Educate Me on MP5 Lowers and Trigger Packs

I am looking for some assistance in understanding the various types of MP5 lowers, trigger packs, and trigger pack components. I see terms such as “SEF, semi, foul auto, Navy, FBI, ambi, etc” when lowers and trigger packs are discussed. To add to the complexity, I was looking at possibly assembling a trigger pack from individual components, but there appears to be “series 90” parts as well as “full auto” springs or “semi-auto” springs, etc. I am trying to determine what combination of lower, trigger box, and parts is compatible.

I would likely be using a Magpul lower and an AP5 upper so if you can tell me what combo of parts works for that setup, I’d appreciate it. Thanks.

Hoser said:

I am guessing this is going on a Registered Receiver or?

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It will remain a semi auto SBR built from a Century AP5. As I look at swapping parts for 922R compliance, I am trying to determine the feasibility of replacing the entire lower half (the housing, the trigger box, sear, trigger, all the springs, etc). I am confused as some parts say full auto, which suggests that they will only work with guns set up as full auto, but that is not necessarily the case as a full auto M16 BCG can work in a semi auto AR15.

I want to ensure that 1) I don’t inadvertently possess or construct a lower half that is capable of shooting full auto and 2) that the combination of parts (certain type of lower housing, certain type of trigger box, certain springs, etc) are compatible, thereby preventing me from having to purchase additional different parts.