Bent decapping rods, crooked sizing buttons and wobbly ammunition- a reloader’s nightmare!
Are bent decapping rods (stems) bad? All of the experienced reloaders I’ve talked to have said they are. They know a bent decapping rod on a die is a recipe for increased run-out (decreased concentricity) on your loaded cartridge. If the decapping rod has a sizing button on it (most reloading dies do), when the bent sizing button makes its way back through the case neck, you are going to have a problem that leaves your neck crooked. Potential issues with sizing buttons are one of the reasons some dies avoid using one and simply use a sized bushing without one to side step this (limiting the work hardening of the brass is another).
Ammunition concentricity is measured using a method called Total Indicator Runout (T.I.R.)- sometimes referred to as Total Indicator Run-out, or Total Indicator Reading- of a cartridge. To measure T.I.R. the cartridge is rolled on a tool, called a concentricity gauge while an indicator is used to measure how many thousandths of an inch the bullet is off center (the distance the needle swings represents the runout). T.I.R. of 0 is perfect and would represent a perfectly straight round. As the T.I.R. value rises, the cartridge is said to be less concentric, or can be described as having more run-out. Most shooters who check for concentricity will cull ammunition with a runout of more than .003-.004″ (I typically cull at .002″ T.I.R. if I have to shoot a match). For more information about what concentricity is and how it is measured, read Bullet Concentricity and Related Issues at Sinclair, by Bob Kohl.
Concentricity is often cited as a factor in rifle accuracy. According to Frank Green of Bartlein Barrels, this is the reason some shooters believe the best accuracy is found with a barrel twist rate just fast enough to stabilize the bullet. A faster twist rate could exacerbate any problems with an crooked, non concentric bullet entering the chamber. Selecting a twist rate that is just fast enough would help mitigate potential problems from concentricity. (To read more about concentricity and accuracy, check out: Ammunition concentricity and accuracy: Testing the effect of run-out on precision)
The other day I was at my loading bench working up some 224 Valkyrie loads for my Palmetto State Armory rifle (above) equipped with a Zeiss Conquest V4, Spuhr ISMS and Aimpoint T1. I had some once fired federal brass and a set of full length RCBS dies. I forgot to flip the shell holder jaws on my Forster Co-Ax press, got a case stuck and ended up bending the decapping stem on the die when I removed it.
From the bottom you’ll notice the decapping stem and sizing button aren’t located in the center of the die. It isn’t supposed to look like that.
When I pulled the stem, you will notice it isn’t straight. It should be. At this point it would be advisable to call the die manufacturer and order a new decapping stem before proceeding.
Not wanting to waste an opportunity, I decided to use the bent decapping stem in my die set to load 40 rounds of ammunition. I would then measure the runout on the newly loaded rounds.
Here is an example of how I measure the run out. In this image (above) I found the low spot on the case with the dial indicator and concentricity gauge. In this case the indicator reads .043″.
In this image you’ll note the indicator reads .034″. To figure out T.I.R. I just need to do a little basic math, .043″-.034″=.009″. So, for this cartridge T.I.R. is .009″, or far beyond the .003 or .044″ T.I.R. most shooters will cull ammunition at.
How bad could the runout be for the entire lot?
I used a Sinclair concentricity gauge to measure the run-out on each of the 40 rounds to the nearest 001″. Average T.I.R. for the 40 cartridges was .010″! The three straightest round had .004″ T.I.R.! You can see the distribution of the cartridges in the bar graph below.
Bent decapping rods are bad. If yours doesn’t look right, call the manufacturer and get a new one. Straight ammo shoots better!
To order a concentricity gauge, check out Brownells!