Yeah the SIG 55X has longer barrels. I believe G&G, King Arms (repacked by Cybergun) and ICS are making some.
You have the choice between the 551 (M4 length more or less), 550 and the 556.
I prefer the last one given you have a retractable stock.
I recommend a scope like this one also :
http://shop.ehobbyasia.com/accessori...cal-scope.html
The flips are pretty crappy but the scope itself is very good and has a very decent eye relief, my face is about 15/20cm away from the scope and perfect sight, also the 1-4 zoom makes it possible to use it either as a scope or simple red dot.
I have it on the SR25, best scope ever bought (aside from the one on my 98B but not the same price).
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Gun doc in Montreal on AEG, GBB, GBBR and Bolt rifle (gaz and spring)
Heavy custom builder (including painting)