That is about it. I have improved range I can usefully engage by increasing accuracy but its not like I am suddenly popping heads at 40m, its not that good even after a tonne of upgrades.
I wouldn't even bother with a gun that couldn't reliably perform headshots at 40m, but then my area of expertise is AK's not M4's so I can only advise you in general terms and from the reputation of stuff I've read about.
The thing to understand about range is that, given perfect hop up performance, it is a function of FPS, both at the muzzle and at extreme range. The heavier the BB, the better it will maintain velocity at range, so certainly use 0.25g BB's and experiment with heavier. "Perfect" hop performance would be where the backspin starts and remains exactly perpendicular to the effect of gravity on the BB. This presents a number of problems even if your hop unit is extremely good, because every time the BB touches the internal barrel during its transition, it has the potential to be deflected. If you can afford it then, a barrel with a good interior polish will produce better results than a standard barrel and very good polish even better results still, not because the good barrel adds anything, but rather because it takes less away. I have found that PDI barrels (from x-fire.org in Japan) really do work considerably better than most TBB's - whether the performance is worth the cost depends on your wallet, but if you cannot afford PDI I would suggest that chromed brass or chromed copper are better than stainless steel (and that is after experience of both compared to Systema stainless steel), so long as you clean them carefully, making sure that you do not force even the slightest speck of dirt down the barrel from the muzzle when you do so, because whilst chrome is a shinier surface than steel, it is also a coating so it can be abraded off.
Ideally your hop chamber will have the absolute minimum of potential wobble, so simply wrapping a few layers of PTFE tape around the hop rubber helps and once greased also improves air seal, obviously not covering the barrel window bump, so that the barrel jams into the hop chamber very tightly. Also you want the nub to fit tightly, so that there is no gap between its edges and the part of the chamber above the barrel window which holds it. Often nubs are supplied with a bit of rubber moulding still attached to one or both ends - this should be trimmed off if you are going to use a typical cylindrical nub, but I would always suggest trying an H-nub. It is likely that RATech 'hourglass' shaped hop up cushions (as they are called instead of 'nubs') are even better although I still have not got around to trying one - you can also get them from ASPUK. If either of these types proves too stiff to allow the hop to be set low enough then a Madbull Fishbone spacer is squidgier and will produce slightly better results than a standard cylinder. IMO it isn't worth faffing with the funkier nubs, flat hop etc. available for standard FPS AEG's because there is insufficient benefit available at lower power.
Airlab are big fans of Lonex hop chambers for M4's so I'd start there IIWY. As for what rubber/bucking to choose, the stickier it is the less you will have to put it 'on' in order to get the same amount of back spin. This is important because the more the hop is on, the more braking effect is has on the BB and less FPS means less range. Maple Leaf 60 degree rubbers are very popular and having tried them I can confirm that they are good - the main reason however is because they have ridges on the body of the tube which create a better airseal between the barrel and hop up chamber than a plain rubber and this principle is better realised in the design of Modify Hybrid barrel/rubber combinations. Of course just wrapping PTFE tape around a good quality rubber, such as a Prometheus (Soft) purple bucking will also achieve the same increased seal. Maple Leaf also make a 50 degree rubber which ought to be better still @UK ambient temperatures, although you should also bear in mind that the softer the rubber, the faster it will wear out...
