hollow punch is best
you can get a set on fleabay but make the hole 1mm-2mm bigger than required = 6mm punch aprox
as when compressed the sorbo squishes and thus the center hole shrinks a smidge
yes freezing is best coz it is a bastid to cut clean - important to cut hole clean or bits will shoot out
(trying to drill/cut or burn a hole with a fag don't work too good)
ensure you top off with 2mm neoprene on top - rubber washer coz sorbo shreds
(found out just a thin 1mm neoprene don't provide enough protection)
sorbo on head first then top off with neoprene
cylinder is a bit big - need to trim it not to say 23.8mm but more like 20mm
coz it expands when squished - also sorbothane gives off a tiny bit of heat as it absorbs to energy/impact
(f*ck knows about physics or energy/forces - I care more if it works and stays put ok - superglue bonds rubber/neo/sorbo very well, key the cylinder head blah blah blah blah blah)
what you can use ready cut to size except for sorbo hole is this:
http://www.ebay.co.uk/itm/252351609259?var=551196799151
http://www.ebay.co.uk/itm/BLACK-NEOPRENE-RUBBER-PENNY-WASHERS-M5-M6-M8-M10-M12-20mm-25mm-30mm-40mm-50mm-/201460144292?var=&hash=item2ee7f5d8a4:m:mhuuDynrv_cDmkHcjFAvu2g
(20mm diameter m5)
sorbo cut to 20mm discs but not sure if they are recommended 70d sorbo
http://www.ebay.co.uk/itm/SRM-TECH-SORBOTHANE-ISOLATING-DISCS-PACK-OF-8-GREAT-VALUE-/331855090038?hash=item4d441ad176:m:mdbA_QRjCTbzsysAd1xUjOg
don't get self adhesive sorbo discs - it is crap self adhesive
there are some self adhesive neoprene penny washers about but I just super glue it all together
though them neoprene glued discs did stick very well - that glue was mega tacky
but now clean it all off with alcohol wipes n washing up liquid and just super glue the lot together
as it just bonds/fuses them sods together - sand/scratch/key the head and you should be all sorted
yes I guess you could sharpen up and old barrel as that is 6mm internal
but cylinder might be a bit large outer diameter cut @ 23.8mm, but could trim down a whisker with sharp scissors etc....
main bastid thing is getting hole as clean as possible or shreds of sorbo may start flying out towards your enemy
ok if you are low on ammo but think range/grouping is not too clever using sorbo ammo