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6mm Creedmoor Load Development – Part Two – Seating Depth

With the charge weight sorted, it was time to have a play with seating depth to see if we could finesse the load a little. The main consideration here was the smallest group size and seeing if I could reduce the SD any further. Ultimately though, remember, it’s consistency over absolute smallest value.

Jump jump…

Seating depth is basically the distance you load the projectile/bullet into the case. This is also relative to the distance from the rifling lands – how far the projectile needs to ‘jump’ from it’s starting point to where it first engages the begging of the rifle barrel and engraves – meaning starts to enter into the barrels raised groves (which, incidentally are slightly smaller than the projectile that passes through it). This then imparts the spin onto the projectile which in turn helds the bullet stay stable over it’s flight.

People have always had their theories on what is the best option – previous common practice being to get the jump as small as possible – some competition shooters even starting with the bullet slightly pushing into the lands – this, in theory, providing the best accuracy out of the system by removing the jump totally.

In practice, for practical shooting though – this provides two issues – firstly – to seat a bullet out this far, often means you can still load the rounds into the magazine – and magazines are kinda important when it comes to ten round strings of fire in under a minute. I can single load fast, but not while moving and transitioning targets!

Secondly, by jamming the lands, if (which happens quite regularly in a field shoot) you need to unload a round without shooting it – there is the distinct possibility that you will lead the bullet in the barrel and end up pouring gunpowder all through your action. A good way to force yourself to have to stop shooting and clean out your gun.

Changing times…

In addition, recent trends with handloading have started to jump the bullet more – this is a combination (I think) of shooters wanting to be able to do so, and bullet manufacturers designing bullets where you can.

Having a longer seating depth has the side effect (sometimes) of also reducing the need to ‘chase the lands’ – meaning – the need to constantly adjust your load and seating length as the throat of your new barrel slowly erodes – particularly when shooting hot, overly hot-rodded rounds through it.

So, if I can do something that lets me (mostly) set and forget my load development and just focus on shooting, I am all for it. Again – consistency is more important to me than eeking the last bit of precision out of a load. I am already at that point where a lot of stuff just gets lost in the noise of my shooting capabilities and soaked up by target size. More practice, not more time in the reloading room is what I need.

The starting Point

So. I needed somewhere to start. I figured the factory ammo I had already been shooting would be a good point.

I simply took one of the Hornady 108 ELD-M rounds I kept aside and put it up into the Redding Instant Indicator. This gives me a ‘baseline’ that I can then set the seating for my Berger 109 rounds off.

This measurement is from/to the Ogive of the projectile – the part that first engages the lands. It’s important to understand here, that I am not measuring overall length, as that can change a little from round to round. More important (by my thinking) to me is the place where the bullet first interacts with the barrel – so that’s where I take the measurement from. So – I am measuring CBTO (cartridge base to ogive) instead of COAL (cartridge overall length) – the instant indicator (and Hornady’s Lock-N-Load Bullet comparator) let me do this.

Once I have the ‘known’ recorded – the micrometer adjustment on the Redding Competition Bullet Seater Die allow me to initially seat it ‘long’, measure with the instant indicator, and then dial in (by the thou) to whatever relative length I want. In this case – I started with the same (which turned out to be 2.225 inches) and then put together a set of test loads that progressively lengthed out two thousands (0.02) for each step. In hindsight, I probably should have also gone the other way – but we will talk about that more later. All the loads were still able to comfortably fit into the magazine.

Test test

Testing was fairly straight forward. I simply went out, and, well… shot them! I shot each ‘set’ on a separate point of aim, record velocity, SD/ES and group size for each group. While the picture just has one target up – I actually put up enough so I could just shoot them all without having to go up to patch. I had one slightly sticky bolt lift – but that was in the middle of a pile of rounds that posed no issue – I think because Greg chambered the rifle quite tightly – that I might find I need to cycle the gun with a bit of, ah, aggression to get the brass out – it’s not a ‘have to hit it with a bit of wood’ tightness that you get with overpressure – more a ‘snug fit’ – I doubt it is doing to slow me down in regards to reloading the rifle – and really – my reload par time is not the weakness in my shooting anyhow. Really.

The results

Because that is all you really care about? Right?

Well… there were kinda what I expected, but still very interesting.

As I got progressively closer to the lands – the speed (and likely pressure) went up as well. This is a function of pressure having to build up more behind the projectile before it engraves and starts heading down the barrel. Think about it like this – if you roll a vehicle right up to the edge of a curb – you generally require a few more revs to get up and over it, compared to a rolling start – same concept. The increase in pressure also results in an increase in velocity – but – I am not really chasing more speed – so I was more interested in consistency of SD/ES and group size. Thought – it should be noted – that group size at 105 meters, does not automatically equate to group size at any other distance (but at least it gives me some good ‘Facebook Group’ posts).

CBTO (inch)Group Size (MOA)Velocity (fps)SD (fps)ES (fps)
2.225.5330032341
2.230.27302163
2.235.27302948
2.240.333029510
2.245.60303248
2.250.39303624
2.255.533038710

Interestingly, basically, the closer out I set the projectile, the bigger the group. The SD of 2 at 2.250 is interesting – but – as you can see in the chart – a little more and it significantly increases – meaning – I would have to get the seating depth band on each time to stay ‘in the node’ – and – it was a slightly bigger group than shorter lengths anyhow.

If I was looking for the most consistent length out of all of them, I personally tend to lean towards the 2.235 load – small group size, small SD/ES and a small jump in velocity to the loads either side. Weight all the things up – it should do what I want it to do, without too much fuss.

Caveats

Of which, there are plenty.

Looking at the LabRadar data – in the first set – with the high SD – the first round was ‘significantly’ slower than the rest – so also considering that might be the fouling shot needed after the rifles last carbon clean. I should have included a couple of fouler rounds in the test load.

It’s a small sample size in regards to only three rounds per group. I realise this. Hell – statistically, I should probably do all the test groups at around ten rounds per sample. But I am not going to. Not only can I likely not shoot that well for that sized string, I can’t be bothered. I mean – take a step back and consider this all from a field shooting perspective. I potentially have a load that groups at .27 MOA, with an SD of 4 that loads into a magazine. I am going to be shooting that off a wobbly position on the side of a tank trap – where I will have to engage three targets at separate distances with two rounds each in 90 seconds, physically moving between shots. It’s not my SD or group size that is the weakest link.

I was shooting quick. I suppose if I sllooooowwwwed down everything I might have shot better… than .27 MOA. I could also have shot five groups and just selectively photographed the best one. But, of course, no-one on the internet would do that sort of thing. Would they? </sarcam>

Now what?

Well… now I load up a pile more and go shoot it. I am still ‘fireforming’ the brass – so expect a slight change in velocity for the second fire (maybe) – if that is the case, then I just adjust the load a little to keep that in line.

Essentially though – I am going to load up another twenty-odd rounds – shoot five over the chronograph to finalise the velocity starting point and then see how far I can stretch out the rifle to true up the data. Then, it’s just a case of more positional and wind reading practice!

No. A smaller SD or 1/10 moa groups wouldn’t have helped me here either. Video from Simon @ Gillice Practical Events. Learning to throw a ball, bringing zoom back down to minimum after shooting, remembering the safety, not knocking out the second round from the two rounder… a few things might have helped.

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