NXS 5.5-22×50 or NXS 5.5-22×56

[308trot]

which of these scope is better for tactical use and why?

 

[USAPatriot]

Specific use completely aside, a larger objective gathers more light and gives a larger field of view. Assuming equal glass quality and coatings, looking at the same object with both scopes set at the same magnification, a larger objective will give you a brighter image than a smaller objective. It may not be important in full daylight but it will be in low light conditions, near dusk or dawn, in the rain or snow or under black clouds, etc.
If you plan on shooting under those conditions or think you might have to/want to, then bigger is better, no matter what the purpose is...hunting, tactical matches, law enforcement, etc. -Rod-

 

[Spotcheck_Billy]

A 50mm objective will allow a lower scope mounting than a 56mm objective, all other things being equal. This will give you a better cheekweld on most tactical stocks, especially when shooting from the prone position. The only reason that I can think of for you to choose a 56mm objective in tactical shooting would be if a large amount of your use would be in poor light conditions.