What is the purpose of having one with 18 teeth and one with 51 teeth.  Is one preferred over the other for certain situations.  I searched but couldn' t find any info.  Thanks guys.

you may want to look or post in the Armory/Suppressors section to find out. Also, silencertalk.com is a great site. I know that the 18t is for the M4-1000 and the 51t is for the 2000..other than that, I do not know the relevance from a functional standpoint..sorry..

18 Tooth is for the M4-1000 51 tooth is for the M4-2000

If you dont have a can it doesn't matter unless youre going to  get one

this

The 51 tooth has a ratchet and CANNOT come undone unless unless you press the arm and release and get it screwed off past the teeth while holding the latch and unscrewing at the same time.  It mounts with a bit less slop and is held more securely as you cannot just try to unturn it and have it come off.  The 18 tooth uses a spring and no latch (the spring is the latch).  Its easier to get off because all you do is unscrew the can by hand and pop the little spring over the teeth.  The 51 tooth latch must never be inside a FF tube becasue you must press the latch to unscrew unlike the 18 tooth mount where you just twist the can and can hold it anywhere vs having to hold the back of the can where the latch is.  The SPR/M4 and SCAR-H SD mounts have a MITER MOUNT 51 tooth, one turn mount so you only have to pinch the can and lever and rotate 180 degrees to get the latch to clear the teeth and another 180 just spinning the can to get it the rest of the way off.  Of course the MITER mounts also give 5 different mounting positions for you to find the one with least POI shift.  The old 18 tooth mount cans used a single spring to hold the can on the mount.  The new ones use double springs on each side. M4-1000, SCAR-SD, and 762-SD use the 18 tooth mount. M4-2000, 249-SD, 416-SD, and 240-SD use regular 51 tooth. SPR/M4 and SCAR H SD use the 51 tooth MITER.

Some people had issues with the older single spring 18 tooth mounts where, because of the fewer teetth and less fine fit the spring would be at the top of a tooth when fully tightened.  This made the can very loose when untightened to get the spring into a vally between teetch.  The double springs are set up so that no matter what at least one spring will hold the can tightly and you cant have this issue... if one is on top of a tooth the other will be mounted, holdig the can in place with less wobble.

I never liked the the 18 tooth mounts.  I think the 51 tooth is better and just OK.  I realy like the 51 tooth MITER.

After using my M4-1000 for a while, I'm wishing I had went ahead and at least got one with the ratchet mount, and preferably the MITER mount.  That 18 tooth mounting system just doesn't work all that well.

like what?

Thanks for the info guys.  I appreciate it.