Bit late to the thread but my primary is a fusion engine'd G&G PDW15. I'll give my two cents on the body itself (PDW15) and the fusion engine, though Hamster's guide gives you all the info you'd need. I've added some more detailed info about alignment because I feel this is an often overlooked aspect of tuning HPA guns.
Donor Body: G&G PDW15 AR
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Compatibility: The fusion engine dropped right in and sits perfectly in the body. I was expecting to have to do some work for have the engine sit and also line up with the hop unit properly but was pleasantly surprised when it was perfectly inline and level with the upper and hop unit. This is something you want to keep in mind with all HPA systems but can be a particular challenge with the fusion engine because unlike other drop in units that fit into the gearbox that comes with the gun (and hopefully lines up correctly with the hop unit) the fusion engine is a complete replacement of the gearbox. I've illustrated the issue below.
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This can be a oft overlooked part of getting any HPA setup tuned correctly. If you shine a light down the barrel, you want to see your nozzle sat directly in the centre with an even spacing around it to the barrel inner wall. Off centre will give you consistency issues and potential feeding issues. Off angle will give you feeding issues, limit rate of fire and wear your nozzle and barrel where they rub. Both issues can be corrected with correct shimming but it does take some work and effort to get perfect. You can also get clips that clip your hop unit to the front of the engine to ensure they line up, its just a matter of finding what works for you.
Features: My body is fully ambidextrous with selector switches on both sides and mag release buttons on both sides. I really like this because it lets me switch hands and shoulders and still retain full functionality. I can reload left handed or right if I go dry, can switch to safe or full auto etc. This depends on your shooting style buts getting comfortable with firing from either shoulder/hand is something I highly recommend learning. Its a huge advantage when you come up against cover that would otherwise force you to expose more of yourself to shoot past.
Build quality: My gun is CNC machined, though I'm not sure of the alloy used. However, whatever alloy they have used is solid. My gun has no wobble or flex at all and has taken the heavy use it see's well. Admittedly, I don't go punishing my gun and throwing it around but I'm not afraid to crawl with it; butt it up against cover to get steadier aim etc. The biggest thing for me, however, is there is zero and I mean zero wobble/flex between the upper and lower. Most AEG's because of the design rely on the front pin and the top/cylinder portion of the gearbox sitting inside the upper to keep it snug against the lower, there just no room for tabs at the rear of the upper to interface with the rear locking pin. Most gearboxes are cast and designed to work in a wide product family, so they don't have a snug enough fit in the upper to prevent wobble/flex. G&G got around this issue by creating a extended shelf that sits up against the buffer tube portion of the lower receiver (see image below). This extension ensures that the upper is properly held down against the lower. Again, this helps keep the alignment between the hop unit and nozzle/engine consistent while I run and gun.
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Fusion Engine
My fusion engine is setup with the standard poppet valve and a green nozzle (primarily used for CSB at my local site). I get around 280(+/- 2-3) fps using 0.3g (1.09J). I was using Geoff's 0.3g bb's and it would shoot like lasers. I found 0.3g to be a nice compromise between weight and cost for CQB. Now I'll be the first to admit that 0.3g is a overkill for CQB but having my hop set to 0.3 meant I could go to a outdoor sight and be ready to roll without any fucking about with my hop or where my sights where zero'd at. It also meant that even if the guy I was shooting was wearing a think hoodie/PMC jacket with plater carrier, they'd feel my shots hit them even at the longest ranges I would be engaging at my local site (RIP The Stan).
I'll be changing my setup to a red poppet and amped purple nozzle for increased shot efficiency and a higher FPS (aiming for around 310fps with 0.28g, 1.24J ) since I only have outdoor sights available to me now. I'm also switching to Valkyrie BB's on a trail basis since I've heard good things and a few friends who've tried them say they get better consistency with them. I'm going with 0.28g because they don't do 0.3's and their 0.32g are a bit too expensive for me right now.
Regulator: Wolverine Storm - 40 - 140 PSI range. Nothing to say other than works perfectly with my fusion engine.
If you've got any specific questions, feel free to ask and I'll do my best to answer but from what I've read so far you're on track for a good polarstar setup. The fusion engine was designed around VFC M4 bodies when it was being designed so should be as close to, if not, just drop in and go as you can get for a fusion engine though the Avalon is a newer model but I doubt much has changed in terms of internal dimensions.
Finally, thank you for give me an opportunity to talk and gush about my hpa setup! ? I don't get to do it often but I really do love to do it when I can (I suspect we all just want someone to listen to us talk about airsoft endless, its why we're all on this forum after all!)
