These are just our observed results, so maybe not true in 100% of cases, but might be a useful guideline to have.
The only major brand we're missing is CYMA, which by some accounts, is a G&G clone. It is to note though, that JG is a Marui clone, but did not necessarily take all the same mags.
Overall, the most co-operative gun was the G&G MP5 and the most
versatile mags were TM and SRC. G&G and ICS mags were too loose to
catch and stay put in the JG but could possibly be shimmed with some
tape.
Guns were: G&G MP5SD, SRC MP5 Gen 2, JG MP5, TM MP5K HC, ICS MP5SD P*.
Mags were: TM short double HC Hicap, JG Hicap, ICS Hicap, G&G Hicap, CYMA stubby mid/lowcap, SRC Hicap and Lowcap, Angel Custom MP5-10 Straight Hicap.
Guns were: G&G MP5SD, SRC MP5 Gen 2, JG MP5, TM MP5K HC, ICS MP5SD P*.
Mags were: TM short double HC Hicap, JG Hicap, ICS Hicap, G&G Hicap, CYMA stubby mid/lowcap, SRC Hicap and Lowcap, Angel Custom MP5-10 Straight Hicap.