Excessive charge?

After cartridge fired: Measuring head space using the Hornady gauge can tell you a lot about your load and rifle

After cartridge fired: Measuring head space using the Hornady gauge can tell you a lot about your load and rifle

Byron Pace returns to powder charge development in reloading, looking at the optimal charge weight method

Before going on to look at an alternative to the ladder test, I think it a worthwhile digression to look at head spacing. This is not only an essential aspect for loading cartridges, but also in rifle building. The head spacing of a particular cartridge design is vital from a safety and accuracy standpoint.

There are a number of head-space inserts available allowing you to check different cartridges

There are a number of head-space inserts available allowing you to check different cartridges

The ‘head space’ ensures the head of a case is supported by the bolt face during firing. Too long, and the case is allowed to stretch excessively and potentially beyond its elastic limit. This can lead to stretch marks above the head on the body of the case, and in extreme instances, total head separation. You may also see signs of a flattened primer much like excessive pressure. As the case expands, the walls push out, gripping the inside of the chamber before the case pushes pack against the bolt. If the head space is too large this can mean the primer is pushed out before the case slides back. When the case does contact the bolt it pushes the primer back in, often leaving it squashed.

The head space measurement on a case is taken between the face of the head and a datum line on the shoulder of the case. This can be measured with using the Hornady Lock ‘n’ Load head space gauge tool, which is an extension of the OAL gauge we used for checking our seating depth to ogive. If you find that you have signs of head-spacing issues, taking this measurement is one way of establishing if your case is over-stretched, and hence your spacing is wrong. We shouldn’t see a drastic difference between our fired case and our loaded case.

Pre-fired case: This should be within the SAAMI spec for the cartridge

Pre-fired case: This should be within the SAAMI spec for the cartridge

Here I have taken a factory example in the Hornady .243 Win 75-grain SST. Pre-fired cases show a head space measurement of 1.6265in, and a fired case through my Kimber Montana shows a case increase to 1.629 – a 0.0025in stretch. Reloading for hunting ammo, you would probably set the shoulder back about 0.002in, so for a factory round this is pretty good. We will see later how you can use a neck-sizing bump die to control the shoulder set-back and why you would want to do this.

There are two primary causes for head space issues, and one is expensive to rectify. It could be that the barrel has simply been set incorrectly with the wrong head spacing. If this occurred with a Savage rifle, that is easy to alter, simply by undoing the barrel locking ring and screwing the barrel in to reduce the head space. On a fixed-barrel rifle, there is no other option than a visit to the gunsmith.

If you haven’t previously had issues using factory ammo, then the problem likely lies with your reloading die or method. In particular this can occur when using a bushing die to bump the shoulder back instead of a total resizing. If the shoulder is pushed back too far, the loaded cases will be too short for the head spacing of your rifle. We will be looking at dies much more in the coming months.

Case inserted into a removed barrel: The shoulder to head measurement is what is important

Case inserted into a removed barrel: The shoulder to head measurement is what is important

To recap on where we are with load development, we have narrowed down the seating depth and saw an example of how to hone in on the optimum powder charge using the ladder test. This is however not the only way to achieve optimum barrel harmonics, and nor is it necessarily the best, especially for hunting applications. In truth, it can be difficult to interpret the results, and a lot of hunters don’t necessarily have the skill or equipment to deliver suitable results at the range with which the ladder test is optimally completed. For this reason I would consider the ‘optimal charge weight’ method, credited to Dan Newberry. I will give you an outline of how this method is used, but full details are easily obtainable from his website. For me, this is the most user-friendly method.

A lot of hunters don’t have the skill or equipment to deliver suitable results at the range with which the ladder test is optimally completed

With lengthy discussion, Dan explains why the standard ladder test can prove problematic when it comes to evaluation. In his charge development, the focus is on point of impact, settling on a powder charge that offers consistent POI across powder increments. This test can successfully be undertaken at 100 yards, although for completeness loads should always be tested at ranges beyond this. He does however state that using this method at 100 yards has proved very successful in delivering loads that shot equally well at 300 yards and beyond, many holding sub-MOA. So all is not lost if you don’t have access to a range greater than 100 yards. The shorter distance also helps illuminate wind variation, which as we saw with my ladder test, can make a big difference to bullet placement even on a very still day.

The testing starts with establishing a starting point for a powder charge (assuming we have chosen our powder already – see previous articles). Dan recommends decreasing the max book powder charge by 7-10 per cent, beginning a work-up from there. Each increment is given as a certain percentage above the previous load, which I think makes it more complicated than it needs to be. Considering what I stated in previous articles about increment values depending on case size, pick an increment in grains to increase each load by. For my .243 Win load shooting 87-grain V-Max, I would start at 45.5 grains, working my way up from 46 grains in 0.4-grain increments to just below my 100 per cent fill load. Given this is a load for the field, I don’t want to get into the realms of compressed loads. As we have not tested this cartridge for a max pressure load, it is also important that the first case from each incremental load is inspected – discussed last month.

Top left: The lowest load showed pretty average grouping but similar POI to the next few loads

The lowest load showed pretty average grouping but similar POI to the next few loads

Seating depth should be the same for all these tests, as per our seating depth optimisation. Dan discards the importance of seating depth, but as I have shown, it can make a big difference, and I think it is important to start the powder charge development with a cartridge close to the intended final make-up. Of course there is nothing to stop you following up again with one more seating depth test to tweak the load after the powder charge has been decided.

The next four loads. It is clear to see the two tightest groups share the same POI.

The next four loads. It is clear to see the two tightest groups share the same POI.

Depending how many charge increments you have loaded, set up the appropriate number of targets on the range – all identical and at 100 yards. After shooting a fouling shot, allow the barrel to cool. Now shoot the first shot from the first test charge group at the first target. Allow the barrel to cool again. Repeat this for the next charge, at another target. So on until you have fired one shot on every target, i.e. one shot from each charge group. The only caveat is if you pull a shot or have a flier – that shot should be discounted and replaced with another shot (always load four rounds just in case).

Likely that the one shot away from the two together was shooter error

Likely that the one shot away from the two together was shooter error

Repeat the whole process until you have three shots on each target. This ‘round robin’ shooting was also used in the ladder test and helps eliminate unfair bias owing to barrel fouling or environmental conditions.

Join up each shot, triangulating to provide the centre point of each group. This position relative to the aiming point needs to be recorded. It makes life easier if your targets have a square grid background of known scale. From here it is a simple case of identifying the loads that come closest to hitting the same point of impact – these should be consecutive loads. We are not interested in the tightest group, because this will not necessarily provide the most consistent and stable loading. Further seating depth testing can also help pull in group size if unsatisfactory. However, more often than not the tightest group will fall among the selected powder charges using POI. The optimum load will sit in the middle of the targets chosen.

Tight group within a 1cm square, sharing the same POI as the previous load

Tight group within a 1cm square, sharing the same POI as the previous load

In my .243 Win test we can see that the first three groups have a similar POI, with the second and third group almost identical. My final load will be between the 46-grain and 46.4-grain load.

The reason for going through this process is to provide the shooter with a load that is resilient to variation. There are a lot of reasons why our loads will not perform exactly the same as previous loadings. Temperature changes, brass inconsistency, neck tension, powder charge inaccuracies – the list goes on. What we are trying to achieve here is a load that will be sympathetic to minor changes, allowing the shooter to place a bullet consistently over a long period of time in variable conditions.

One way to check just how well the process has worked is to force variation on a test group. Take your optimum load, measure out a charge 1 per cent heavier and 1 per cent lighter. Loading these two bullets along with the optimum charge, shoot a group at the furthest range which the cartridge needs to perform. You should find that it will still hold a MOA despite the difference in powder charge. Next month we will start with this test before looking at concentricity.

Information on all reloading components available from Edgar Brothers: 01625 613177, edgarbrothers.com. 

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