CA use a custom long mechbox similar to G&P's, needs a custom piston, cylinder, sector gear, and a couple other parts that you have to by from CA/G&P and there are no aftermarket parts for. Some models of the SR-25 from G&P have a standard ver.2 mechbox instead of the long one (the long mechbox wasn't released until after the SR-25 was so the original SR-25 releases have a standard mechbox.) Using a standard mechbox requires an extra long nozzle, special cylinder head, and a metal ring that does around the nozzle bridging the gap between the mechbox and hopup. Other than that its all standard ver.2 parts. I have this setup with a standard ver.2 mechbox and have all SystemA internals in my mechbox. Also the G&P SR-25 out of the box is setup for semi-auto only. There is a stud on the body that prevents the selector switch from going over to auto and the selector plate that comes with it, I'm told, has no full auto setting even if you do move the selector over there. I have a standard ver.2 selector plate and ground down the stud so I can use full auto. My gun has been converted into a 14.5" HK417 assault weapon instead of a sniper rifle and is only shooting ~360fps.
Stock G&P guns often shoot near 400fps, some shoot over. Several local players bought G&P SPR's and they all came out of the box shooting around 420-425fps.
The SR-25 hicaps are designed for semi-auto fire so when shooting full auto they completely unwind after 10-20 shots. Semi auto allows for a hundred or so shots from my experience on one wind. The 170rnd midcaps are nice for full auto but when you only load 23rnds to represent a realcap they don't feed well since their midcaps. I was pondering the idea of using the midcaps shells and mounting STAR VN M16 realcaps inside them with some sort of rig to hold them in place in the front of the midcap shell. Haven't gotten around to it yet though.
Also the mags in my gun had to be slammed incredibly hard to get them in, and they where so hard to pull out I would pull the magpuls off. Found out the ring that the mag catch shaft slides through protruded into the magwell (normal M4/16's don't have the mechbox that far into the magwell) and was jamming up my mags. Just had to grind the front of that ring down. It was a G&P reinforced ver.2 mechbox if anyone is interested.
All in all, unless you really want it for the looks (I was after an HK417 so didn't have a choice) stick with the normal M4/16 series. So many more parts, mags, bodies, etc. to choose from.
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