The hop up system in Airsoft is actually very delicate, requiring many parts to mesh together perfectly to produce good performance. For example, the air nozzle must be able to hit the bucking squarely, as even a little deviation of the position or angle can cause loss of air seal and thus poor performance. The quality of a hop up chamber has a great impact on the performance as it houses the bucking, produces the backward spin on BBs, and feeds the BBs.
The VFC SCAR hop up chamber is known for several issues: 1) its build material lacks rigidity and is fragile, 2) the inner chamber diameter is a bit tighter than other makes, 3) the factory hop up spring is weak, and 4) the hop up dial can loosen from vibration and lose its setting. With poor rigidity, the hop up chamber can bend upward from a fully loaded midcap’s spring tension, causing the bucking to change position and thus poor air seal between air nozzle and bucking. In some cases, the gun couldn’t even fire. The tighter inner chamber diameter makes it difficult to push many makes of buckings far enough into the chamber, and some may still clog up BBs even though they seem to be in correct positions. The weak factory hop up spring can prevent the hop up chamber from sitting tightly on the front end of the gearbox, and thus cause poor air seal between air nozzle and bucking. Finally, the issue of hop up dial loosening can cause change in BB trajectories in the middle of a game.
To get the most out of VFC SCAR hop up chamber, I recommend addressing the four issues mentioned above: 1) refrain from loading midcaps to maximum capacity, 2) use thinner buckings and/or inner barrels with smaller outer diameters, 3) replace the factory hop up spring with O-rings or better springs, 4) put locking washers underneath the hop up dials. In terms of bucking, I had good experiences with Maple Leaf 70 degree buckings. Even so, I’d check for constriction in the bucking lip after installing the bucking in the hop up chamber: https://youtu.be/156t4FrN2KA?t=4m19s If it takes a lot of force to push BBs through it, I’d re-install it or replace it with other makes. O-rings can be put on the inner barrel in front of the hop up chamber, which replace the function of the hop up spring. O-rings can be bought from hardware stores or Amped Airsoft. Also, replacing the hop up spring with O-rings can make assembling/disassembling the upper and lower receivers a bit difficult. I recommend loosening the six screws holding the outer barrel, pushing the outer barrel forward a bit and then put in/take out the lower receiver. Here’s a video showing disassembly: https://youtu.be/Idyy_H1zDQE?t=4m57s
To get better performance on VFC SCAR, I’d recommend replacing the factory hop up chamber. I personally despise dial-style hop up chambers, so I installed a ProWin M4 hop up chamber after extensively modifying the lower receiver and mag catch. Without the modification, it would be impossible to separate the upper and lower receivers, and BB feeding would be compromised.
Step 1: Remove Gearbox and BB Feeding Piece
Remove the gearbox (instructions can be found on YouTube), as well as the BB feeding piece (top picture). The BB feeding piece is held in place by a screw, the lower receiver pin, and four tabs by the side. Take off the retainer clip of the lower receiver pin, and push the lower receiver pin out. Take the screw off, and use a flat-head screw driver to pry the tabs off the lower receiver. The lower receiver without the BB feeding piece is shown in the bottom picture.
Step 2: Cut Off Part of the Lower Receiver
Use a Dremel tool to cut a notch in the front of the lower receiver approximately the size of the lower receiver pin lug on bottom of barrel assembly (top, middle and bottom pictures). Care must be taken on this step because if you mess up, you would ruin the lower receiver. I'd recommend getting a replacement lower receiver from VFC just in case.
Step 3: Modify BB Feeding Piece
Take the BB feeding piece removed earlier and remove the two ears at the front (the parts which the lower receiver pin goes through). Glue these pieces in their original position. Use a strong and durable adhesive, such as Loctite Plastic Epoxy. The two ears take up the gap on either side of the lower receiver lug and keep the lower receiver pin retainer clip in place. Notice that only one ear is shown in the picture.
Step 4: Modify the Mag Catch Window
Some of you might know the gearbox in VFC SCAR sits slightly higher than TM M4 clones. This is the reason why after completing the modification mentioned above, some magazines will have some vertical play, as well as feeding issues. In my case, the vertical play was so much that my G&P 130rd mid caps refused to feed. I thought about shimming my magazines, but I'd have to shim all ten of them, and it'd cause problems when I use them on my M4s. After careful consideration, I decided that modifying the lower receiver and shimming the mag catch would be the best solution. First, insert a magazine all the way into the mag well. Measure and mark the amount of play between the notch on the magazine and the mag catch (top and middle pictures). Use a Dremel diamond drill bit to cut a small slot on the lower receiver, so that the shim on the mag catch would not impede movement (bottom picture).
Step 5: Shimming and Modifying the Mag Catch
Put some JB weld on the mag catch, and let it cure for a day (top and second pictures). Shave off excessive JB Weld little by little until the mag catch can lock the magazines in place and there's no vertical play of the magazines. Some magazines may also require further shaving of the mag catch to allow easy insertion as shown in the third and bottom pictures.
Step 6: Assembling the Parts
Put the gearbox back in the lower receiver. Loosen the six Torx screws securing the barrel assembly in the upper receiver (top picture). Push the barrel assembly forward as much as possible and pull it down a little (keep the faux gas tube inserted). Put the tab in the back of the lower receiver in the upper receiver, and insert the air nozzle into the hop up. Push up the front of the lower receiver and the barrel assembly simultaneously into the upper receiver. Be gentle on this step, as it is possible to break the air nozzle if care is not taken. Slide the barrel assembly back, and tighten the six Torx screws. Push the lower receiver pin back into the lower receiver.
When everything is put together, the ProWin hop up should look nice and pretty as shown in the third and bottom pictures.
Step 7: Acknowledgement
Some pictures and texts are from ChopperMech on PolarStar Talk forum: http://www.pstartalk.com/showthread.php?2088-VFC-SCAR-L-P*-Upgrades