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The V2 mechbox in my MP5A5 was upgraded in December last year, with a "normal" PDI 150% spring (I say normal because there are a lot of bad PDI springs out there that put out up to 450fps at times) and I was shooting about 350fps where it should be. Only shooting I did was in the basement (winter, never shot outside) and noticed a crack inside the mag well on the bottom of the mechbox. I opened it up to find the top part had separated. So I put a Hurricane reinforced mechbox in and has been happy ever since.
So, as has been said in the posts above, they give out after a while with even a moderately upgraded spring.
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