So a quick update. Tried out a variety of other pellets this morning. Turns out mine loves the JSB Exact Heavy Diabolo in 18.13 grains a lot. Here are five back to back 5-shot groups at 25 yards shooting off a Primos tripod rest in a seated position. Gun is stock from the factory with no tune. 


 
Keep an eye on the two small stock screws and the trigger guard screws. Mine have loosened twice, the second time after having applied blue loctite. It totally ruins accuracy and is quite frustrating.

The guys in my club are saying to detune it to 12 FPE with a Vortek kit to smooth it out. And not to shoot the heavy pellets that it likes, as doing that will cut the spring life short. 
 
"dr_beretta"Thanks for the tip Smaug. I tend to check all screws after each shooting session and tighten them up if needed. I used blue loctite on all the screws prior to shooting it. At least I'll know what to look for should accuracy suddenly change on me. So I'm guessing the gun's pellet of choice changes when you change to a Vortek Kit?
I don't know. I'm still shooting it stock. I'm going to hold out awhile and see if it'll hold together in the stock 16 FPE tune with some more loctite and patience.
 
I looked into the Vortek Kit for the gun and while tempting, I can't get myself to purchase it and go tinkering with a gun that isn't broken. I'm glad that at least parts are easily found to purchase here on this side of the pond including OEM springs and basically anything else to rebuild the gun. Maybe if the gun goes to hell and breaks down then I'll either see if the cost of me replacing things would be cheaper than having it worked on by the folks at Pyramyd Air where I purchased the gun. I know you mentioned that the spring may fatigue due to shooting the heavier pellets over time, so I checked the Vortek site and they was selling them for about $18 so I'm not too worried to use them.
 
Congrats......the tx200mk3 is a fine springer and looks like yours is performing as it should. Mine liked both the jsb 15.89 and 18.13 the best with a slight edge to the 15.89gr. Give em a try if you havent. I did vortek mine and it improved the accuracy alot. However.......mine out of the box was not shooting as well as yours is out of the box. I have to agree....no need to vortek yours right now. The stock spring on mine had a slight bow in it lengthwise. That bow randomly changed the recoil of the rifle depending on how the spring was rotated from shot to shot. The stock spring has no spring guide so it can recoil both forward and out in any direction if its not perfectly straight. After the vortek, which has a spring guide, mine was lights out accurate.
 
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