Born Again wrote:Choke is a function of bore size and choke size and many manufacturers have a different interpretation. Your best bet is to do some pattern testing, a steel plate with whitewash and a measuring tape are required. Once you have seen the pattern your gun throws you'll be a better shot too.
Pattern is determind in a diffrent way.
If i remember correctly the 30 yards and 30inch circle and you are lookinf for 90 % of the shot fired to within the 30inch circle to give a set choke size but i cannot remember the size but i think it was 1/2choke.
For testing perposes English trade uses Eley GPrix 7shot as the standard.
Over and above the density you are looking for holes in the pattern and this can be ajusted by reaming the lenght of forcing cones ect.
Having establishing the best pattern at this standard trap cartridges are worth and exrta 1/4 choke approx