Measuring Crimp

First of all, you didn't say, but I assume you are talking about Taper Crimp for an auto cartridge. So then, 'No', it's not really that easy.

1. For that you need to understand that the TC is only applied on the last 0.040" of the cartridge. That being the case, you need to use the narrowed portion of the sliding caliper jaws (NOT the rotating jaws of a micrometer), as shown below.



2. Secondly, You need to know that if you are working with any of the 9mm series (9x19 Luger, 9x21, 9x23, etc) that these are tapered cartridge cases. As such, the taper makes the diameter increase the further you get away from the case mouth. So it's VERY important on these calibers to stay away from anything but the portion of the case adjacent to the mouth.

Hope this helps.

It's hard, your measuring a small area that should be tapering inward while trying to maintain the vertical and horizontal plane with the caliper. I measure a few times and take the average of the numbers that look close to what it should be and disregard the way off numbers.

I expect that its hard to do. I can't honestly say that I ever measured a crimp. I just adjust the die to crimp until it looks right by eyeing it. On revolvers I roll it over the crimp groove or over the front driving band and on autos I just take the bell out plus a little more.

After backing out the bullet seating plug, I screw the die out a few turns. Place a seated bullet in the shell holder and raise the ram. Screw the die down until I can feel it contact the case mouth. Lower the ram. Then I screw the crimp die down a little at a time until I get the desired amount of crimp. Lock down the die and measure the gap between shellholder and die. Using rfwobbly’s example. Record for future reference.
If there’s ANY difference in case length, it will make a difference in amount of crimp. For me, I believe the difference is negligible.
I still try to keep my brass segregated as to number of times sized.
I’m certainly not an expert reloader or shooter. Some may have a better idea.
To me, consistency is key in reloading.

I like to do about the same ,I noticed on the new untrimmed starline brass that I skipped trimming for my 41 mag , some i don,t feel the same pressure on my press handle when I roll crimp with my lee crimp die.getting lazy as I most times i always trim all my new brass after belling,making sure they all crimp the same place in the canulure.i can,t tell much difference in accuracy as long as they all have some crimp an all the same.

I "measure" my crimp with the "Plunk Test". Just enough crimp to "deflare" and get a good "plunk". When I first started reloading semi-auto ammo I tried measuring crimp, and occasionally when working on a load or problem I'll measure. It became too frustrating, so I just plunk now...

The method described by rfwobbly is correct, and will get you good measurements

I would drill a hole in a piece of wood that is just thick enough to let the crimped portion of the case JUST poke through the hole. Then you could measure with your dial calipers flat on the piece of wood and be much more consistent on exactly how far up the case you are measuring.

But I am not sure why you are trying to measure the crimp in the first place?

Mike Excellent idea. Using a free hand to measure this to about .375 is VERY difficult as the original poster states.

Tensaw said:

I'm primarily trying to measure the taper crimp on 9mm.

Click to expand...

I wouldn’t bother measuring a 9mm. Its going to change depending on the brass length, which you probably don’t measure anyway. I just put two rounds together. One facing up and one facing down. Hold it up to the light and look at the gap between them. Adjust the crimp a bit at a time. When the gap (light) just goes away, your golden. Me, I don’t want a crimp. I just want to get the bell out. Over crimping a 9mm has its own issues.

Tensaw said:

I have a hard time measuring crimp and getting consistent results. I can measure the same round several times and get slightly different measurements. I'm using a digital caliper. Is it just me?

Click to expand...

Open and LOCK the caliper to the target size (like 0.380 for 9mm)
Place the caliper over the case mouth as if measuring it, if the caliper slips down over the case edge, the edge is smaller than 0.380.
If it doesn't slip down over the case mouth/edge, take the case out of the caliper, unlock it and reset it to .0381 and re-measure the case.
If it DOES slip down over the edge of the case mouth, take the case out of the caliper, unlock it and reset it to 0.379 and LOCK it, put over the case mouth again and see if it still slips down over the edge.
Continuing this routine until the caliper no longer slips down over the case edge/mouth will give you the diameter of the crimp within 0.001".
It's what I do,

Edit: If you don't sort brass by brand, the wall thickness variation must be considered in calculating the optimum crimp "diameter" and that it may change with the wall thickness.
That wall thickness variation can produce a "crimp" that just touches the bullet or cuts thru the plating or coating with heavier walled cases. at the same setting.

Tensaw said:

By the way, I'm starting to load a new to me bullet profile and the contact at the manufacturer said it was critical that the crimp was a specific amount. In the past I always crimped just enough for the round to plunk and fit a case gauge.

Click to expand...

In my humble opinion, this is incorrect thinking.

• Taper Crimp is to erase the belling.
• Done correctly, Taper Crimp allows every cartridge to fully head space on the end of the tapered 9mm chamber.
• A good test for correct Taper Crimp is to see if the finished round will fully fall into, fully head space on, and then fall back out of, the gun's chamber using only the weight of the cartridge.

Knowing all that, then it is easy to see that the final diameter range of the Taper Crimp is one that satisfies the requirements of your barrel. Therefore, the correct Taper Crimp is a function of your barrel. That is to say, that the correct taper crimp is fully dependent upon the finished dimensions of your barrel's chamber.

From that idea we can leap to the insight that there is no need to change your crimp, because all these rounds (which may have many different bullets) will all be fired in the same barrel. It may be many bullet styles, shapes and diameters, but it's still just that one single barrel you need to 'make happy'.

Consequently, there is no such thing as "light" or "heavy" taper crimp. Neither is there ever a need to vary the Taper Crimp. All this because you are quite simply going to fire all the rounds in the same barrel. Thus, there is only the Taper Crimp diameter range that satisfies the requirements of the barrel.

I realize this goes against common, accepted thinking, but it's the only way I can justify the reality of the 3 commonly accepted truths listed above.

I have measured taper crimps mostly for my own information (and I don't think it's right to assume a reloader is doing an unnecessary/foolish task). I have measured crimped case mouths during some trouble shooting, some out of curiosity, and some just because. I don't consider any of these times measuring "wasted". There are processes used by some that are considered a "waste of time" by others. There are some very personal methods being used and I couldn't care less if a reloader paints happy faces on their case heads (but happy faces on the bullets are another story ).. It's their ammo, their guns, their time, their hobby, if it is good for them, it's not up to anonymous screen names on a forum to condemn them for their methods.

Personally, I think all the folks swaging primer pockets are wasting time and energy pursuing an "improvement" that is in actuality a "feel good" move; only an improvement in their mind...

How's that for judgmental?

I think Tilos has the best approach if you just must attach a number to your crimp. I quit wrasslin with trying to measure an edge with an edge some time ago. I just eyeball my crimp vs factory. Roll crimp for revolver is not hard to see.
Everybody talks about "just enough taper crimp to remove the flare" but my sample Black Hills .45 SWC has appreciable taper at the mouth. It is also loaded a bit shorter than the usual recommendation.

Here is the secret to measuring crimp: DON'T.

Inherently difficult (from a repeatability perspective) and totally unnecessary. All you're trying to do is take out the flare.

There are a lot of things in reloading that must not be eyeballed - powder level, COAL, brass length in bottleneck rifle cartridges, etc. Taper crimp is not among them. Case and/or chamber gauge will tell you whether you're crimping enough. It will also tell you if you're grossly over-crimping. If you're trying to avoid serious damage to something like a plated bullet, you may want to make a dummy round and then pull the bullet to see what kind of impression you're leaving on it. Those sources of information are sufficient, and far more trustworthy than most people's caliper skills.

Tensaw said:

taper crimp on 9mm ... I can measure the same round several times and get slightly different measurements. I'm using a digital caliper. Is it just me?

Click to expand...

No.

One could get varying taper crimp measurements from the same finished round due to following:

• Worn calipers
• Inconsistent use of calipers
• Out-of-round bullet / tilted bullet during seating
• Inconsistent case wall thickness
• Inconsistent resized case length/Progressive reloading/etc.

Proper use of calipers and checking for accuracy/wear - Like using check weights for scales, I recommend use of known standards/gages for calipers especially since caliper gears can wear and loose accuracy from use. Since measuring cylindrical objects could have different "feel" depending on the amount of pressure applied to caliper jaws, I prefer to use pin gages to check my calipers with my eyes closed so I can get more consistent feel/readings (as bullets and finished rounds are cylindrical) and improper use of calipers and worn calipers will result in inconsistent taper crimp measurements.

You also want to use the same size pin gage as the items you are measuring since different parts of the caliper gears can wear at different spots. Since I mainly reload 9mm/40S&W/45ACP, I have .355"/.400"/.451" pin gages.

Pin gages can be quite affordable to have for each caliber you reload for. Here's Vermont Gage .355"+ pin gage for $4.61 - https://www.thehighroad.org/index.p...ks-for-digital-calibers.821135/#post-10545265

If you verified with pin gages that calipers are inaccurate, this could be one of contributing factors to inconsistent measurements around case neck.


Out-of-round bullets / tilted bullet during seating - Due to manufacturing process and/or shipping/handling damage, bullets can become out of round which can contribute to oblong finished rounds at case neck. And if you see noticeable bulge on one side of case neck, it could indicate tilted bullet during seating that will also elongate finished rounds and give you inconsistent taper crimp measurements around the case neck.


Inconsistent case wall thickness - Depending on headstamp, case wall thickness can vary quite a bit at case mouth on the same case where we apply and measure taper crimp. In this myth busting thread, case wall thickness was measured at 12/3/6/9 O'Clock positions .100" below case mouth above which we typically measure taper crimp and I found case wall thickness could vary by .002" to .003" - https://www.thehighroad.org/index.p...nd-bullet-setback.830072/page-3#post-10712225

And .200" below case mouth where most of neck tension is created from thicker case wall, case wall thickness also varied up to .002" to .003" (And BTW this why increasing taper crimp at case mouth won't significantly increase neck tension as most of neck tension comes from reduction of case neck further down from case mouth from resizing die) - https://www.thehighroad.org/index.p...nd-bullet-setback.830072/page-3#post-10713822

So adding inconsistent case wall thickness to improper caliper use/worn calipers/out-of-round/tilted bullet can aggravate the inconsistency of taper crimp measurements.

Other factors (Inconsistent resized case length/Progressive reloading/Brass condition) - These are less of an issue but when stacked on top of other factors can contribute to inconsistent taper crimp measurements.

• While most reloaders do not trim auto-loading pistol brass, they often use mixed range brass and resized case length can vary depending on headstamp and number of firing/work hardening and this could result slightly varying amount of taper crimp applied to case mouth.
• If reloading on progressive presses, depending on the shell plate load/tilt/deflection when varying resizing effort allows daylight between the bottom of die and top of shell plate and/or cause shell plate to deflect/tilt on sub carrier, push on the station that is applying the taper crimp could vary.
• It is also my opinion that depending on work hardening/condition of brass, brass spring back could contribute to out-of-round finished case mouth/neck measurements. Measure some resized brass and see how many of them are out of round.

Tensaw said:

the manufacturer said it was critical that the crimp was a specific amount. In the past I always crimped just enough for the round to plunk and fit a case gauge.

Click to expand...

Reloaders have access to measuring tools that can measure to .001". Since most case wall thickness at case mouth averages .011", I usually add .022" to the diameter of the bullet for taper crimp amount.

So for .355" sized bullets, .355" + .022" = .377" is the taper crimp I measure at case mouth which essentially returns flare back on the bullet flat and very slightly more. If the case wall is slightly thicker, it will apply slightly more taper crimp.

And for .3555" - .356" sized bullets, I use .378" taper crimp.

Using too much taper crimp can cut into the copper plating (if using plated bullets) and deform/reduce the bullet diameter which can decrease accuracy.

lightman said:

I just adjust the die to crimp until it looks right

Click to expand...

This is why reloading is an art form.