Alright everyone, just did my first proper field test. It actually went pretty well. And the longer I played the better the blaster got. Not to many ball jam ups at all. Even tested on full auto a whole 350+ mag and it Jammed twice so. I was getting a low fps using 7.3mm gels at a consistent 180-200fps and some down spin at times but I’m thinking maybe the barrel is 7.5mm so could most likely be the issue. Funny enough for the first time I used it there was a player that just bought a genuine LDT ATM rifle but a v1 and he said my bolt felt better. So there’s that, the nylon is fine also. But the gun was heaps of fun to use in general. The guy I met also recommended to upgrade to a brushless motor like him as his fire rate was alot better. But I’m keen to re shell eventually and see what else I can do. Maybe go to a smaller 7.3mm barrel if it is 7.5mm and apparently his had a spring upgrade but I’m not sure as I heard it’s not modifiable. But futures looking bright for the braness hk416d ATM rep 
plans also for reshell are a shorter outer barrel with a tracer unit with inner barrel length to match with a cqb hand guard and a wat metal receiver. And a new stock
Also was all in an indoor cqb style field.
Don’t feck around with a smaller barrel, simply find the gels that work best for your setup and don’t get blindsised about FPS numbers!
All you need is a bucket full of gels that you can grow with your eyes shut and a Blaster that will send Gels down range reliably without any mucking around. 
Something else to keep in mind is the fact that when LDT released the HK416D years ago it came with a 7.6mm inner barrel… designed to work with LDT heavy milkies which typically grow a little bigger.
Might be worth measuring that inner barrel. Maybe being brandless the manufacturer copied the same barrel specs. 
But as DocBob suggests, a blaster that will run on any gel without jamming or stuffing around with controlled grow times etc is your ideal goal.
7.5mm inner barrel all day every day. 