That's my understanding. I think we agree? Your level can only hurt you (if you are less than the enemy), not help you. If your level is equal to, or above the enemy you are hitting....then all damage is THEN based on the gun you are carrying, and the level of your opponent.
I couldn't make the curve very well so I just used diagonals. The theory still holds, though. Something about the attack/15 cap may be wrong or these there's something going on with that 49 attack weapon causing it to cap early. However it is very easy to explain why your 300 attack weapon always does more damage.
The low attack weapon is already capped at level 2 enemies. It never does any more damage in any of your testing, which means that you never tested it on any enemies below it's damage cap. The 300 attack weapon isn't capped until level 20 so it is always doing more damage than the incredibly low attack weapon. Your conclusions are drawn from a sample bias because you didn't test a full range of attack weapons merely the highest and lowest you could find.
Look at the range of values for the 300 attack weapon. If I take that 88 damage value at level 2 and extrapolate +7% compound damage per level then guess what I get? At level 6 I get 115.35, at 18 I get 259.79 (that's a bit off, sure but then this is a ballpark...). At level 22 you get 297. Now at 7% compounding I would get 340.53, but that's also over the level 20 cap. What would the compounding % be at level 20? Why 297.43, pretty much what you got at 22 and 26.
In other words the low attack weapon you tested was simply too low for the level enemies you tested against to show both weapons having the same damage on low level enemies. A level 300 weapon vs an identical 330 weapon would not have the same results.
Conclusion: 300 attack weapons hit the damage cap at enemy level 20. 330 attack weapons should not hit the damage cap until 22 which gives 330 attack weapons a maximum of 14.49% more damage than 300 attack weapons. (Granted, this is all ballpark figures without knowing where exactly rounding happens, ect.)