Sig Sauer MCX SBR: De­signed Ground-up for Sup­pressed 300 BLK


SIG Sauer’s mod­u­lar-plat­form MCX is en­gi­neered to rede­fine out-of-the-box shoot­ing per­for­mance.

It’s easy to see how modern ri­fle de­sign­ers, man­u­fac­tur­ers and shoot­ers alike have em­braced the con­cept of mod­u­lar­ity. This ease of parts-in­ter­change­abil­ity is de­liv­er­ing us from tra­di­tional plat­forms, whose cus­tomiza­tion and mod­i­fi­ca­tion re­quired com­pli­cated, la­bo­ri­ous gun­smithing work, to com­pletely mod­u­lar de­signs such as the AR-15 and its con­tem­po­raries.

Be­ing able to take an AR-15 up­per from com­pany X and slap in a bar­rel from com­pany Y then in­stall it on a lower from com­pany Z is cer­tainly a con­ve­nient way to go. With a mar­ket full of in-spec com­po­nents, the sky’s the limit when it comes to cus­tomiz­abil­ity op­tions. Even if you have a ri­fle cob­bled to­gether with parts from mul­ti­ple man­u­fac­tur­ers, you’ll know that you can rea­son­ably as­sume it’s safe to fire and may even be as ac­cu­rate as one made from a sin­gle man­u­fac­turer.

But in a world where mis­matched “Franken­ri­fles” have seem­ingly be­come the norm, we may not im­me­di­ately re­al­ize that the ben­e­fit of the mod­u­lar­ity of ri­fle com­po­nents, how­ever in­ge­nious the con­cept, might not be per­fectly ex­e­cuted on each and ev­ery build.

Why does one ri­fle shoot more smoothly or ac­cu­rately than another? Some will point out that they tuned their ad­justable gas block or buf­fer. Oth­ers will say it’s their spe­cial recipe for their ammo reloads. When it comes down to it, it’s the in­tan­gi­bles that make it dif­fi­cult to re­ally know why. When you’re mak­ing a stew, all the in­gre­di­ents al­low it work—or, in some cases, not work.

Com­plete Sys­tems Provider

SIG Sauer is a name that needs no in­tro­duc­tion in the firearms in­dus­try. The es­teemed man­u­fac­turer thought long and hard about mod­u­lar­ity and what it meant to shoot­ers when it de­vel­oped its new­est and ar­guably most pro­gres­sive ri­fle to date. The de­sign­ers re­al­ized that only through metic­u­lous en­gi­neer­ing and pre­ci­sion man­u­fac­tur­ing could the true ben­e­fit of mod­u­lar­ity be achieved.

The SIG Sauer MCX isn’t just another AR vari­ant or an AR that’s been up­dated or tweaked. It’s a com­pletely new ground-up de­sign that re­flects decades of col­lec­tive hands-on ex­pe­ri­ence of shoot­ers of all types. Be­fore we delve into the in­tri­ca­cies of the MCX, let’s re­veal what SIG has been up to lately.

If you haven’t no­ticed by now, SIG has greatly ex­panded its ca­pa­bil­i­ties and of­fer­ings over the past few years. Al­ready well known for finely crafted pis­tols and ri­fles, SIG is now also pro­duc­ing its own sound sup­pres­sors, op­tics and even am­mu­ni­tion.

Un­like other big-time con­glom­er­ates, all di­vi­sions at SIG work hand in hand to make sure their prod­ucts are de­signed to work in con­junc­tion with one another. If you are look­ing for a com­plete weapons sys­tem that in­cludes ev­ery­thing from the ri­fle and sup­pres­sor straight through to op­tics and am­mu­ni­tion, look no fur­ther than the MCX.

The Sys­tem

Like oth­ers, when we first laid eyes on the MCX it was with a mix­ture of love at first sight as well as, “Oh, that’s just another AR.” Read on to find out ex­actly how wrong we were about that last sen­ti­ment.

The com­plete MCX SBR sys­tem shown here is a cul­mi­na­tion of mul­ti­ple di­vi­sions and hun­dreds of co­op­er­a­tive man-hours. At first glance, you’ll no­tice the MCX SBR ri­fle it­self as well as the SRD762TI sup­pres­sor. Up top, you’ll see the SIG Romeo red dot op­tic, but the SIG ammo isn’t im­me­di­ately ap­par­ent.

What is im­pres­sive is that ev­ery el­e­ment of the MCX was de­signed to achieve its mis­sion of si­lence, light weight, com­pact size and hard-hit­ting ac­cu­racy out to 300 me­ters.

The Lower

The MCX looks sim­i­lar to an AR-15 be­cause SIG wanted to cre­ate a ri­fle with Ar-style con­trols since so many shoot­ers are al­ready fa­mil­iar and com­fort­able with them. No need to rein­vent the wheel, so to speak. An added bonus is that af­ter­mar­ket AR parts such as pis­tol grips, safety se­lec­tors, mag­a­zines and trig­gers are all com­pat­i­ble.

The MCX’S lower re­ceiver is ma­chined from alu­minum al­loy and fea­tures a flared mag­well and an over­sized, in­te­grated trig­ger guard and houses an Ar-style trig­ger as­sem­bly. The lower fea­tures am­bidex­trous con­trols, so both your strong and sup­port hands can op­er­ate the gun with­out any ex­tra ef­fort. Its mag­a­zine re­lease is larger than that of an AR and ap­pears on both sides of the re­ceiver.

Another am­bidex­trous con­trol is the safety se­lec­tor switch, whose levers re­side on ei­ther side of the lower. How­ever, we no­ticed that it lacked a bolt catch re­lease on the right side to make it fully am­bidex­trous. It’s not a big deal, but is worth not­ing for those who ab­so­lutely need dual-sided con­trols. What it does have are two quick-de­tach (QD) sling mount points mir­rored at its rear end that are great for at­tach­ing sin­gle-point slings.

An ad­van­tage the MCX has over tra­di­tional

ARS is that its butt­stock can be folded tightly against its left side. Un­like an AR, there’s no buf­fer tube to worry about. SIG of­fers a va­ri­ety of dif­fer­ent fold­ing and re­tractable stocks that are pain­less to swap thanks to a quick-change rail sys­tem. It even of­fers an adapter that ac­cepts an AR buf­fer tube so you can fit any of your fa­vorite af­ter­mar­ket stocks.

A tele­scop­ing and fold­ing 3-po­si­tion stock is now avail­able and comes stan­dard. The low­pro­file skele­tonized stock at­tached to our test SBR is op­tional. Both stocks are light in weight, slim in pro­file, and con­tain QD sling mounts on both sides.

The Up­per

The MCX SBR fea­tured here has a 9-inch bar­rel and is cham­bered in .300 AAC Black­out (300 BLK). The MCX is also avail­able in 16inch bar­rel length car­bine, 11.5-inch SBR, 11.5-inch pis­tol and 9-inch pis­tol mod­els with dif­fer­ent cal­iber choices. If you don’t know which one to choose, you’ll be glad to know that you might not have to.

The MCX’S up­per re­ceiver is con­vert­ible for cal­iber and bar­rel length. You don’t need to be a gun­smith to con­vert it, ei­ther.

SIG’S en­gi­neers made sure that any­one, me­chan­i­cally in­clined or not, can per­form the con­ver­sion re­li­ably and safely. By re­mov­ing just two screws, you can re­move and re­place the bar­rel as­sem­bly and hand­guard with dif­fer­ent units.

The MCX op­er­ates on a short-stroke gas pis­ton sys­tem and has a two-po­si­tion gas valve lever to op­ti­mize it for sup­pressed and non-sup­pressed fire. Gas pis­ton op­er­a­tion is a proven sys­tem renowned for its re­li­a­bil­ity. The ri­fle ben­e­fits from re­tain­ing its op­er­at­ing sys­tem in its up­per re­ceiver, mean­ing there’s no need for a buf­fer tube and re­coil spring as in an AR. This helps keeps the ri­fle short and al­lows for a fold­ing stock.

The mono­lithic and free-float­ing bar­rel de­sign of the up­per re­ceiver al­lows the ri­fle to be rigid and ac­cu­rate. A full-length Pi­catinny rail runs across its top, ready for op­tics and ac­ces­sories mount­ing. Our tester came equipped with a set of SIG’S flip-up iron sights for backup sight­ing. As men­tioned pre­vi­ously, the alu­minum-al­loy hand­guard can be in­ter­changed with longer lengths to ac­com­mo­date longer bar­rels, as can even lighter car­bon-fiber hand­guards.

You’ll find Key­mod mount­ing points at the 3, 6 and 9 o’clock po­si­tions on the hand­guards. Robby John­son, Gov­ern­ment Pro­gram Man­ager at SIG Sauer, ex­plains: “We de­cided to go with the Key­mod hand­guard sys­tem be­cause it’s a proven de­sign. With Key­mod we have a cham­fered mount­ing sur­face, so our straight pull strength is up­wards of 280 pounds.”

Real­iz­ing that gen­eral use can cause ex­tra wear to the up­per re­ceiver in crit­i­cal and com­mon wear ar­eas, SIG im­ple­mented steel in­serts in the up­per that can be user-re­placed as needed to in­crease the longevity of its ri­fle.

To push its ca­pa­bil­i­ties even fur­ther, the MCX’S up­per re­ceiver is fully com­pat­i­ble

with mil-spec AR-15 low­ers. How awe­some is that? If you wish to in­stall one of SIG’S fold­ing or col­lapsi­ble stocks on an AR lower to use with the MCX’S up­per, the man­u­fac­turer of­fers a con­ver­sion kit that re­quires only three sim­ple tools and a few min­utes to in­stall. The re­moval of the stock AR buf­fer tube is re­quired, how­ever.


You could say that the MCX was born to shoot Black­out ammo, but that doesn’t mean it’s com­pletely set in its ways. Not one to turn down ammo to munch on, the MCX can be con­verted on the fly to feed 5.56x45mm and 7.62x39mm car­tridges. This same con­ver­sion process also al­lows you to change bar­rel lengths.

Bar­rel and cal­iber con­ver­sions are not new to mod­u­lar ri­fles, but what sets the MCX apart from the rest is that its con­ver­sion process is de­signed to take only a minute and a half. We tried it, and even with­out be­ing fa­mil­iar with the process, we were able to get it done in the same amount of time.

Si­lently Op­ti­mized

The MCX was de­signed to meet the U.S. De­part­ment of Defense’s re­quire­ment for a com­pact ri­fle whose fo­cus is on sound sig­na­ture re­duc­tion as well as com­bat ef­fec­tive­ness out to 300 me­ters. With that in mind, SIG specif­i­cally de­vel­oped the MCX to run sup­pressed on 300 BLK cal­iber am­mu­ni­tion. The Black­out round was op­ti­mized to take down tar­gets at midrange dis­tances and run through sup­pres­sors ef­fi­ciently.

Most sup­pres­sor com­pa­nies do not make firearms. To that end, they cre­ate si­lencers that are made to fit any num­ber of guns. Spe­cific is­sues for each in­di­vid­ual end user such as pres­sure, heat, and even cal­iber us­age can amount to be a guess­ing game. SIG’S big ad­van­tage in this area is that not only do they make the guns, but also sup­pres­sors that they can tai­lor to each ap­pli­ca­tion. This is a cor­ner­stone of what makes the MCX such a com­plete sys­tem. The ri­fle and sup­pres­sor were de­vel­oped to work in to­tal har­mony.

Be­cause SIG’S pri­mary goal for the MCX was to run as si­lently as pos­si­ble, it was pri­mar­ily en­gi­neered to run a sup­pres­sor right out of the box, un­like many other ri­fles with add-on sup­pres­sors. A chal­lenge of mount­ing si­lencers to firearms is re­tain­ing un­sup­pressed ac­cu­racy. Due to man­u­fac­tur­ing pro­cesses and sup­pres­sor de­sign, SIG

“The MCX SBR is a com­pletely new ground-up de­sign that re­flects the col­lec­tive hands-on ex­pe­ri­ence of shoot­ers of all types.”

has been able to at­tain bet­ter ac­cu­racy with less point-of-im­pact shift from the MCX and its si­lencer. This re­sults in the MCX run­ning just as well sup­pressed or un-sup­pressed in terms of con­trol, ac­cu­racy and re­li­a­bil­ity.

The MCX is op­ti­mized for both sub­sonic and su­per­sonic ver­sions of 300 BLK cal­iber am­mu­ni­tion. This al­lows su­per­sonic am­mu­ni­tion to re­tain ac­cu­racy and lethal­ity at 300 me­ters even when sup­pressed. In­cred­i­bly, when sub­sonic Black­out is fired through a suit­ably con­fig­ured MCX, it can give you the equiv­a­lent sound of a si­lenced .22 cal­iber round. Good­bye, sweaty ear­muffs!

The SRD762TI si­lencer we tested is made of grade 5 ti­ta­nium and fit­ted di­rectly to the ri­fle’s bar­rel threads. It weighs just over a pound and mea­sures a tad over nine inches long. Thanks to a unique baf­fle de­sign and ad­vanced ro­botic weld­ing tech­niques, SIG was able to greatly re­duce the si­lencer’s weight by elim­i­nat­ing its outer tube by us­ing its own baf­fles as its tube. This re­sulted in not only a lighter si­lencer, but also a gain in internal vol­ume and re­duced internal pres­sure, which helps in­crease si­lencer dura­bil­ity.


Boldly en­croach­ing onto Aim­point and Eotech ter­ri­tory, SIG re­ally did set out to make sure that its ri­fles could be com­pletely out­fit­ted with SIG Sauer–de­vel­oped ac­ces­sories. To that end, we got to check out its Romeo4 red dot. Sim­i­lar in size and shape to an Aim­point Mi­cro-se­ries op­tic, the Romeo4 dif­fers in that it is ac­ti­vated with rub­ber­ized push but­tons on its top sur­face and is switch­able be­tween a 2 MOA dot and an Eotech style, 65 MOA cir­cle dot ret­i­cle. Like other red dots, it prom­ises un­lim­ited eye re­lief and is par­al­lax free.

The Romeo4’s body is ma­chined from alu­minum and has a listed bat­tery life of 50,000+ con­tin­u­ous hours run­ning on a sin­gle CR2032 bat­tery. We didn’t have a chance to try it, but we would be in­ter­ested in see­ing how the Romeo4 would fare in a longevity and dura­bil­ity test. Sev­eral Romeo4 mod­els are of­fered, the new­est be­ing the Romeo4s, which is equipped with a solar cell that can in­crease its bat­tery life to 100,000 hours.

At the Range

We headed to the range with a cer­tain amount of ex­cite­ment, as we haven’t had

“en­gi­neered to run a sup­pres­sor right out of the box.”

the chance to shoot through SIG’S si­lencers be­fore. Shortly af­ter ar­riv­ing and set­ting up a se­ries of AR500 steel gongs at dif­fer­ing ranges, we went to work by load­ing up mag­a­zines full of SIG’S Elite Per­for­mance Match am­mu­ni­tion.

Us­ing a mix­ture of U.S.G.I. and Mag­pul PMAG mag­a­zines, we loaded both SIG Elite 300

BLK sub­sonic rounds and their su­per­sonic coun­ter­parts. The sub­son­ics fea­ture 220-grain Sierra Match King pro­jec­tiles while the su­per­son­ics are loaded with 125-grain Sierra Match King bul­lets. Yes, fancy stuff—we are spoiled at times. We started with the sub­sonic rounds.

Un­loaded, the MCX SBR weighed in at less than six pounds. With the added op­tic and sup­pres­sor, it still felt light and very well bal­anced. You would think that a sup­pres­sor on one end and a thin butt­stock on the other would mean it would want to take a nose­dive, but that wasn't the case. The ri­fle was well bal­anced, with just a smidgen of the bal­ance felt for­ward of the pis­tol grip. Be­cause of its AR con­trol lay­out, we in­stantly felt at home with it—ex­cept for the fold­ing stock, which took some get­ting used to.

For our first string of fire, we de­cided to go sans ear pro. The sub­sonic rounds are made for low noise lev­els, and we wanted to see just how quiet they are. Clack, ding! Clack, ding! That’s all we heard. It was a sur­prise: The sound of the bul­lets hit­ting the steel man­aged to sting our ears, but not the shots fired from the MCX. We ac­tu­ally had to shoot into the dirt to clearly hear the sound of the gun, as the ring­ing of steel had drowned it out. No kid­ding. If SIG set out to make a quiet ri­fle, it cer­tainly was achieved.

“We’re will­ing to bet that no amount of Franken-build­ing will get you closer to the level of Sig Sauer’s de­sign, en­gi­neer­ing, build qual­ity and shoot­ing sen­sa­tion.”

The MCX was a plea­sure to shoot and han­dle. Its trig­ger was clean and not at all gritty. Its pull mea­sured a bit over 7 pounds on our Ly­man trig­ger-pull gauge, which isn’t bad for a stock trig­ger. That said, we think an af­ter­mar­ket trig­ger would be an out­stand­ing ad­di­tion to this ri­fle.

Nat­u­rally, the sup­pres­sor got re­ally hot af­ter we fired a few hun­dred rounds through it. The MCX we tested had the shorter SBR hand­guard at­tached, so we were wary of get­ting burned by the si­lencer. Gloves on! We sus­pect if we in­stalled the longer hand­guard that par­tially cov­ers the sup­pres­sor, it would bet­ter pro­tect us from in­ci­den­tal burns—of both per­son and prop­erty.

Un­for­tu­nately, we didn’t have an op­por­tu­nity to shoot to 300 me­ters on this par­tic­u­lar range day, so for the su­per­sonic rounds, we had to set­tle for 100 yards. Nat­u­rally, the 6-inch steel gongs were no match for the 125-grain Sierra Match Kings we launched at them. Even sup­pressed, we no­ticed that the hits were all point of aim, point of im­pact.

The day started sunny and very bright, so we had to crank the ret­i­cle bright­ness to a higher level on the Romeo4, but not to the point of max­ing it out. The ret­i­cle was switched be­tween shoot­ers from the plain 2 MOA dot to the 65 MOA cir­cle dot de­pend­ing on per­sonal pref­er­ence. The red-col­ored ret­i­cle was easy to pick up no mat­ter how bright or dark the con­di­tions.

Fi­nal Thoughts

We’re will­ing to bet that no amount of Franken-build­ing will get you to the level of

SIG Sauer’s de­sign, en­gi­neer­ing, build qual­ity and shoot­ing sen­sa­tion. The MCX SBR is more than just a ri­fle: It is the ba­sis of a com­plete sys­tem when com­bined with SIG Sauer de­signed and man­u­fac­tured ac­ces­sories such as the out­stand­ing SRD762TI si­lencer. It re­ally is a tough com­bi­na­tion to beat.

Each of the four shoot­ers who fielded the MCX SBR with the SRD762TI that range day claimed it would be their next ri­fle and sup­pres­sor combo. In this line of work, we are for­tu­nate enough to shoot a lot of ri­fles, so that’s not a state­ment to be taken lightly. SIG Sauer went above and beyond with the de­sign, en­gi­neer­ing and ex­e­cu­tion of the MCX SBR, and we be­lieve that even the most dis­crim­i­nat­ing of shoot­ers will en­joy this ri­fle.

One added MCX bonus is that af­ter­mar­ket AR parts like pis­tol grips, safety se­lec­tors, mag­a­zines and trig­gers are all com­pat­i­ble with the SIG Sauer sys­tem.

The MCX lower re­ceiver fea­tures am­bidex­trous con­trols, so both your strong and sup­port hands can op­er­ate the gun with­out any ex­tra ef­fort.

The MCX is avail­able in 16-inch bar­rel length car­bine, 11.5inch SBR, 11.5-inch pis­tol and 9-inch pis­tol mod­els with dif­fer­ent cal­iber choices.

The MCX was a plea­sure to shoot and han­dle—clean, never gritty. Trig­ger pull mea­sured a bit over 7 pounds, which isn't bad at all for a stock as­sem­bly.

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