Don't post on that topic, but that is the gun damage formula.
[(GunAP - 11) + 20(AC - 48) ] x M
Is the formula for Powershot and shotting spree, as you may be able to deduce from there, 20 gunAP is equivalent to 1 AC in terms of damage so if you have a gun with 1k Ap and 20 AC, you would be doing the equivalent of adding 70AC. This gun would have a similar effect to a gun with 400 AP and 50 AC.
The key to this is balance. A flashbat gun may not be fully capable of getting as much AC but it will easily (with a few refines) achieve some nice ap. Another advantage of the flashbat set is the potential extra DP on the helm. This may make a slightly noticable effect on your defenses and mean you may use a few (10% maybe) less HP pots.
The thread I have linked above has provided a good spreadsheet, use that to compare the many options you have and that should tell you if your skills will do more damage.
Given your Gun AP I'd either go for the flashbat set or level up 45 more times and good for a thorny rose set.
Someone please correct me if I am wrong.