Slugs for Hatsan Flash Wood QE

Hi all, I am new to the forum and a fairly new air rifle owner. I have recently become aware of slugs for air rifles. Since my barrel is not a dedicated slug barrel, is it silly for me to shoot slugs out of my gun? Will they be less accurate or harmful to my gun? I have the .22 caliber model and I have seen some slugs advertised as .217 caliber and .218 caliber, which one should I buy? Thank you for any info, I apologize if these are stupid questions.
 
I bought an H&N slug sampler, we tried slugs in a few .22 pcp rifles. One of the rifles was a flash pup .22, we had the .217 sample and it liked the 25gr well enough that my buddy bought a can of them to check out. He has had a good result, they shoot as well or better than his favorite pellets. You can also order small samples of NSA slugs to try in your rifle, I did that for a .177 Walther pcp rifle and had great success with 12.5 gr slugs in that one.
 
I definitely suggest ordering a bunch of different sizes and weights to see what your barrel likes best. 

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My hatsan BT65SB QE prefers the .218" 24.8gr NSA's.