Forgive me if I'm off topic of your question but this is how I can think to answer your oddly phrased question.
According to the wiki:
Killing an enemy with a warframe ability (molecular prime) will grant 100% of the experience from each enemy killed to your warframe. If you use weapons, it splits the experience 50/50 between your warframe and weapon.
When you're playing co-op and an ally kills an enemy, the experience gained is equally divided between your frame and each weapon.
According to personal experience:
When I first started playing warframe I had my friend explain the experience mechanics to me. He was probably completely wrong but it actually does make sense. He's told me that when you kill an enemy with an ability/weapon, you get 100% of the total experience toward your correlating weapon/frame, and then it divides the experience between your other gear. This CAN NOT be 100% correct (since I've never actually levelled a secondary by using my primaries) but I don't think hes too far off of the actual answer.
In all this leads me to believe that, when you kill an enemy with an ability, it gives you 100% of that total experience to your frame and then an additional bonus to your weapons. In regards to your "leeching method", its an inaccurate test according to this:
Kills: You get a bonus 50% experience from all kills at the end of mission.
If you want an accurate comparison, multiply your experience from the kill run by 2/3 (100% + 50% = 150% total, 2/3 of this would be 100%).
That is not easy to calculate. You get 50% bonus XP for all enemies killed by that weapon, but for team XP, you get no bonus. (Team XP is when your team kills an enemy, and you are in a certain range to that team member or enemy killed, of these I'm not sure).
So if you claim a weapon, equip it, but never use it (not even damaging the enemy), then you need all the 450.000 XP as that weapon will get only team XP and no bonus.
However if you claim that weapon, and you kill enemies with that weapon only, and you are doing Solo mission to ensure to get no Team XP, then you need 450.000 / 1.5 XP (=300.000) which is level 25 (rounded).
Of course the solution is somewhere in the middle between the two edge cases.
Best Answer
Just lookup the weapon in your Codex:
The bars below weapons (Archwings, Warframes and Sentinels as well) will show you your progress.
If you've mastered a weapon before - even after selling it - it will keep that status and will still show that progress in your codex.