Any M11A1 is a pain in the butt to carry, large baton pouches fit the mags though, but overall it's not a worthwhile sidearm due to size being awkward as fuck.
Open bolt means you get crap in there (you need to keep it closed bolt, rack it then shoot, but keeping it closed bolt makes the firing pin put a bit of pressure on the gas release valve, so it ends up slowly leaking throughout the day, making you end up with no gas left when you need it) and if you put a supressor on it (they CAN work very freaking well if you modify it right), adds an extra length to deal with.
Mags are an issue also (if they KJW are similar to the KSC), they have an exposed feed channel all the way to an inch of the mag base, BBs and channel will get dirt, cedar bits, other crap inside, causing serious feeding issues. Might find a cinderblock easier to deal with really, unless you want to carry it in an extra large Ziplock freezer bag, assemble when you need it, then go BRRRRRRRRRAAAAAAAAAPPPPPPPPPPPPPPP! Which in itself is fucking cool!!!!
I have had a KSC M11 since 2003, love it, great supressor I made for it, but it's a cumbersome bulky thing even when slung, and is a lube pig, even then it's reliability is questionable.
Nutshell, get an MP5K and run that, much less issues, and short mags are easier to find too, as well as having cheaper options for back up mags ($10 per Star 30rd mag vs. ~$70 per 50rd M11A1 mag, unless they came down in price) Also, the rate of fire with an M11A1 is scary, literally you go through 50 rounds in less than 3 seconds, so an extremely skilled trigger finger is required for short bursts. With the 1200rpm fire rate, took me a while to be able to get practiced to do just three round bursts, and even after 3-5 round bursts are the norm.