Steyr LP50 Standard?

Yes, the LP-S used the action of the LGB1 Biathlon rifle and the same magazines. The trigger is from the LP50. Correct, the mags for the LP50 don't fit the .177 Hunting 5 rifles, those mags insert from the opposite side of the action and are quite a bit bigger, probably a much more robust action for the additional power and to accommodate the .22 cal . Both mags in pic are .177. 

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When Steyr discontinued the LP-S Silhouette pistol in 2016, they also introduced two new models of the LP50. The RF (Rapid Fire) and HP (High Power) were meant to replace the LP-S and add more versatility for shooters. Both come in full size and compact barrel length. The RF is rarely seen as most vendors choose not to carry it for some reason. It is in the middle, power wise to the Standard and High Power. Standard being up to 5 joule, RF being up to 7.5 joule and HP up to 14 joule. Silhouette shooters can also choose the same grip used for the LP-S, see pics. Bullseye shooters can choose either for training in rapid fire and timed fire at 25 yds or closer. They also offer a weaver rail for optic mounting and a benchrest plate for Bullseye Benchrest or Benchrest Silhouette. 

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That's a tough find. Never seen one for sale anywhere and that includes the P55 & 56 . Coldair on this forum has a 58 I believe. Look him up maybe he will sell. He also has an LP50 so maybe he'll part with the FWB, never know. 

From what I know they are very expensive ($2600+) new which you would think would trickle down to used. Mags are approaching $100 each new even at Krale. Lots of "stories" about issues but believe most are sorted with the 58.

Suggest a WTB on TargetTalk, much more likely to find one there. 

Good luck!