After hours of case inspection and the inevitable sore fingers of neck turning, a chance to go to the range is a welcome respite. But the shooting you do there, at least as far as your accuracy project goes, won’t be for groups yet. The goal will be to make cases that fit your rifle’s chamber perfectly through the process of fireforming.
Before loading the first cartridge, it is necessary to find your chamber’s actual maximum overall length and make a dummy round that matches this dimension. Like everything else in the accuracy world, there are nifty tools available that help with the task. The standard for this type of tool was the Stoney Point Chamber-All, which is now marketed by Hornady. This tool uses a modified case to measure the point where the lands of the rifling would engage a bullet. It is a great tool and well worth the money. https://www.dillonprecision.com/hornady-formerly-stoney-point-bullet-comparator_8_8_24083.html For shooters just getting into the accuracy game, there is an expedient that works quite well that doesn’t require a special investment.
Measuring Maximum Overall Length
To measure maximum chamber length without a specialized tool requires a cleaning rod, a blunted jag, a dowel (or another short rod), calipers and tape. The only really unusual part here is the blunted jag. It can be made from any jag that will fit down the bore, cut and sanded smooth so that it will lay up accurately against the bullet tip.
Place the empty rifle in a vice with the bolt closed. Carefully insert the rod into the muzzle until it makes contact with the bolt face. Using a piece of tape, mark the point where the rod meets the end of the barrel. This mark represents the base of the cartridge. Once the tape is in place, remove the rod and pull the bolt from the action.
Angle the rifle downwards and drop a bullet of the type you plan to shoot into the chamber and re-secure it in the vise. Using a dowel, gently press the bullet into the chamber until it makes contact with the rifling. While holding pressure on the base of the bullet, use your third hand (this is really a two person job) to gently run the rod back down the bore until it contacts the bullet’s tip. Using tape, mark the point where rod emerges from the muzzle. Measuring between these two pieces of tape will provide the maximum case length for your chamber. There are two important caveats here. This measurement only works that specific bullet and only in the chamber being measured. If you change the bullet or rifle all bets are off.
Creating a Dummy Round
This dummy round is going to provide some very important measurements for your accuracy testing. A mistake here is going to be amplified down the line, so make it as precisely as possible. If you have a case that you turned the neck on and then eliminated for some reason, this is a great place to put it to use.
Use the same bullet that provided your maximum overall length for the dummy round. Work in small increments and measure after each adjustment. If you overshoot the mark us a kinetic bullet puller to move the bullet forward and try again. Scuffing the bullet doesn’t matter as long as the ogive remains undamaged.
Using a Comparator
All bullets, including match bullets, vary slightly in length, even within the same lot. We reused the bullet because its length was used to set the chamber depth, but what we are really looking for here is an accurate measurement from the ogive to the rifling. This measurement will be provided by one of those tools you do have to buy. You will need a comparator. http://www.midwayusa.com/product/231904/hornady-lock-n-load-bullet-comparator-basic-set-with-6-inserts
Comparators use a caliber-specific gauge held in calipers to measure from the base of the cartridge to the ogive. Comparator measurements ignore the Cartridge Overall Length, instead providing a true measurement of the first portion of the bullet that will engage the rifling. Knowing where the rifling begins and the distance from your load’s ogive will allow adjusting bullet engagement during accuracy testing. It is also going to allow our fireforming loads to just touch the rifling, a measurement that could not be replicated by simply measuring cartridge overall length.
Using the comparator, measure the length of your dummy cartridge. I tend to write both the measured overall all length and the comparator length on the dummy itself with a Sharpie, and include the information in my reloading notes. You will be referring to these dimensions quite a lot as you develop your accuracy load.
Time to Load
If everything was measured correctly, using the dummy round to set the seater die should make ammunition that lightly engages the rifling. Doing so will center the case within the chamber and allow it to expand concentrically upon firing. This is the main goal of fireforming for accuracy, making precisely fitted cases. Don’t worry about group size yet. You are still just prepping cases. It is just louder and more fun.
Fireforming with the powder intended for testing will give the handloader a baseline to monitor for obvious pressure signs with later loads. With bullets up in the rifling, pressures are usually higher than those anticipated by published data, and should be reduced. As a guideline, I use the lowest published load, with the idea that all data will work up from that point if the pressures look acceptable. These loads only need to produce enough pressure to fully expand the case and neck within the chamber. More pressure than that is a waste of powder and a limiter of case life.
Inspect fired case for cracks or excessive smoking.
Shooting Your New Cases
Check each case after it is fired for neck or shoulder cracks. Check the neck itself for excessive smoking, which indicates inadequate pressure failed to seal the chamber. If you are getting groups that look great, good on you, but don’t give up if they are disappointing. Accuracy will come later with neck sized brass and ladder testing for the best accuracy combinations.