I received a new M3 357 several weeks ago that was under powered as well. From the vendor, the front reg was at 150, second at 135, Micro 4.5 Macro 16. JSBs were going 825-830 Nielson 93s 750ish.
I walked the gun up to 175 front reg and 165 rear reg and couldn't get above 860s with the JSBs and 800 with the Nielsen 93s. I then went down to 120 front and 100 rear, walked it up to the same 175/165 split and ended up with the same speeds. I stopped and called FX and explained the situation. They offered to walk me through some steps to see what was going on, and I said no thanks, I would like to send the gun to you so that it can be made right on your time, not mine. I was informed this would take several weeks, and I agreed.
I spoke with the tech a few days ago, and he explained it was missing some parts necessary for the .357 power. He stated that the gun is now sending JSBs at 925 and Nielsen 93s at 865-870. This is occurring with the front reg at 180 and second reg at 170. 4.5 Micro, 16 Macro and valve at 13.5mm. Additionally, he stated that the gun is basically running at max and I am not going to get any more out of it as is-stock. It is suppose to arrive back to me at the end of the upcoming week, and I will verify, but I was told the gun is shooting very well and consistent. If so, I will be very pleased.
He stated that the gun will be happiest with the regs within 10-25 bar of each other, and 1/4 turn on the front reg would match well with 1/8 turn on the second reg. I also asked quite a few questions about stepping down the gun to .30, .25, and .22, as I intend to do so. Basically, he said at .30 it will run hot, but very well. Same with .25, although it's gonna favor the heavy stuff in both of those calibers. He went on to share that he wasn't aware of many step downs with the M3 .357 to .22 because the gun is new and caliber kits are hard to come by. He said it may be possible, but it would most likely require heavy for caliber ammo and there would only be so far it could be turned down without removing the .357 specific parts in .22, which sounds ideal. My intent with this gun is to have it be a multi caliber slug gun throwing fast and heavy.
If your gun is under performing, I recommend making sure the hammer, springs, and spacers that are unique to the .357 are in place. I am glad I just sent the gun to service and waited, as I would have wasted time and pellets/slugs chasing this situation and these are all precious commodities. For the kind of money this gun costs, it should work as advertised out of the box.
FX service has been very easy to deal with, communicative, and timely with their repair and warranty. I wasn't happy boxing up and sending off the gun after only having it a few hours, but I am happy with the resolution and my decision to let them make it right. It sounds as though they have, the problem was parts related, and no amount of tuning would have corrected the missing part problem.
Hopefully this gives you another data point, as I certainly spent some time sifting through the limited information available regarding tunes, performance, and possible problems relating to performance issues on the .357 on this forum.
