Originally Posted By TSH77769:
There is plenty of parts compatibility. However there is more parts compatibility on the Gen 4 and E4 (all E4 are Gen 4) then previous generations. Mostly because of the stocks. If I was going to purchase an MSR today it would be a Gen 4 and only a Gen 4 and especially an E4.

On Gen 4 models...

Muzzle devices: compatible either way assuming correct thread pattern. MSR barrels were 13x1 left hand. I've heard rumors of some half by 28 but I've never seen one.

my E4 is threaded 1/2-28.

Barrels: not compatible either way but read on down below about one interesting exception. Also note that MSAR barrels were contract manufactured. I forget the contractor. They are high quality and Chrome lined unlike current production Aug barrels and they are fluted and lighter as well. They are also 1 and 8 twist!

Gas plugs: I can't be 100% sure but if my memory serves me they interchanged either way.

Gas piston: as per previous entry.

Gas plugs and pistons are compatible.

Vertical foregrip or replacement rails: an MSR grip will fit on an aug but we'll have a tiny bit of side to side play because the stem is a hair thinner. An Aug grip will not fit on an MSR unless you shave it down a few thou because the stem is slightly thicker. Same deal for the replacement rails such as by rat works and others. Good news is that on many of those replacement rails they are two pieces the part that actually interfaces with the barrel is a separate part and they are aluminum and thus easy to fit. By contrast the stem of the vertical foregrips is steel iirc.

Spring inside the grip is usable either way.

The pen that the grip rotates on is compatible either way. It is different in it's retention method. The AUG has a tiny little groove on the pin that the spring interacts with. The MSR pin is retained by a flange on one side and a little eclip on the other side. The diameter is the same and either one will work either way. For that matter a regular pin without the groove would work fine if it was tight enough or peened over on the ends.

Grip caps I don't recall last time I tried it however if I recall correctly will go either way.

Receivers and the optics rails: very different and not compatible either way. However some people have modified Aug rails to fit on an msar although it required drilling screw holes through the AUG rail.

Charging handle: the charging handle itself the part you actually grab is interchangeable and you can put aftermarket ones on the MSR. The shuttlecock itself the part inside the receiver is different because the MSR does not have forward assist. They are conditionally compatible in the sense that you could put in Aug one in there iirc but you wouldn't have forward assist even with the AUG one in there because the bolt carrier for the MSR doesn't have the groove on the thruster cap to interact with.

Stocks; A Gen 4 msar stock will fit on an Aug and vice versa. However if you want to use your last shot bolt hold open you need to have the matching trigger pack.


Butt plates: very different not compatible.

Mag release: for waffle mags compatible either way including aftermarket. For NATO mags very different and not compatible.

Trigger shoes and trigger bars: compatible either way. Arid triggers are known to work.

Safeties: compatible either way. However some aftermarket ones not so much.

Ejection Port covers: very different not compatible either way.

Bolt carrier bolt cage and Bolt: not compatible either way. However rat works makes MSR bolts that use AR extractor's ejectors and springs. They also make the bolt cage and cocking rail peace.

NOTE: I have not testfired this however I have test fitted this... The reason why the bolt carrier itself not talking about the bolt but the bolt carrier is not compatible appears to be because MSR use the larger diameter pieces as found in the full auto Aug. Theoretically this might mean it might be possible to use mill surp bolt carriers.

NOTE: I just now took an MSR Bolt and only just the bolt and put it in an aug full auto carrier with Aug cage and it fit and cycled full stroke and appeared to lock up fine.

[color=#ff0000][/color]good to know!!!!

Cocking piece is still completely different though definitely will not fit! But as mentioned elsewhere rat works makes them.

Firing pins: test fitted but not fired they appear compatible either way.

can confirm they are interchangeable.

Sling pin: compatible either way. MSR sling pens have qd sling Mount cups. However they do not have the additional grooves that allow you to remove the butt plate while retaining the fire control group or allow for adjustment of the gas settings.

Fire control group: most but not all of the parts are compatible. For example the cassette itself is different because the last shot bolt hold open is different but if you don't care about the last shot bolt hold open it would work and then also you can modify the cassette pretty easily because it's polymer. Titus arms makes a 3D printed cassette with an AR trigger for the AUG and I believe has said they would be willing to make MSR cassettes if they got enough orders.

Upgrade options for MSR fire control group include the Red springs the trigger tamer the neu trigger although I've heard they're out of production now and the 20/20 seer absolutely fantastic but definitely out of production.

Okay special note about the barrels!
Someone on another forum has proven with video that a heresy design not Steyr but heresy design 9 mil conversion kit works on the Msr! Apparently that is because the barrel flange that interacts with the receiver is just different enough there is no gas piston and the barrel isn't actually locking into the rear trunnion or locking up together with the bolt since it's straight low back.

That's all I can remember off the top of my head right now in the middle of the night on no sleep.

EDIT: The bender zee over travel stop should also work as a FCG upgrade. His waffle mag well is also known to work on the STG556. Not sure about the Nato mag well or his brass deflectors.

Originally Posted By jntmjt1:
I'd like to talk about the barrels a little bit... I've always read the barrels are not interchangeable.  I am a gunsmith and had a cutomer's AUG barrel in my shop.  I grabbed my MSAR and put the AUG barrel in it.  To my surprise, it locked right in place as it was made for it.  Then, I took my headspace gauges and checked it out.  The AUG barrel in my MSAR passed both the go and no-go checks perfectly.  So... I'm not sure if there was something I'm missing, or if it's just internet lore that's been passed down over the years that the barrels will not interchange.  Obviously, since the AUG barrel wasn't mine, I didn't want to test fire it.  But, I spent quite a bit of time trying to figure out why it wouldn't work and couldn't come up with anything.  Any insight?