Barrel and cylinder match issue

[Neko]

I recently left my m4 with local techs. I asked them to throw and m120 in there for me. I chronoed in at 330 fps with .2s at a recent OP. I was told the chrono was giving weird readings that day, but I can't be sure of that. When I confronted the techs about it, they said it was because I had an m4 (4/5ths) cylinder in my gun. I immediately wanted to call BS on that but let it slide for the time being. After looking inside the gun myself, I found it to have good compression, so that's not the issue. Are they talking out of their asses or would a zero cylinder give me a higher velocity? From what I understand, there would be too much air volume in that cylinder for a 363mm barrel and the BB would be out of the barrel before the piston finished a full stroke. Does going from a stock CA spring to an m120 require changing a cylinder?