DROOP?? Only on a M1 Series MBT (USA) lol It's 120mm smooth bore with MRS(Muzzle Reference System on the end of the gun) due to the characteristics of the main guns length, weight, metal type, the gun droops at varying degrees depending on ambient temp. Keeps you closer to the original boresight thus retain better accuracy over the course of the day. Think of it this way, with cold temps the gun contracts(not visible to the naked eye without MRS) hotter weather expands thus droops. When firing lots of rounds down range the tube will also expand due to heat even though it could be winter. Now going back to your question, I am not 100% sure but I feel gun droop would not apply to either traditional firearms nor air rifles. They are light, tiny in comparison comparison to the above. Even if a metallurgic engineer can confirm there is some it would so tiny, I mean tiny, that it is negligible. If your zero changes from day to day or once in a while, I would most likely attribute that to several possibilities. Example would be in my mind would be ambient temp, any crosswind in the first 10yds of the initial muzzle blast affecting flight as well as crosswind down range. There may not be crosswind at the firing position but at target area there could very well be. Ammo is always a factor. No two rounds/pellets are ever the same therefore slight deviances do occur as grouping shows you. (quality, grains or dimensions etc) loose scope mounts(it happens ,torque to specs) elevation where you live matters especially for example shooting in the south on sea level or below sea level ground elevation. Example firing you Georgia zeroed rifle be it air/traditional shooting the same zero in Colorado would result in rds flying higher due higher elevation and thinner air vs thicker humid air in the south (i.e Georgia or Florida examples) I mean reasons can be vast and unique to your experience. I have experienced taking a perfectly zeroed air rifle out of the home storage where inside is maintained 70 -73 degrees (weapon is cold) and on a summer muggy morning upper 70's or 80's bench shoot and its off! But if you let the rifle rest long enough in that temp and it catches up to ambient temp it will shoot accurately again after a few shots. Nothing wrong with the zero. Not droop, just metal contracting and expanding we just can't see it with human eye. Yes you probably guess, I was a Tank Commander/Platoon Sergeant for 20yrs(active) seen combat, fired many weapons during my time both US and NATO weapons including inferior Russian Assault rifles and one sniper rifle(old SVD 70) I thought one was kind of cool just the one time. Sorry for a long explanation. I never heard some use the term DROOP here before with all of these shooters in this forum.(DROOP sparked a hidden enthusiasm for my former occupation) Most probably can't relate to what I am saying if they didn't have the honor and experience I had. A combat Vet could. I still reminisce. Peace out. 
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