I fell in love with this MP5/M4 hybrid the first time I saw it, and finally managed to find one for a good price [It's harder than you'd think, as the SOB line was discontinued years ago].
List of current upgrades, and tentative upgrades/mods/fixes/I-don't-have-a-clue-what-I'm-doing are below...
External upgrades to date:
1. Magpul Ladder Rail covers in olive drab
2. Black Stubby Foregrip [Magpul OD AFG in picture]
3. Magpul 9mm Magpuls
4. Aftermarket metal iron sights and QD flashlight with strobe function [flashlight in picture came with the gun]
5. Clear fingernail polish coating over the selector switch markings [which have a nasty tendency to rub off, eventually, I'll have to replace the markings with a more permanent, cosmetically appealing alternative. In hindsight, fingernail polish probably wasn't the best way to retain the markings...lol]
6. Self-adhesive loop side of velcro inside stock adapter
Internal upgrades to date:
1. G&G Green [temperature resistant] hop-up bucking and nub
2. 370mm 6.03mm AirsoftGI chromed inner barrel [to replace the original brass one, which was only 229mm long; barrel in picture is 363mm brass barrel]
3. 4.5" M4 outer barrel extension [to cover the extended inner barrel]
4. 2" Madbull outer barrel extension with o-ring [NOT shown in picture]
5. 9.6v 1600 mAh nunchuck battery [came with an 8.4v nunchuck battery]
*note, no upgrades have been performed on the GB to date.
After these upgrades, the SOB1 chronoed in at approximately 430 FPS with a .20g BB. It also exhibited a decent increase in range and accuracy [not that it had actually been lacking in either before]. Most SOB1's had characteristically bad stock wobble, and mine has also developed slight RIS wobble as well. But I got it used [and abused], so that's to be expected. Some of my tentative upgrades will be aimed at eliminating all wobble.
CURRENTLY BEING OVERHAULED, DISREGARD ABOVE DESCRIPTION [or don't, it's funnier that way.]