Cutting mkv without reencoding

[Clnthl]

Hi all, hope you can help I've been trying to cut the beginning and end off of an mkv file all day and ive now given up so come on here for help. I have tried using avidemux as I've read you can do it on there but when I then try to save it starts encoding the video.
I have also tried to use mkvtoolnix but I can't get it to cut the correct point it jumps from cutting 14 seconds to cutting 18 seconds this is if I put 00:00:14.5 or higher!
Please help me either do it on one of these properly or on another programme!
Many thanks,

[Baldrick]

Did you select Video Copy in avidemux?

But you can only cut at keyframes(can be some seconds a part) with avidemux and mkvtoolnix without reconverting.

Some editors can reconvert just at the cut point(smart rendering) like mkvcutter (ALPHA version) or solveigmm video splitter(trial).