SIG MCX: When is an AR not an AR?

[M4Guru]

Like others, I am eagerly awaiting the conversion parts. The 16" I have does not really interest me; I bought it for what it could be, not how it ships. Hopefully SHOT yields some aftermarket support in the form of hand guards as well.

I will probably chop the barrel down to 14" and pin a Surefire hider on it, but I would like an 8-ish .300 and a 11-ish 5.56 to round out the kit. I want the HK-style collapsible also, because it looks cool if nothing else.

[Drummer]

Just out of curiosity I pulled a GI bolt carrier group and weighed it against my 300BLK set-up minus the spring components. Weights of the actual bolt parts appears identical. I would guess the 5.56 MCX parts are slightly heavier due to the longer piston rod interface.

Attachment 36341

Interesting. Thanks for the info. If you remove the bolts are the weights still similar? The ratio between the bolt and carrier is part of the equation.

Also, the SIG doesnt have the weight of the buffer behind it or the dead blow effect from it.

[Tokarev]

Interesting. Thanks for the info. If you remove the bolts are the weights still similar? The ratio between the bolt and carrier is part of the equation.

Also, the SIG doesnt have the weight of the buffer behind it or the dead blow effect from it.

You are correct that the MCX lacks any additional mass behind the bolt carrier. It relies/depends solely on the twin recoil springs that run at 10:00 and 2:00 along the bolt carrier. I speculated earlier in this thread that LAV's experiences may have been issues with bolt bounce from the lack of a buffer.

Anyway, I didn't weigh the stripped carriers or the bolts themselves but they should be about equal. The SIG bolt is probably just a tad lighter without the gas ring flange.

[Drummer]

I think we're tracking as far as the but bounce issue. It's interested that piston-driven ARs usually need H2 or H3 bolts.

[JoshNC]

You are correct that the MCX lacks any additional mass behind the bolt carrier. It relies/depends solely on the twin recoil springs that run at 10:00 and 2:00 along the bolt carrier. I speculated earlier in this thread that LAV's experiences may have been issues with bolt bounce from the lack of a buffer.

Anyway, I didn't weigh the stripped carriers or the bolts themselves but they should be about equal. The SIG bolt is probably just a tad lighter without the gas ring flange.

It seems this could be rectified by a counter weight buffer in the carrier, similar to that of the FN P90/PS90 or conceptually similar to the 22lr bolt group sliding counter weight.

[MountainRaven]

Sounds like a high speed camera is needed for testing/observation.

[Sqr]

Our range held an OEM day with SIG being the major sponsor by demonstrating a number of hand guns and a couple of AR with the MCX 5.56 being one. Great rifle to shoot sans the folding stock that was to short for my arm length. The rep did mention that a longer version was available.

The piston action was smooth along with a straight back recoil allowing for a quick double tap.

[krm375]

Saw on SIG's Instagram account that the 16 inch 300blk MCX are being shipped.

[WatchTheWorldBern]

Does anyone have the slow mo setup to do tests on carrier bounce? Pretty much everything else about this rifle has me wanting to pick one up (ok, would prefer m-lok—if we're lucky, maybe at SHOT 2016?), but I can't shake LAV's assessment of it.

[interfan]

I just got the barrel taper adapter today. I have been travelling, so it actually arrived on Monday. That is pretty quick for Sig's CS. I am going to swap the flash hider for a Surefire Warcomp either tonight or this weekend, depending on time availability. The taper adapter gives you a square shoulder to index on, so it looks pretty straightforward.