a lot of the cylinders that systema makes are not uniform in output. oftentimes the cylinder is the biggest culprit of varying fps differences. for example, i've heard of red cylinders ranging between a tad under 500 fps all the way up to 540 fps.
to answer you question, i don't think you'll see much of a difference in fps due to barrel length between the m4 length inner and the cqbr length.
