Rolling Average

Having completed the pellet counts, although not the full analysis, for the three cartridges I tested earlier this evening, it is difficult to escape the conclusion that a rolled turnover (RTO) closure is exceedingly damaging to cartridge performance.

I have heard it said in the past that the kind of crimp – 8-point, 6-point or RTO – make very little difference to overall performance and, beyond making life easier for the cartridge companies and their loading machines, is most often used as a means of avoiding unsafe chamber pressures via the adjustment it allows to “resistance to opening” – i.e. the amount of force which must be applied to open the case and propel the shot column out of it, up the gun barrel.

Although I understand the well-established relationship between crimp type and chamber pressure, I have never been convinced that the type of crimp applied to a cartridge is simply an irrelevance when it comes to performance and, since beginning to test .410 cartridges on a regular basis, I am more sceptical than ever.

Once again, the ubiquity 12-gauge guns and their in-no-way-marginal performance probably obscures for most the knowledge of a significant factor affecting pattern quality.

Losing 5% of the pellets in a 12 gauge pattern is rarely significant. Even a 20% loss in some loads is neither here nor there. One will always have enough shot in the pattern to cover and break or kill one’s target. Again, however, we find that the .410 – although not principally different – highlights a feature of cartridge design which significantly damages cartridge consistency: the rolled turnover.

Non-Standard Deviation

Looking at today’s data and the data for the other cartridges we have previously tested, it is immediately clear that those cartridges with fold crimp are far more consistent than those with a rolled turnover, almost irrespective of the other components used to create them.

The only exception to this rule (to date) has been the Lyalvale 2″ cartridge which appears to show good consistency but, perhaps importantly, was largely tested at ranges of 20 yards only; perhaps more 30-yard patterns would begin to paint a different picture?

I would suggest again – since I think I have mentioned this theory previously – that the piece of card which closes the cartridge must be interfering with – and sometimes spreading – the shot in the column.

This interference likely occurs because the card acts as though a very small sail, slowing rapidly as it enters the atmosphere (from the gun barrel). In doing so, it is doubtless impacted by the shot, which, though it has much greater momentum, is not comparatively massive – especially where the RTO card is thick. These collisions – essentially random – deflect the shot and the card, leading to further interference until the shot has spread beyond the diameter of the card.

The result of all of this combined interference is essentially random. Sometimes a small number of pellets will be affected as the card flies quickly out of the cloud; at other times, the card will be buffeted along by the shot and take longer to fall to the ground, affecting / deflecting more pellets before it does.

I presume that this variance correlates with the number of pellets remaining in the useful pattern area and the larger range of variation seen with RTOs (as opposed to fold crimps) with the standard deviation in pellet counts for a particular pattern test.

In Short

It would appear that cartridges completed with a folded crimp will give best performance. Today’s cartridges, all having RTOs, gave very poor performance, even with the supposed advantage of larger shot and relatively low velocities. They were also noticeably inconsistent.

On the other hand, the #7 version of the Eley “Fourlong” cartridge (the #5 and #6 versions of the same cartridge were tested today) has a folded crimp and produced some of the best, most consistent performance the SmallBoreShotguns team has ever seen in a .410.

From inspection, the #7 loading appears to use a different powder, but is otherwise identical (wad, primer) to the #5 and #6 loadings. This is strong evidence as to the value of a good, firm, folded crimp over a rolled turnover.

Aside: Keeping the Pressure Off

Although several notable sources of reloading data indicate RTO (i.e. less resistant) crimps for some particularly thumpy reloads, I am not in favour of this approach, for the reasons described above.

I have always thought that if one fears excessive chamber pressure, it is likely an indication of trying to use the wrong powder for a given load. It would be far better to change the powder or quantity of powder and use a good, strong, 6-point fold crimp on the case, than add a card and an enormous degree of randomnimity to the loading’s performance.

I suppose the evidence is on my side: does anyone reading this know of a top-of-the-range clay cartridge made with an RTO? I can’t think of a single example.