First off - Good to meet you CanGiaMilla and welcome to the forum. Air guns are very fun to tinker / build. As for the Sig, the mpx is identical to the mcx except for the barrel, shroud, etc. My wife has the mpx and I have modded the crap out of it its identically inside to my mcx. If you want power ( velocity or ft lbs depending on pellet weight ) then the way to go is to do the hammer mod I mention in this forum. I can walk you through it if you need. Its pretty easy and will make the hammer hit hard enough and still cycle the semi auto correctly. Secondly you will need to remove the brass screen that comes on some valves. Its very easy to drill out and it stops flow drastically. I believe its there to keep the UK versions under 500fps but some American ones have this.
Once you have done the above I would recommend converting to HPA. With the hammer mod I would run at least 1400psi but 1800psi seems to be the magic number if you ask me. With that combination you will be moving at least 4 times the volume of air with no valve work other then removing that brass screen. One thing to understand about air guns is that air expands at roughly 1650fps. Always. Weather its 100psi or 5k psi. So more psi inst always more power. The hammer must be capable of opening the valve fully. This is why you will hear complaints about people switching to hpa and the power goes down. That's because the hammer was set for c02 at a max of 850psi. Then hpa is at 1100psi and the hammer can not get the valve to open fully. More psi but not more volume of air. I have learned from my experiences that porting valves, etc only gives minimal results. More psi with the hammer power to open the valve makes more power. Also barrel length greatly improves power up to a certain point.
Sorry to go on and on there but it is very import information if your getting into air guns. So back to the sig, you can expect to get around 900fps with 8 grain pellets with the mpx and around 1100fps with the mcx. Good luck and let me know if you need any other assistance.
Once you have done the above I would recommend converting to HPA. With the hammer mod I would run at least 1400psi but 1800psi seems to be the magic number if you ask me. With that combination you will be moving at least 4 times the volume of air with no valve work other then removing that brass screen. One thing to understand about air guns is that air expands at roughly 1650fps. Always. Weather its 100psi or 5k psi. So more psi inst always more power. The hammer must be capable of opening the valve fully. This is why you will hear complaints about people switching to hpa and the power goes down. That's because the hammer was set for c02 at a max of 850psi. Then hpa is at 1100psi and the hammer can not get the valve to open fully. More psi but not more volume of air. I have learned from my experiences that porting valves, etc only gives minimal results. More psi with the hammer power to open the valve makes more power. Also barrel length greatly improves power up to a certain point.
Sorry to go on and on there but it is very import information if your getting into air guns. So back to the sig, you can expect to get around 900fps with 8 grain pellets with the mpx and around 1100fps with the mcx. Good luck and let me know if you need any other assistance.
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