Do you have a set of dial calipers and know how to use them?

My guess is that the barrel's grooves were machined to minimum specs and that is the root of your problem.  Another possibility would be to try another gas cylinder or three to find one that fits.

I am assuming that this is a new barrel, and the grooves for the GC have not been peened for tighter fit.  I am also assuming that there is no interferance between the Major Outside Diameter (O.D.) of the barrel, and the Major Inside Diameter (I.D.) of the GC.  This is basically the outer surface of the barrel and the surface of the GC that corresponds. Should be a small difference between the  two dimensions, allowing the GC to slip over the barrel.

If so, measure the center portion of the female grooves on the barrel and the male splines on the gas cylinder.  The groove should be wider than the spline by a small amount.  If this is so, inspect the leading edges of the male splines to see if any have  become deformed, with small burrs on the edges.  Measure the leading edge of the spline and compare with the measurement of the middle of the same spline that you made earlier.  A difference indicates that the leading edge of the spline has been deformed.  If you find this, use a jeweler's file to carefully remove enough metal off the leading edges of the damaged male spline(s) to allow the leading edge of the splines to engage the female grooves.  A very small bevel is OK.   Do likewise if the leading edge of the groove has been burred or damaged somehow.  IMPORTANT:  this is the only instance where altering the barrel is suggested, I.E., to repair a damaged leading edge of the groove.  I would not bevel the grooves.  In all other instances, any modifications should be done to the less expensive part, namely the GC.

If the Minor O.D. and Minor I.D., namely the bottom of the grooves and the top of the splines is incorrect, and there is interferance, you will find it tough to measure with calipers, as, IIRC, there are three splines and grooves.  If I am incorrect, and there are four splines and grooves, then take your measurements and modify the GC to suit the barrel.  

If you have carefully measured everything, and repaired any damage present, then the GC should go on.  Tap it on with a hard plastic, wood, or rawhide mallet.  Tape over any surface that you will be striking to avoid any possibility of marring the finish.

Take your time, measure carefully (TWICE!), and the answer should be apparent, along with how to fix it.