The various MK1 type plastic wadded 21-gram loads were relatively straightforward to assemble, utilising a combination of components used in other existing loadings, but with the shorter 65mm case.
As pleasant as the MK1s were to shoot, the need for a fibre-wadded 21-gram loading was obvious. To gain penetration of the market at both the local club level and shooting schools, where plastic wads were not permitted, a fibre load was essential.
This brought its own set of problems, not least of which was the considerably reduced volume taken up within the 12-bore cartridge case by the very light 21-gram load. To fully appreciate this problem, it is worth reiterating that the plastic wads used in the shorter 65mm and 67mm cases, with 21grams of shot, were designed for 24-gram loads in the longer 70mm case.
Depending on the actual wad used, somewhere around 15 per cent of the 24-gram shot load at the top of the shot column would be outside of the confines of the shot cup (the top left picture shows a 24-gram load overfilling the shot cup).
As the 21-gram load has 12.5 per cent less shot, this meant that the shot cup was just filled sufficiently (with 21grams of shot) to allow a proper crimp closure with the shorter length cases.
The main reason for the extra difficulties with a 21-gram fibre version was the larger internal diameter, and therefore volume, of the 12-bore cartridge case (for any given length), when compared to the smaller measurements in both instances with the average plastic wad’s shot cup suitable for 21-grams.
To put this into a visual perspective, the average 12-bore plastic wad’s shot cup is much closer in its internal diameter to the bore of a 16-bore shotgun (.662inches/16.815mm).
The shot column diagram shows the relationship between length of the shot column for any given weight of shot, which in this case is 1oz or 28.35grams.
The average 12-bore plastic wad cup internal diameter and volume is in effect much closer to that of a 16-bore, so it has a similar shot column height.
With a fibre wad, the full internal diameter of the 12-bore cartridge is available, so the same shot load takes up a considerably shorter length within the case.
In both instances with the same shot weight, the plastic wad’s shot column height will be around 21 per cent longer; a significant difference.
When this is transposed to the very light 21-gram 12-bore loading, using the same shot size as in the diagram, the plastic wad shot column length is .about 16mm, but with the fibre it is 13mm.
This difference can be even more pronounced with thin-walled plastic cartridge cases that have a larger diameter than a true 12-bore measurement, which shortens the shot column still further with any given weight of shot.
Initially the MK1 21-gram fibre load used the shortest length cartridge case of 65mm, and the longest fibre wad combination then available as used for 24-gram fibre loadings.
With reference to the shot column diagram, a 24-gram loading would be 14.84mm in length with a true 12-bore diameter, a 21-gram load would be about 2mm shorter, so existing 24-gram recipes for 67mm cases were loaded into 65mm cases for initial appraisal of the concept.
It was soon found that the powder charges, and type of powder, had to be modified to maintain the sufficient pressure development necessary, to achieve consistency with such a light fibre loading.
The fastest burning powder types were utilised to this end, as their pressure development build was more rapid and rose to a higher level, while the shot and wad was still held within the confines of the cartridge casing. This was especially necessary, as in most instances these cartridges would be fired in 70mm or longer chambers.
In this manner the MK1 fibre 21-gram load was born, with broadly similar ballistics as the MK1 plastic variety, with around 1200 feet per second at 2.5 metres range, being directly comparable to the popular clay pigeon cartridges available at that time. They were fired through the same 12-bore test gun as used for all velocity comparison testing, which has a bore of 0.733inches and a 19thou (tight side of half) choke constriction.
The MK1 fibre loads were very soft on the shoulder and patterned well, if somewhat on the tight side for any given degree of choke constriction (mainly due to the very short shot column), but the overall impression was extremely favourable, especially when instructing new shooters.
Their only downside was their unsuitability for use in semi-autos, as the use of fast burning rate powders with a fast rise time meant that there was insufficient gas pressure available on a consistent basis to operate the gas operating mechanisms of some of these types of guns.