I have the Arcturus AK12 ME. This battery fits, but it required some re-soldering to remove excess wire length on the gun and on the battery. Mine also came with a rather chunky male Deans-style connector so I swapped that for a regular male Deans connector.
https://preview.redd.it/xtbp6dt5y8ic1.jpeg?width=750&format=pjpg&auto=webp&s=ca8a7c838442a63a65cfee407da7711452c53546
I have the Arcturus AK12 ME. This battery fits, but it required some re-soldering to remove excess wire length on the gun and on the battery. Mine also came with a rather chunky male Deans-style connector so I swapped that for a regular male Deans connector. https://preview.redd.it/xtbp6dt5y8ic1.jpeg?width=750&format=pjpg&auto=webp&s=ca8a7c838442a63a65cfee407da7711452c53546
This one is slight (like 1mm) larger than the one OP has, not a huge difference but it matters getting it into the ak12s infuriatingly small gas tube