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You could have your barrel rotated 180deg...but it's easy to have the nub fall out and not notice it.
I'm trying to recall correctly....but I think the Sig hopup unit is very similar to the G3/G36/UMP hopup unit.
There's a big adjusting ring on the front of the hopup unit that you turn to adjust the hopup. The inside of the ring has a slot that is spiraled cut into it. On the hopup body, the hopup arm has a tab that fits into this slot. As you turn the ring one way, the tab gets pushed down (more hopup) and as you turn it the other way the tab rises.
Under the tab is the hopup nub. The nub pushes down onto the hopup rubber/sleeve that goes over your barrel. The nub is always on top of your barrel (same with any AEG/GBB)
The hopup rubber/sleeve has a ridge running the length of it...that fits into the slot along the bottom of your barrel.
You'll notice the barrel (with the hopup rubber/sleeve off)...as a square notch cut right into/through it. That's the top and it's this window which the bump of the hopup rubber/sleeve gets push down into the bb path.
As you rotate the ring...you should see the hopup rubber/sleeve bump protrude down into the barrel.
So, got to get the hopup rubber onto the barrel the right way (ridge goes into slot). Have to insure the nub is in the adjustment arm in the hopup body. Have to make sure the "window" of the barrel is aligned with where the nub will push down into it. Have to make sure that the adjustment ring is moving the tab. Put on the barrel clip to lock the barrel into position....and you're done (almost)!
Best of luck,
Tys
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