Memories of being elbow deep in piles of boxes grabbing packages of random parts in the hope that SOMETHING would bloody fit into a build 
The frustration of trialling one part, only to throw it back in the pile, only to find another part half an hour later that might actually work with that part that was ditched in the first place, just to go find it again to try out with the other parts that looked like they might work with it!
Rinse and repeat until something eventually comes together 
I did this countess times, mainly with alloy aftermarket cylinder heads, that came with double o-ring alloy nozzlesā¦ but when tested for air sealing at full extension, they were fkn useless 
Many customers builds ended up with upgraded cylinders and alloy heads, but ran the stock standard nylon nozzles with a smear of silicone grease that provided a 100% seal compared with the double o-ring alloy āhigh performanceā parts 
Iāve found double o-ring nozzles to be more trouble than benifit. They are usually too tight, and donāt suit high rps, and sometimes even standard 11v slows the return too much. And a good single o-ring nozzle will seal anyway.
Quite often an oring change on the stock cylinder head is perfect too. Most nylon cylinder heads work perfectly then and help resist cracking gearboxes by absorbing the impact better.
Parts matching is definitely a headache, maybe less so in China as we can just order another off Taobao and it arrives in like 3 days. Also thereās quite a bit of info on the Chinese forums as well as in Wechat groups where there are people that just know things.
Another thing that has opened my eyes to stable performance is your gels. Obviously what kind of gels you get is important, but also the water temp you soak them in and how long. Some really serious people will buy a batch, fill a whole bucket and then start taking out gels starting at the 60 minute mark every 10 mins and then test the gels to see the exact time the gels perform the best. For example he might find that 80 mins is the absolute best size for his barrel and hopup. They also take the time to pass their gels through a sorting machine, to get the exact size gels, as the sorting machine will remove gels too big and too small. Then they will use some kind of towel and dry the gels, as wet gels donāt get enough traction on the hop up and so wonāt spin and fly as stable.
One guy came in, with really really decent performance on his PDX, and all he did was switch to a 40 RMB motor, airseal, O-ring switch, and then lots of attention on his gels, and they were flying damn stable.
You also get young guys that come in with similar builds like you guys mentioned - souped up and the gels just get shredded coming out the barrel lol.
Gel ball sorter exactly like this if anyone was wondering.
Great advice and information @Martin 
Always tell new blaster owners that gels are extremely important, and for people new to the hobby having issues with their new blasters, the gels are the cause 98% of the time.
Mainly because they try and use cheap Orbeez, donāt grow the gels correctly, pour them into the mag whilst still full of water or mix them up in a big bucket of all different brands/shapes/hardness and expect them to all work the same.
Those new electric sorters are a great invention, whereas I had the old sorting sieves from ZHENDUO which worked fine, they were just a bit more labour intensive.
Yeah naw that aināt gonna cut it. Talked to some techs and they said theyāll even take apart the mags and buff out the inside of the mags to make sure there are no factory edges that could be putting scratches on the gels which might cause unstable flight patterns. Some guy buys like 20 mags, tests them all then sells the ones that didnāt really work for him on 2nd hand market.
Not saying everyone should do this, but the gels are a huge factor to consider when your flight patterns/groupings seem to be inconsistent for the amount of investment youāve done on your blaster.
Thatās quite interesting, never bothered checking mags and deburring anything other than the GBB Pistol mags that were more susceptible to sharp edges and damaged gels.
Poor gels have a long mechanical pathway to follow with many stress points right up until the moment they are fired through the barrel, but never seen enough problems with constant confetti to have to focus on getting the mags as smooth as possible.
Great advice for anyone having gel issues though as another area to look into