Lozart

I would expect the problem to be more consistent - generally low FPS. If the hop rubber was torn I'd expect more shots to be all over the place. I'm wondering if the hop unit to gearbox joint isn't holding steady, check you've still got the spring on the front end of the hop unit (possibly over the outside of the inner barrel). Chrono wise, Xcortech ones are the usual go to, I have an X3200 but make sure you get a legit one. As with all things airsoft there's knock off ones about that are just paperweights.

It's not a definite that modding the arm would be a problem, it's just that you might be changing the wrong thing to fix the problem you have an inadvertently add another problem! Do you have a chrono? Might be worth seeing if the shots that drop are coming out at significantly lower FPS in which case it may be that the hop isn't sealing brilliantly or there might be an air seal issue in the gearbox, though to be fair I would expect that to be more consistent.

Ha! The ongoing chase of ever decreasing circles eh? That said, if it is new old stock I've not really had any issues with mag compatibility with any of my Maxx hops (using PTS EPMs and, rather surprisingly, Nuprol mids).

Personally, I'd be tempted to get a small bottle of some different ammo and see if it does the same thing (same weight, different manufacturer and different weights too if you can swing it - maybe ask a buddy if he has some spare he can let you try). You could try swapping the hop rubber, I'd usually suggest a Maple Leaf but the Krytacs have a weird nub so I'm not sure it'll work without changing the arm a bit and then you could be introducing other problems.

The TE was the model before the current one (ME) so if it's an actual knock off not just old stock then it's based on an older design. That said, I have a genuine Maxx TE hop unit and it's been great for me.

There's this one for AEG https://locknloadairsoft.com/products/copy-of-vfc-hk-licensed-mp5-front-sight-sling-loop-replacement (ignore the description in the link it's not for a sling loop!) Or this one for GBBR https://www.mlemart.com/vfc-magazine-catch-set-for-umarex-h-k-mp5-gbb/

In terms of other BBs I was more thinking of trying a different weight to check if the mags are struggling for some reason. If the hop is working well with your 0.28s then I would tend to think it's OK. If it was torn they'd be all over the place. When you get a shot that drops sooner, is it definitely just one BB or could it be double feeding?

Do you have any other BB's you can try it with? Does it happen in semi when you fire quickly or does it happen when you fire it slowly and deliberately (ie not spamming the trigger). Is it completely fine in Auto?

Gas or AEG?

Sounds lik it's performance is about on par with a £150 second hand gun. Checking the hop rubber is always a good place to start but before you do, do you know what the original owner has done to it in terms of modifications? Could be misfeeding every now and then. Does it always happen with a particular magazine for example? Have you made sure you've not got some different weight BBs mixed in?

Yeah @L3wisD what's the deal? Hmmm? I'd say that you could get 3 guns in that package. WHAT AREN'T YOU TELLING US? WE WANT THE TRUTH DAMMIT!!!!

(tongue firmly in cheek by the way)

I have their MK12 suppressor and it's LOVELY!

Playable area is outlined on their FB post https://www.facebook.com/Ai500-148728415210679/

Check with your airline, some are fine with it.

I always found that if it was loose enough to not hit the release it was flopping about like a dick in a shirtsleeve! Hence I moved to hard retention/kydex holsters. If you're running the holster loose it would be worth getting a lanyard for the pistol.

Be aware that those universal holsters are bad for tweaking the mag release and losing your mags.

Core are plastic not metal, but yes, comparable to a combat machine if not slightly better being a bit newer.

Errrm...OK?

I'm surprised they didn't mark it as a commercial sample or somesuch. Mind you, the insurance will bump up the fees too.

Super l33t old skool coolness courtesy of @heroshark!

If that's the case, I doubt you'd need the extra retention you'd (allegedly) get from a polymer mag holder like that. Esstac kydex pouches are great but you could do worse than look at the standard open top WAS pouches. They have a grippy liner material and I've certainly never lost a mag out of one.

Aluminium isn't great as a conductor and isn't going to stand up to much mechanical abuse in that application. Better to get a ring crimp like @Hudson suggested.