The following table shows the number of pellets in typical .410 loads for differing shot sizes
Note - For example, A 3" load of 9 shot contains the same number of pellets as a 12 bore 1 oz. trap load (7½ shot) though the pattern of shot in flight will be somewhat different.
The pattern of shot inflight, and hence its 'killing' ( clay or otherwise) ability will depend upon a number of factors.
In general the muzzle velocity is allmost identical to 12 bore loads. The shot pattern may 'string', that is, have a marked distance between the leading and trailing shot in the pattern and not simply be an expanding 'disc' of shot moving towards the target.
Cartridges not using a full length shot cup will result in a large proportion of the shot rubbing the barrel wall and the resulting deformed pellets will spead away from the bulk of the pattern resulting in a rapidly opening pattern with a number of 'flyers' off to the side of the pattern.
12 bores are usually patterened by shooting at a pattern plate 40yds from the muzzle and then drawing a 30" cicrce aroung the main part of the shot.
A pellet count is taken and the general pattern reviewed for holes through which a target may escape.
A .410 shot pattern is unlikely to survive much beyond 25yds to 30 yds and thus it is more usual to pattern at 25yds and draw a 20" circle around the pattern.
Usually a .410 pattern starts narrower than a 12 bore pattern (naturally, as a 12 bore is aprox 0.729 inches, minus a few thou choke) but opens out quickly due to the large proportion of pellets in contact with the bore. Many .410s are 'full choked' to try to hold the pattern together but the pattern fails, as mentioned, beyond about 25 yds. Though remember, the pellets still have plenty of energy and can wound (live targets) or chip (clay targets) well beyond this range.
Webley Bolt Action / Full Choke 2½ using cartridge 7/16 oz #6 (118 pellets), 77 in 30" circle @ 40yds
A large circle at a large distance, No indication of pattern quality.
Eley .410 Adapter in 12 bore (¼ Choke) using 7/16 oz #6 (118 pellets), 107 in 20" circle @ 20yds
No indication of pattern quality and velocity, the adapter will be equivelent to a 3" (aprox) barrel