So How Are SIG LE Guns Different From MIL Guns...?

[SteyrAUG]

Talking about the Swiss rifles.

I have a SG551 and SG552 Commando which were brought in as LE guns but they are NFA because they are SBRs and NOT machine guns or else they would both be post samples.

So what changes were made to the FA to be importable as semi autos?

DISCLAIMER: I am NOT asking for information on how to do a conversion, legal or otherwise. I'm only asking the difference between the semi and select fire variants.

As a "for example" having a sear hole makes an AK a MG, having a push pin hole makes a HK a MG, etc.

But I'm looking at pictures of select fire SIGs and aside from the lower (which has select fire options) I can't see the difference. And I'm assuming you can't just replace the current lower with a select fire lower and have a "functional" MG. But I still can't see what has been changed to make that not the case.

[motoduck]

There are several differences. The full auto vs the semi lower is one. I can't recall if the semi guns have a full auto bolt carrier or not (the semi carriers have the curved trip surface milled out). The semi uppers "receivers" have a small "block" welded on the inside left rail to block the hole where sear trip on the full auto lower fits. If this block is removed the reciever becomes a "machine gun".

[JoshNC]

Upper receiver: Just like the AK, the 55x series has a slot milled in the bolt rail (right side on the SIG), that allows the automatic sear release lever to fit up into that slot in order to be tripped by the bolt carrier. The semiauto 55x has a steel "carriage block" welded in place over the slot in the right bolt rail. Interestingly all semiauto 55x upper receivers start out life essentially a MG, then have the carriage block welded in place to prevent fullauto.

Bolt carrier: Has a trip surface at the bottom/rear right side that engages the automatic sear release lever in the lower.

Lower receiver: Has the automatic sear release lever axle, which is welded in place on the right side of the lower. This is that circular glob of weld on the right side of the receiver. The semi lowers on your rifles appear (based on my memory of previous photos you've posted) to have started life as fullauto lowers that had the axle milled out.

The short answer to your question, what makes the rifle a semiauto, is this - from a BATF technical standpoint, it is the carriage block within the upper receiver.

I have photos of the post-sample fullauto 551-2 we have at the range where I used to work. I will post photos detailing the differences in the auto receiver later this evening.

[motoduck]

JoshNC

Ok ,my description "sucked".

Thanks for posting a much more detailed and accurate discription. My recollection was the sear trip and block were on the left?

[SteyrAUG]

The short answer to your question, what makes the rifle a semiauto, is this - from a BATF technical standpoint, it is the carriage block within the upper receiver.



And Thank You. That is the info I needed.