Compact model SIG rifles have notoriously small battery bays. The Marui is probably already the most generous of the bunch. I believe there's a battery tray you can remove to make fitting a battery easier, but keep in mind that your battery choices will be limited by the physical space you have. An external battery box would be a good choice for a larger battery. A lipo would fit in nicely, but I'm not really sure a stock Marui 552 is up to using a Lipo.
If you ever decide to modify the front hand guard, there are a bunch of cheaper RIS/RAS options these days, although IMO it ruins the aesthetic of the SIG.
One upgrade you might want to consider, if it's not already installed, is a top rail. The drum sights on SIG rifles are pretty low, and I personally have a lot of trouble sighting in with my face guard on. Using an RDS or zoom scope helps with this. If you find that using the scope this way is uncomfortable, try wrapping some gauze around the stock where you rest your check as a DYI cheek rest.
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Last edited by TaroBear; March 22nd, 2012 at 12:26..