I just bought some Calor gas jets, they are small brass items with a 1BA thread and holes that range from about 0.15mm up about 0.35mm.
it would be much easier to modify the end of the extruder nozzle to allow a replaceable jet to be screwed in than drilling very small holes. The jets I bought cost£1.55 each.
I just did a search and came up with this
"These gas jets are available (mail order) from the following (amongst others): -
www.roundhouse-eng.com
www.polymodelengineering.co.uk Then look under the 'BRUCE ENGINEERING' section.
sizes commonly available are: -
Number 3 which has a 0.15mm dia jet bore
Number 5 which has a 0.2mm dia jet bore
Number 8 which has a 0.25mm dia jet bore
Number 12 which has a 0.3mm dia jet bore
Number 16 which has a 0.35mm dia jet bore
All have 1BA thread.
"
TC