I am considering to purchase a CMMG .22 conversion for my AR15 for cheaper ammo for extensive training. Just wonder if this conversion will change zero on my red dot?  How accurate is it at 50, 100 yards?

I am going to answer your question with a question.  Are the ballistics of a .22LR the same as .223?

I don't adjust my red dot with the conversion installed.  If it changes the impact, it isn't enough for me to worry about.  I have only fired it at 20-50Y.  I wouldn't waste time with rimfire at 100Y unless I am using one of my dedicated tack drivers.

It is the group you are concerned with, not the point of impact.  And for steel, it is close enough for a hit.

I also think "training" with a rimfire is stupid.  Practicing fundamentals and marksmanship is a different animal than "training".  Prove me wrong.

old post....but ill throw this in anyway

there are multiple aspects where ive used a conversion kit in rifles during skills or drills i teach. For several things, ballistics dont really play in...basic marksmanship: trigger control, cheek weld, breathing, addressing sights. Magazine changes, tactics and movement. As for accuracy out to 100 yards, ive got a kit in a DSG match barrel patrol rifle...it will easily hit a 2/3rd size steel silhouette, ive run the kit in a A2 rifle....also hit steel easily...

for a public entity, its a very inexpensive way to get my guys shooting and moving....in other words, if i only shot .223...they would have alot less trigger time
there are some drills and qualifications that require .223

I use the conversion in 2 rifles, 16" 1in9 and 20" 1in7.  It seems to get tighter groups in the 1 in 9 barrel, about 2-3" at 50 yards.

I use it as a warmup for tactical drills, like coming up from the ready, shooting from different positions, multiple targets, etc.  Then I swap bolts back to 556 and continue my training.

Just build a dedicated upper. Check right to bear

I’ve had one for a decade. It shoots close enough to 5.56 to ring 6” steel out to 100 yards.