Target Sports

HANDLOADING PART TWO: TO INFINITY AND BEYOND

Created on 14th May 2009

LAURIE HOLLAND continues his overview of Sierra's Infinity V6 programme with a look at comparative ballistic performance and the Tactical Chart View option

I finished off last month by creating a database of five load combinations in Infinity, and found each resulting ‘chart view' tabular output easy to read and useful for tasks such as calculating elevation adjustment changes, or examining the loads' terminal velocities and wind drift values. However, many competitive shooters want to use their ballistics programme to compare different bullet-MV combinations, even different cartridges and calibres. You can extract data from the tables of course, but information presented in graph form is more easily read. In this case you get the best of both worlds by asking Infinity to superimpose numeric values onto the graph line appearing every 50yd in short-range trajectories, 100yd in long. It also lets you select any combination of the five loads you've set up to show their graph lines on one display for comparison.

However, I'm running ahead of myself as I should have mentioned there is a choice of four graph displays: velocity, energy, bullet path (conventional parabolic trajectory) and wind drift (in inches). The last should be of the greatest interest to most target shooters. Why so? Well, we know the exact distance to the target, and should know our sight settings. So, providing the bullet remains stable throughout its flight, the exact path is neither here nor there. Wind effect is a different kettle of fish as each shot requires a new estimate of the conditions, bringing opportunities to drop points. This is particularly so when the cartridge/bullet combination has nearly run out of steam and bullets are passing through the ‘transonic zone', as with .223R and .308W beyond 800yd. So, if Load A sees a drift of a half-MOA greater than Load B in a partially misread wind condition, it will almost inevitably produce a lower score in F-Class with the increasing use of targets employing half-MOA dimensioned rings. There will be a high probability of the same outcome with the larger-dimensioned NRA TR target.

Comparisons

Nevertheless, the traditional parabolic display is popular for comparing different bullets' or cartridges' ballistic efficiency. It remains useful to sporting shooters and those taking part in tactical and practical competitions who may need to hold over or under rather than fiddle with sight settings. However, many otherwise well-briefed shooters have gaps in their knowledge about comparative ballistic performance, assuming that old, familiar designs are still best. For example, Rob Hunter mentions in his description of the ‘ultimate tactical rifle' that Vince Bottomley built for him (November 2007 to January 2008 issues of Target Sports) that he'd taken a little convincing that the 6.5x47mm Lapua cartridge would deliver better ballistics than his natural choice, .308 Winchester. Given the latter normally uses loads with 155gn, 168gn or 175gn bullets in this application, it's certainly difficult to see the smaller-cased 6.5 being superior, especially as it is usually loaded with a relatively light 123-grainer. So let's see how they stack up, choosing 155gn for the .308.

I initially compared bullet paths at 600yd, a typical maximum range for sniper and tactical competitions. This produced relatively little difference, although the 6.5 kept an advantage throughout. I re-ran the comparison at 1,000yd, and for fun then stuck another 6.5 into the mix - 6.5-284 Norma with a 139gn bullet at a relatively modest (by F-Class rifle standards) 3,000fps. All three plots used Lapua Scenars, the 6.5x47/123 load input at 2,790fps and the .308/155 at 2,820fps. The company claims these MVs for its factory ammo. The barrel length is not quoted, but is usually around 24".

Figure one shows the traditional parabola which sees the little 6.5 produce a modestly flattened path compared to the 308. You'll note that the graph is actually for 1,100yd - a useful tip is to specify 100 more than you need, otherwise you lose the superimposed numeric values against the right-side graph wall. You'll also see the big 6.5 is in a different performance league with a much flatter trajectory. Its mid-range trajectory (MRT) is around 42" lower than that of the .308, a reduction of just over ¼. How about velocity curves for the trio? Again, in Figure two we can see a small but worthwhile benefit in retained velocities at longer ranges for the 6.5x47 Lapua, running at higher speeds from 300yd with a 59fps advantage over the .308/155 at 1,000yd despite starting 30fps slower from the muzzle. However, note that Infinity calculates that the 155gn .308 is still travelling at 1,340fps at 1,000yd, a full 200fps above the speed of sound (1,122fps under standard environmental conditions). Anybody who shoots .308 Winchester at long range knows this is unlikely. Most bullets are falling out of supersonic flight way out there, especially at the modest MV of 2,820fps compared to the 2,950fps plus obtained from 30-32" TR or F-Class rifle barrels. I'll return to this issue later.

Figure three looks at the wind effect; this is revealing. While the trajectory plot shows the 6.5x47 giving a 4.5% reduction in the height of the .308's 1,100yd MRT, there is an 8.1% reduction in wind drift at 1,000yd, slightly more at 1,100yd, both in the ‘standard wind' of 10mph at 3 o'clock. In physical terms, this is a reduction of a shade over 7" at 1,000yd or ²/3MOA, modest but still valuable. However, the 6.5-284 Norma load provides a much larger improvement over the .308W, giving a near 30% drop equivalent to just under 2.5MOA on the target - this is the reason we now have Open and F/TR classes. So, the traditional bullet path or trajectory is not a particularly good guide as to performance in the wind, understating differences. The .30 Carbine standard and ‘super-loads' v .223 Remington with a 69gn bullet were shown as a bullet path graph on p.17 of last month's issue. We saw that the heavy bullet .30C trajectories were much closer to that of the common or garden 110gn round-nose bullet performance than to the 0.224" 69-grainer. Running the quartet again for wind effect gives a different picture, though. The 150gn Lapua at 1,700fps performs far, far better than a standard .30C load at 200yd to 300yd range, and narrows the gap with the .223 to just under 1MOA (figure four). Also, the two heavier .30C bullets (Lapua 123gn flat-base FMJ and 150gn Lock-Base FMJBT), had virtually identical bullet paths over 300yd, but have now separated noticeably. More and more curious! Incidentally, if you return to Vince's Project Build articles, you'll notice that much larger benefits were quoted for the 6.6x47mm over .308W. This was based on an assumption that the former can give a 123gn Scenar 3,000fps MV, while here I've stuck to the factory figure.

At this point, I'll stress that ballistics aren't everything, otherwise we'd all be shooting .30-378 Weatherby Magnum or .338 Lapua Magnum rifles. There is often a trade-off between flatter trajectories and reduced wind drift and the costs of the performance that provides them: rifle weight, monetary outlay for rifle and ammunition, barrel heating and wear and recoil. The objective is to get a win-win situation wherever possible, which is what Vince provided Rob with in the 6.5x47mm rifle project. While bettering .308W performance, recoil is down. Calculating this factor is another facility offered by Infinity, needing bullet, powder charge and rifle weights plus MV. Assuming Rob's rifle weighs around 14lb and the powder charges are 39gn and 46gn respectively for the 6.5x47 and .308W factory loads I've been using as examples, the results (table one) demonstrate a significant advantage for the smaller cartridge. This is shown especially in the measure most commonly used - recoil energy. Infinity estimates a 36% reduction from 10 to 6.4 ft/lb. The cartridge also offers savings to handloaders, as Vihtavuori's maximum charges need between 5gn and 10gn less powder for the smaller model. 6.5mm match bullets are also a little cheaper than their .30-calibre brothers.

Tactical

While we're talking about tactical shooting, a ‘Tactical Chart View' output is available in Infinity V6, which is an enhancement over its predecessors. Let's input our factory 6.5x47mm load for a 250yd zero, 500yd maximum range with 50yd steps and see what it gives us. We have three output options: ‘Field Print', ‘MIL' and ‘MOA', shown as figures five A, B, and C, all printouts that can be cut out and taped to the rifle stock. These calculations assume an 18" ‘target' size and while the ‘Field Print' shows estimated bullet strike as usual, the other two show hold over or under. The ‘MIL' printout is particularly interesting as it includes adjustment hold-values for various shot angles differing from the horizontal. This means that our Lapua cartridge would need a bit more than a Mil hold-over at 400yd (1.2Mils) with a 250yd zero in a horizontal shot. These figures would reduce to 1Mil exactly on a 20° slope which applies whether the shot is up or downhill. (Specifying a 500yd range is fine for Rob and his international sniper competitions, probably on the low side in fact, but is too much for police marksmen. FBI studies into thousands of law-enforcement rifle shots taken over many years produced a mean figure of 57yd and 200yd shots were found to be exceptional. Military practice is another matter and snipers are expected to have a high probability of a first-shot hit on a head size target at 800m.)

Light v Heavy

Let's finish our use of the graph and load-comparison facilities available in Infinity by having a look at the F/TR issue of the moment. Does .308 Winchester perform better ballistically at 1,000yd with a slow-twist barrel and the best of the 155s, or with the heaviest and highest BC bullets that make sense given the .308's case and powder charge sizes - 210gn VLD form bullets from Berger and Sierra? Vince Bottomley touched on this in last month's ‘From The Bench', comparing the 210 at 2,450fps against the 155 at 3,050fps. However, certain shooters say their 155gn loads chronograph at a full 3,100fps (incidentally, just shy of the MoD's 4,500J High Muzzle Energy starting value). Meanwhile the chief protagonist of the heavy bullet brigade tells me he's chronographed his load at 2,600fps - a figure disputed as being unreasonable by his main light-bullet opponent. A professional ballistician advised a useful take on this issue that can also be applied in a wider context. If you have an MV from one bullet weight, calculate the ME, and it's a reasonable, but not foolproof, assumption that you'll be able to obtain similar energy values from another bullet weight without significantly increased pressure (rifle, barrel etc the same throughout and throating suitable for a longer, heavier model too). 155gn at 3,100fps produces 3,307ft/lb ME, this equating to 2,663fps MV for a 210gn. Even 3,050fps from the 155 lets us hit 2,620fps with the 210-grainer, and with the claimed 2,600fps obtained from the longest (34") barrel seen on the F/TR circuit, I'm relaxed about this figure.

Figure six shows wind drift for these two combinations using Berger VLDs, but in order to keep everybody happy I've also included Sierra's equivalents at Vince's 3,050fps and 2,450fps. Figure seven shows the four bullets' remaining velocities over 800yd to 1,100yd, to see how they compare to the speed of sound (1,122fps). According to Infinity, there is absolutely nothing at all in wind drift between the two 210gn combinations at their respective MVs, but they move 10" or 1MOA less than the 155gn Berger VLD at 3,100fps MV and 17.81" (1.7MOA) less than the 155gn Sierra Palma MK at 3,050fps, all at 1,000yd. Velocity-wise, the two Bergers have nearly identical remaining values calculated for 1,000yd at around 1,415fps, rather faster than the Sierras. The slowest of the quartet is the 155gn Palma at 1,316fps, but this is still comfortably supersonic. On this basis, either of the 210gn options are a better choice than the 155s. Of course you never get something for nothing, as heavy bullets at maximum loadings will increase recoil and barrel wear. Table two lists recoil levels for the four, assuming the rifle weighs close to the allowed 8.25kg (18.18lb). They're all still modest given the weight of the rifle, and even the ‘hotter' 210gn load only increases recoil energy by 11% over the milder of the 155gn examples. The graphs in figures four, six and seven are examples of the Infinity ‘Zoom' facility in which part of the display is selected and enlarged for clarity.

Garbage in = garbage out

The PC and software combination gives a super-fast adding machine that holds various formulae, letting us produce results in minutes that would have once taken a ballistician hours or days with a slide rule and pad of graph paper. I have complete faith in Infinity's ability to do the maths, and in the formulae used by the two consulting ballisticians who wrote the programme. But if the values that are inputted either from the programme's database or by you are incorrect or have limitations, the results are equally incorrect or limited. There are two problems: MVs and ballistic coefficients (BC). The former are easier to understand. Many people guesstimate MV on the basis of the highest velocity they can find in any reloading manual, adding 250fps for a 1" longer barrel plus the extra 1.5gn powder they're stuffing into the case, or what Fred says the load should do or even down to plain wishful thinking. You can work out what the MV is using ballistic programmes by inputting the additional elevation needed on the sights for increased range, going from say 300yd to 600yd, but that assumes your scope click-values are accurate, which is often not the case. (You can adjust these values by shooting groups at different settings and measuring the actual effect on the paper between group centres, but the tolerances and opportunities for errors start to become significant.)

Next month, I'll finish looking at ballistic programs with a brief glance at other drag profiles/BCs and will see how the Lapua Scenar is predicted to perform over 1,000yd with a G7 drag curve based BC compared to Infinity's results. I will also ask the question of whether it's vital for bullets to remain supersonic at the target as is often stated.