All the world is a compromise. With a bullpup you get short, but at the cost of wonky handling characteristics and generally rather second rate triggers.
With an SBR (especially an AR) you get high gas port pressure, and all the issues that come along with it. Even with the 14.5” barreled M4’s, over-gassing is a persistent issue that few ever talk about. Much less an issue in semi-auto, but can be a full on nightmare in full auto! In full auto, the issue is very high cyclic rates, which causes all sorts of issues. Magazine timing issues, overheating, extractor bounce, early opening, etc. In semi auto you can still have those issues, but they become much less pronounced.
The issue is much less when the weapon is new. But as gas rounds off the gas ports, it causes the gasses to travel at an even higher velocity, which just exacerbates issues.
In 2008 at the battle of Wanat there were a large number of weapons that became unserviceable due to heat related issues. When they traced it all down, it was found that the issues were related to high cyclic rates on full auto. Earlier detection of worn gas ports on the weapons could have helped greatly. Given the circumstances they were under, it’s likely they were going to experience overheated weapons no matter what, but M4’s with non-worn gas ports would have delayed the point of failure; which is never a bad thing.
When you SBR a rifle, especially an AR, then you have to know that you will wear out gas ports much earlier, and you will have to address over-gassing. Most are using adjustable gas ports which is a good start, but even they can be worn out. So you just need to know what you’re getting yourself into with an SBR AR.
Issues related to over-gassing on something like an AK are much less critical, as the AK has much farther travel of the bolt carrier group, which adds a touch of a delay in the cyclic rate, and also gives magazines more time to advance the next round.
