OK, I know that on an HK94 (or clone) the trigger group detaches when you field strip it.  I also know that there are a number of different options for trigger groups - FBI/navy group, the so-called SEF trigger group, the trigger group that just has safe and fire.  Many people seem to prefer the FBI/Navy group and are willing to pay a premium for it?  Why?  

Also, it is my understanding that the chief difference between a full auto MP5 and the semi-auto HK94 is the trigger group and that the full auto group attaches with a pin, while the semi-auto version does not.  Are all the groups I listed semi-auto.  Will they attach to a HK94 without having to be modified?  Kind of like the look of the Navy group and the fact that it's ambidexterous (since I shoot lefty), but  don't want to buy it if it won't attach to my HK94 clone or if it turns my gun into full auto and me into a felon.

Trigger groups and lower housings are two separate parts, and do not make a machine gun by themselves. The plastic Navy lower with an SF (safe or fire) is readily availble for the HK94 and its clones. The cheapest route for this is from Top Notch Parts. If your current lower features a single-sided selector, you'll need a new trigger pack with an ambi selector to use this plastic lower though. I paid around $100 for a used metal lower with a trigger pack for mine.  

Full-auto lowers, or the housings intended for the MP5 family, are held on with a push pin at the front. These will NOT fit on ANY semi. The BATF regs that allow the production of a semi 94-style carbine specify that the push pin hole on the receiver must be completely blocked, and replaced by a "shelf" for the lower to "clip" onto. Hence, you will hear the term "clipping and pinning" a lower to make it fit a semi. This is the machining process to cut a groove in the FA lower to fit the shelf on the receiver, then cut a pushpin in half and glue it into the holes on the lower to make it look like a real FA.

The semi Navy lowers sold by Top Notch are already clipped, but he pin holes are molded solid. Many people say they like this look better, as it's not such a "poser" gun for the airsoft-type crowd of FA wannabee's. That's a matter of opinion only.  Since you already have a clone 94, using a real HK German lower is, in my opinion, a waste of money, unless you are having reliability issues and intend to replace the bolt/carrier and other parts with HK stuff to make it run right. But that's a whole 'nuther can of worms.  

I have a Bobcat BW5 that I bought to be a shooter. I purchased the plastic SEF lower because I'm right handed, and I like the thumb-rest, the looks of it, etc. Again, I had to replace the "guts" of the trigger housing with those from a single-sided metal lower, and the whole shebang cost me about $200, and I still haven't paid somebody to "clip and pin" it for me (probably another $100 or so).

FWIW, my Bobcat experiences FTE problems about every 20 rounds or so, and replacing the Top Notch lower (that came on the gun) with a German metal lower has made no difference on the functioning problems, telling me it's probably either a bolt/carrier issue, or just crappy tolerances in the overall build.

As long as you have a semi receiver without a push-pin receiver, you'll never be able to make an illegal machine gun just by added what are otherwise labeled as machine gun (MP5) parts. However, you are not supposed to run machine gun parts in a semi rifle (ie. M16 bolt carrier in an AR15 is a no-no). The reality is though, many factory HK94's have MP5 bolt/carriers in them, but without the sear/trigger pack, it doen't do anything different functionally.  Just having a three or four position lower (burst, SEF, etc) does not make your 94 clone an MP5.

Quoted: OK, I know that on an HK94 (or clone) the trigger group detaches when you field strip it.  I also know that there are a number of different options for trigger groups - FBI/navy group, the so-called SEF trigger group, the trigger group that just has safe and fire.  Many people seem to prefer the FBI/Navy group and are willing to pay a premium for it?  Why?

The only reasons I can think of why someone would want a ambi pack over a standard SEF pack is, (1)  Cool value is very high (2)  Burst pack option (3)  Comfort (4)  Left handed shooters

Yes they are expensive, yes they are a little harder to take apart, yes the burst packs go together like a Swiss watch.
If I was looking for new lower, would I purchase a ambi pack over a SEF, it would have to depend ont he gun I planned to use it on.  If you have a German HK94 and you where looking for a MP5, yes.  If you own a Bobcat, no.

Also, it is my understanding that the chief difference between a full auto MP5 and the semi-auto HK94 is the trigger group and that the full auto group attaches with a pin, while the semi-auto version does not.

There are quite few differences between a HK94 and MP5.  Namely, flapper mag release, 8 1/2" three lug barrel, auto carrier, trigger pack and trigger housing(lower)

Are all the groups I listed semi-auto.  Will they attach to a HK94 without having to be modified?  Kind of like the look of the Navy group and the fact that it's ambidexterous (since I shoot lefty), but  don't want to buy it if it won't attach to my HK94 clone or if it turns my gun into full auto and me into a felon.

All MP5 lowers will need to be modified to be used on a HK94.  The auto parts need to be removed and replaced with semi, and the trigger will need to be clipped and pinned.  The modifcation for the SEF are similar to the Ambi packs.

Which ever one you decide to purchase, send the entire lower to a trusted gun smith and have them convert it to semi-auto.  I would guess $150-ish for the conversion.

tag