[TokyoSeven]

Many points have been covered already this morning and Im not sure what I have to add. I will attempt to provide some insight as best I can as many have already stated helpful tid bits.

If you want keep that over 400 mark, I would say change the spring so something like a guarder sp100 or systema m120 and just upgrade your barrel, airseal nozel, and make sure youve got a good air tight hopup. But you wish to keep your PDI 170, so those options maybe out for you. However out of my two M4s, My M4 RIS has a higher FPS than my M4 MRE,My RIS has a guarder sp100 while the MRE has an sp110, but the RIS also has an upgraded deep fire airseal nozel and tighter barrel. So even though the MRE has a stronger spring, the RIS still has a higher fps due to having more efficient parts.

Unfortunetly my RIS also has an aluminum piston head on a systema red polycarb piston and aluminum cylinder head while the MRE is just stock piston head on a red systema polycarb with a stock cylinder for now, eventually my RIS may explode, it may not. I cant really say. As preventative maintenance I should replace just because.

The thing about reenforced mechboxes is that they arnt designed to last forever, they just take longer to break than regular mechboxes. Ultimately your goal should be to find appropriate parts that would compliment the other parts in reducing the stress.

Although if its in your budget and you can get one, a pro win mechbox may suite your needs if your goal is to continue running with that PDI 170.

Unfortunetly, unless I am blind, I did not see what type of aeg you are running this pdi170 in. If its anything wit a version 2 mechbox, well to be honest, I cant confirm this but some people just say v2's are garbage regardless how they are set up, thats not my opinion, just what some people have stated. I dont know, I havent had any major issues with any of my M4s. Nothing that wasnt fixable atleast.