shells enter the various dies, there is a very small amount of “play” that has to be in the ring to permit it to rotate on the down stroke. That “play” has no effect when the ring has five shells because all five are engaged and the force on the ring is balanced around the ring. The ring is pressed downward equally at all five points and it remains perfectly horizontal.

But consider the forces if you take the extreme case of removing four of those shells and have only the one that’s going to set the COL in place. When that round is raised and pressed into the die, the only force being exerted is at that one place in the index ring. The “play” that must be present in the index ring’s central bolt will then allow the ring to tilt down and away from the die ever so slightly. That particular round will, therefore, be a smidgin’ longer.

How much longer? Well, it depends on how much play you have in your index ring — and remember there has to be some for the machine to turn (index) that ring.

So, while we normally have all five shells in place while reloading, if the occasional shell doesn’t line up with the de-priming and resizing die, it will be crushed a little or a lot, depending on when you catch it. Or maybe when you seat a primer you notice it doesn’t feel right and, checking the shell, you discover the primer went in sideways. When something like either of these events happen, we typically remove that shell but then continue with an empty slot in the ring.

Those four rounds need that ±0.005″ tolerance in COL to come out of the process and still be acceptable, to be “within tolerance”.

Another source of variation in length will be due to slight differences in shape from one bullet to the next. (Think “quality” here.)

As you may know, the insert in the seating die in the Dillon 650 can be flipped over for SWC (Semi-Wad Cutter) or ball (round nose) bullets. The depth to which the bullet is seated depends on where that insert touches the bullet so, assuming you are using the correct insert orientation for the bullets you are loading, a specific bullet’s shape and regularity at that (those) point(s) of contact will determine how deep that specific bullet is pressed in to the shell.

Are all bullets exactly the same? Or is there some tolerance in their shape as well as in their weight?

For example, an SWC round with an irregular collar shap might be pushed farther into the shell than one with a smooth collar. Similarly, a round nose bullet with a slightly elongated shape would also be pushed deeper than one that is not elongated.

Tolerance in COL has to allow for such variations while permitting us to make a reliable (and accurate) round.

Again, do the tolerances for COL and crimp really matter that much? Do we really need ammo that’s made that precisely?

While the powder throw primarily affects muzzle velocity, the dimensions we’re talking about here (COL and crimp) primarily affect the reliability of the handgun’s functioning. That is, if the rounds are too far out of tolerance, incoming rounds won’t chamber correctly and you’ll have a malfunction on the line.

If that happens during Rapid Fire, you’ll then be shooting an alibi string. And I hope you don’t have another miss-feed during the alibi string itself — you only get one alibi per match, you know?

The answer is that, yes, you do need ammunition made to those tolerances so it will function correctly.

Case in point is my wad gun.

The chamber of the Kart barrel in that gun is a tad on the tight side. I presume that Kart made it that way on purpose to more closely control the positioning of the cartridge in the chamber — the “jump” distance the bullet has to travel to engage the lands in the rifling is a major contributor to accuracy. If you control that distance to a high degree, you can get highly repeatable performance.

But my wad gun was jamming a lot more often than I thought it should.

So, I tried hand-chambering some commercially manufactured rounds, dropping them into the chamber of the barrel after removing it from the wad gun. And I discovered that while Remington and Federal ball dropped in just fine, Aguila did not.

Checking the outside dimensions of these brands of commercial ammunition, I discovered that while all three were within the specifications required by the SAAMI* spec, the Remington and Federal ammo was just a tad smaller, by about 0.002″ in COL.

My wad gun with its very precise Kart barrel and fitted into the slide and frame much closer than factory 1911s, could tell the difference.

That firearm is a thoroughbred.

It isn’t something you would throw into the sand, slosh about in a creek, set it on the dirt to dry and then expect to repeatedly shoot the X ring at 50 yards.

Most likely, it’s just not going to work after such treatment.

My wad gun’s mechanical tolerances are too tight for that.

Instead, this fine thoroughbred of the Bullseye sport needs to be kept in a clean stable along with other race critters of similar breeding. It demands a diet of health-promoting, precisely-manufactured ammunition and it expects, no, it demands to be treated as a fine precision creation.

So, while the tolerances are tight for wad ammo, they need to be tight because the guns we use are made tighter to shoot tighter.

The tolerance for COL and crimp allow for variations in the reloading equipment and process and, as with the throw, the tighter tolerances listed by the Bullseye shooters really are what we need.

As with a good digital scale and calipers, there are some other doo-dads and gizmos you might add that will claim to reduce variations from one round to the next.

But do they really make things more precise?

For that, you have to look and see how the machine operates and exactly what the new gizmo is supposed to do. And then test it to see if you really get the results.

I have two such additions on my 650, one to secure the head holding the dies and the other on the powder throw. Barring a discussion of backlash considerations on fine-pitch screws, my conclusion is that one of these is worth its weight in gold but the other is no more than a “feel good” addition.

In the next (fourth) installment in this series we look at bullet weight.


(Click here for part four.)


* Note: The ANSI/SAAMI Z299.3-1993 standard specifies the dimensional standards for center fire pistol and revolver ammunition. It is a voluntary standard intended primarily for commercial manufacturers.

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