Sig MPX range report

[herkyguy]

third time out with my MPX and comfortable enough with it to post some thoughts.

I've got the Gen II 13.5" barrel with a 2.5" flash suppressor and collapsing stock. Lucky it's not the earlier version with the tuning fork flash suppressor - that thing was absurdly annoying to listen to it ping.

I got a great deal on it through gunsamerica and paid $1500 plus shipping & FFL. It came with one mag, several rail attachments, a QD mount for the forend plus a sling.

First two times out shooting were indoors at 20 yards with zero failures, although there were a few pierced primers with factory ammo. The only thing I can see is a tiny burr on the firing pin, but internet searches reveal that I'm not the only one with the occasional pierced primer. Anyhow, zeroed about an inch low at 20 yards and had a blast with it those first few times.

Getting it home, I found it insanely easy to disassemble. If you can take down an AR, it's a few steps easier with the MPX. Forend slides right off.

Today I finally got it out to shoot some steel and a random toilet lid (which actually makes for a surprisingly durable target). I shot 100 rounds of Berry's 147 grain flat nose bullets sitting on top of 3.8 grains of 800X at a COL of 1.05" - I made the mistake of seating them a bit too deep. I had previously tested 5 rounds out of my XDs and they wouldn't cycle the slide. I was hoping they'd work with the MPX's longer barrel and medium burn rate and I'm happy to report that they were flawless. I forgot my chrono, but they were a light-recoiling and fairly accurate load, spitting the brass all within a foot of each other. No signs of pressure spikes. I also shot 50 rounds of Federal steel cased 115 grain, again with no failures but an impact about 4 inches higher than the 147s at 50 yards.

Pros:
Trigger is crisp. Minimal creep. One of the best features.
Minimal recoil, followups are fast.
Plenty accurate at 50 yards (3" group standing unsupported).
No failures yet at 250 rounds total through it.
Mags are solid, albeit pricey.
Reliable. Reliable. Reliable.

Cons:
Burr on firing pin - several pierced primers during first outing.
Little bit of wiggle between upper and lower.
Stock attaches to rail, had potential to wiggle loose.
Not the smoothest finish, but doesn't show any tool marks.
Windage is halfway to the right to cowitness with my Leupold Prismatic.

Overall, I dig it. I'm really interested in working up a hand load of 147 grainers with medium burn powders to see what I can get from the extra barrel length.

Lastly, Gen I and Gen II mags have different lips for feeding. I've played with two Gen I mags and they wouldn't feed one brand of ammo, but have worked with all the others. I'll stick with Gen II for most of my shooting.

 

[MCMXI]

Nice report on your MPX. I just picked up my MPX SBR (8" barrel) this morning so haven't had a chance to go over it yet. I ordered two additional 30-round and two 20-round Gen 2 mags that will be here on Wednesday. I'll be running an AAC Ti-Rant 9 on it and will be dropping in an auto sear at some point. I ordered mine sight unseen and all I did was take it out of the box during lunch. I like the trigger, I like the iron sights, I like the ergonomics and I was surprised by how heavy and solid it is. I'm looking forward to taking it apart to see how it's similar and also different to an AR. I might shoot a few rounds tonight out back ... no, change that ... I will shoot a few rounds tonight out back! I will add that it looks awesome and feels just as good. I've been a SIG fan since my first P220 in '92 and that positive experience has continued with many more SIGs over the years including the P320 TACOPS that I bought recently. I think I'm going to like this MPX SBR just as much ... and maybe more.

 

[MCMXI]

Here's my new MPX SBR with an AAC Ti-Rant 9. The P320 also shown has the newer AAC Ti-Rant 9M screwed on which is modular and shown in the shorter version. I took the MPX apart last night and I agree that the ease of removal of the forend is very slick with the front take down pin locking it in place. It didn't take more than a few minutes to remove the factory flash hider after pulling off the forend and securing the upper in a barrel vice. I haven't delved into it yet but the barrel appears to be held in place with two captive screws making barrel swaps easy. The overall design seems to be well thought out but SIG makes the point that you shouldn't use standard AR triggers in the MPX due to cycling stresses on the trigger. My gf and I shot a few rounds each this morning using 124gr Remington factory ammunition and we were both impressed. She likes the stock in the middle position, I like the stock fully extended for a "nose to charging handle" shooting position. The LOP does feel very short but it's a "submachine gun" after all so that's to be expected. The trigger is ok, felt recoil is basically non-existent, the staggered magazine is easy to load and the MPX seems to function just fine. I'm looking forward to putting together some 147gr handloads for the ultimate in quiet. It's hearing safe with the 124gr supersonic stuff but it'll be even more pleasant to shoot with subsonic ammunition. The upper to lower fit is more than acceptable for me given that this is not a precision rifle. I noticed the action already feels smoother after only 20 or so rounds and I expect that both the trigger and safety will smooth up a little with use. At this point I give SIG and the MPX a solid 10 out of 10.

MPX SBR with 8" barrel and collapsed stock



MPX SBR with 8" barrel and extended stock

 

[herkyguy]

Looks good. I found the 147 grain rounds with 800x had less felt recoil than the steel cased 115 grain factory stuff. No plans to suppress mine anytime soon (we'll see if suppressor legislation changes), but the subsonic heavy rounds with medium burning powder gives virtually no muzzle rise..... follow ups we're almost too easy.

 

[MCMXI]

I found the 147 grain rounds with 800x had less felt recoil than the steel cased 115 grain factory stuff.

I've just come to the realization that I probably don't have any powder suitable for 147gr 9mm loads. The only pistol powders I have are Win 231, H110 and Trailboss. That sucks!! I need to order powder and bullets. Time to fire up the old Hornady L-N-L.

I did receive four Gen 2 magazines today and had to Google the difference between the Gen 1 and Gen 2 mags to make sure that I received the correct ones.

 

[vtsteve]

I run mine with 3.6gr of titegroup over a plated 147gr. It's nice and quiet with a can, cycles great, and has none of the blowback I've heard others complain about when shooting an MPX suppressed. Mine is a gen 1 FWIW.

I'm still wondering if SIG will ever deliver on the caliber conversions we were promised. I'm not holding my breath.

 

[MCMXI]

Mine is a Gen 2 and gas blow back is a real issue with many suppressed firearms, particularly if they're running full auto. With such a short LOP and the "nose to charging handle" hold it's very easy for the gas/oil mix to blast the shooter in the face.

I have 1,000 147gr plated bullets inbound but still haven't picked a powder yet. I have masses of 231 for the standard 9mm stuff and am thinking about VV N330 or VV N340 and according to Hodgdon's website I should be looking at Winchester WSF, Hodgdon Longshot, Winchester 572, Hodgdon 800-X, IMR SR 4756, Hodgdon HS-6, Hodgdon CFE Pistol, Winchester AutoComp, Hodgdon Universal, IMR SR 7625, Hodgdon Titegroup and Hodgdon 700-X. I like choices but this borders on ridiculous.

Has anyone tried Winchester WSF? It's supposedly a shotgun powder but it works well with the 147gr 9mm subsonic loads.

 

[MCMXI]

I picked up 1lb of Winchester WSF this afternoon for $25. I figured I'd try it for the 147gr loads before ordering 8lb of it from Powder Valley. Fortunately the local farm store has a decent selection of reloading components. The bullets should be here early next week so I'll be able to provide my own feedback on the MPX shooting suppressed. I have the P320 shown above so ideally I'd like to use the same 147gr load in both. I like the WSF for the fact that it is capable of producing velocities in the 950 fps range from a 4" barrel at max charge. Subsonic is subsonic whether you're at 1,050 fps or 700 fps but I don't want to limit the performance of either platform by slowing the bullet down too much. Somewhere around a 1,050 fps from the MPX and 950 fps from the P320 would be ideal.

 

[MCMXI]

After resolving a case sizing issue using a Lee carbide die in a Hornady LNL press I was able to shoot some handloads today. I'm shooting a 147gr plated bullet from Accura with 4.3gr of Winchester WSF. Velocity is around 1,030 fps from the 8" barreled MPX (suppressed) and 990 fps from the 4.6" barreled P320 (suppressed) and all rounds functioned without issue. Velocity is easily subsonic and these rounds are quiet. I was shooting an IPSC sized steel target at 50 yards and have the MPX iron sights close now. I'll be adding a red dot next week.

So far I'm really liking the MPX. The trigger is average but might get better with use. The controls are excellent and the quality of the parts, fit and finish are commensurate with the price (excluding the nylon ejection port dust cover). I think that MPX is an improvement over the venerated MP5 or at least a viable alternative. I don't know how it compares to a 9mm AR though.

 

[Gtscotty]

Very nice setup! A good short suppressed pistol caliber carbine is a glaring hole in my collection that needs to be addressed at some point. I'm curious as to why you couldn't use 231 for 147gr 9mm loads? I use it for everything from plated 135gr 9mm subs to 180gr .40s, to 240gr cast .44 mid-mags. Was it an issue with port pressure?

Edit: after reading a little, I guess 231 is a bit fast for 147gr bullets... It sure does work well with 135s @ 900 to 950 fps though.

 

[MCMXI]

Very nice setup! A good short suppressed pistol caliber carbine is a glaring hole in my collection that needs to be addressed at some point. I'm curious as to why you couldn't use 231 for 147gr 9mm loads? I use it for everything from plated 135gr 9mm subs to 180gr .40s, to 240gr cast .44 mid-mags. Was it an issue with port pressure?

I have a lot of 231 but when I looked at Hodgdon's load data for 147gr bullets they didn't list it at all. They list 231 for lighter bullets but not for 147gr so I assume that it's not a good choice given the number of powders listed. I want velocities around 1,000 fps and WSF does that for me. I don't know if 231 would work well in this application.