Air Venturi .249 vs .250

A link to the slugs you are referring to will help. You may be comparing apples to oranges.
Here’s an example H&N make a 38 grain slug in .249.250 and .251 all the same price . FX make a hybrid slug in the same dimensions only they are double the price of the H&N .

The OD of the slugs are different only so that they are the correct size for the barrel on your rifle. If I was buying off the shelf slugs for a newly acquired rifle , I would try to get my hands on a variety pack with different weights as well as different sizes from .248 through .253. Finding the best accuracy with the combination of weight and size. Then buy a ton of the most accurate.
 
I'm shopping for slugs. What is the difference between . 249 and. 250 caliber? Why do the .250 cost twice as much as. 249? I assume. 249 will work fine ln a 25 caliber?
Thanks for your time!
NSA slugs? It depends on barrel size. Its only .001" smaller. They may fit fine, or they may be a little too loose. I would try and see what other people with your rifle are shooting.