Surefire m500: annoying rotation

I bought a surefire M500A ( used bit in excellent condition) for my colt 6520. I mounted the handguard without difficulty. only problem the handguard, when held, rotates slightly left and right a few degrees.

I would like to understand how to solve the problem. I report a passage of the instructions:

"2. Install the halves of the housing by first inserting
them into the hand guard cap then pressing them
together while pulling back on the hand guard slip
ring, install top part then the bottom part, when they
are seated release the slip ring. The rear groove in
each half of the housing contains "crush dimples"
designed to flatten under pressure. After the housing
is installed, you may need to use the palm of your
hand or a rubber mallet to tap the two halves together
to create an exact fit. One tap on the rear end of each
housing, just above the dimples, will suffice."

maybe the problem is due to the fact that it was installed on another rifle?

Thanks for your help

My experience with the M500 is that they just don't fit together that well. My agency had a bunch of them that were finally replaced a few years ago and they all had that sloppy fit.

someone else who solved the problem?