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?
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.
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.
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.