I was thinking about that earlier. Anyone know if there are undesirable effects of mounting a cylinder backwards? Like a cylinder taper that I don't know about... xD

Yeah, if the numbers I found over at zshot are correct, there should be just enough volume to flip a 3/4 ported cylinder, assuming ~140mm barrel.

A stock cylinder in backwards would give an fps drop.

ICS makes one. I've wanted one for a while.

You can find it here

Is that because the cylinder tapers towards the front of the gun to allow for a better air seal?

Wow, that's one dog of a gun isn't it? Lack of buffer tube and strange front end (ignoring the RAS).

No, it's because there is less volume in the cylinder than there is in the barrel. A mild vacuum starts behind the moving BB, hence the term barrel suck. Really only if a long barrel is used, if a short one like a 229mm, you'd see a drop in fps.

Yeah. but if you're using a 4 or 5 inch barrel (101mm and 127mm respectively) then as long as you've got enough volume in that cylinder to push the bb out the barrel before the piston starts it's backwards journey, you should be ok, right?

Ya, should be completely fine.

Sweet. God damn I can't wait to be back in Canada to start this project...

Good luck with this, I've built a FMU Patriot
and owned that very same ICS M4 pistol and
the fps wasn't what they listed on that site.

As expected and said many times already—
you can get to 320fps but it's tricky business
when your dealing with such a shorty barrel.
Too much air to push and the bb exits without
reaching top speed. So you'll be finding the
balance between air volume/ length and
strength of spring is a bitch. Colin_S told me
about PDI springs being physically shorter
so they could reach expansion faster. I would
take his advice and the Guarder MP5K kit
plus a couple PDI springs.

Thanks a lot for the info about the PDI springs. That's a big help.

EDIT: Also, using regular pitch (as opposed to the traditional irregular pitch) springs will cause the spring to expand at a constant rate throughout the duration of it's expansion. Irregular pitch springs are designed to get increased spring expansion near the end of it's cycle, which for short barrel users, is too late at the bb has already exited the barrel.

For the lipo battery, I thing something around 1000mAh should fit correctly in there. Forget about 11.1v though. You will have barelly enought place for a 2 parts 7.4v pack, as in one cell on each side of the barrel.

I think you can fit a mini type in the fake buffertube if you cut the post on the body. Again you are stuck ar 8.4v, but you probably would be able to fit a 1200mAh 11.1v lipo in the same space.

The ICS one is awesome. Saw it at the warstore and nearly creamed the jeans. There are a few though I think. Worth looking into.

PDI springs aren't exactly shorter, they are various lengths. Across a few 150% springs I've measured, one was 6 3/8", one was 6 3/4" and one bruiser was a whopping 7" long! All the same brand and rating, but results right across the map. Hence my saying PDI springs suck, avoid at all costs.

I got a TM body left over from an upgrade and a chopped down barrel so i figured why the hell not, initially i was going to make just the upper and drop it into my CQBR but then i decided to go the extra mile and make the whole gun. As for the battery its going to be a PEQ2 solution. I've always wanted a PEQ2 unit anyways.