No info that I've really seen out there on rectal admin of 4-MMcat. Some compounds can cause irritation or burning feelings when administered rectally and perhaps it is one of them. Also, some individuals report pain from 'chemical X' while others do not, so it seems a bit variable. Ed has never felt any discomfort from taking methylone rectally, but other have reported doing so. Ed has felt some intestinal discomfort (could perhaps be described as a cramping feeling) a short while after taking 2C-D rectally, but it goes away pretty quickly and is no worse than what he often feels in his stomach during the onset of 2C-D taken orally.
Ed would suggest that liquid solutions given with a syringe is the best method generally, and that 2cm is pretty shallow, Ed would suggest maybe 5cm or a bit more (?). But not going far enough wouldn't cause cramping, just loss of some of the dose. Changing to gelcaps doesn't seem to me to be likely to reduce whatever pain SWIY felt, either.
Maybe this drug is just not tolerated well when administered this way, for whatever reason.