Hornady says no non monolithic supersonic bullets in MCX 1:5 twist .300blk with can

General Firearms Other Semi-Automatic Rifles
1

I recently picked up an MCX 5.56 pistol. After stumbling onto some papers on .300blk and very short barrels, I’ve been thinking that an MP5-sized firearm that shoots supersonic ammo that fragments out to 50m would be cool.

However, I’ve run across two reports of blown up suppressors that put the blame on Sig’s fast 1/5 twist .300 blk barrels for spinning apart supersonic bullets.

The “Rangehot” page seems to be referencing a 125gr “slotted” solid copper bullet that came apart when fired from an MCX. It would appear to be a 125gr version of one of these. The bullet referenced in the 300talk link is definitely a frangible.

http://rangehot.com/advised-sig-mcx-300-blackout-twist-rate-baffle-strikes/

http://www.300blktalk.com/forum/viewtopic.php?f=140&t=100882

Does anyone have any experience or opinion on whether 110gr Hornady V-max might exhibit similar behavior?

ETA 1:

Ok, I just got a quick reply from Hornady about running 110gr V-max through the MCX’ 1:5 barrel with a suppressor.

My message:

“I’ve seen warnings that your .300 Blackout 110gr Vmax cartridge should not be used with a suppressor in barrels with twist rates faster than 1:7 as the bullet can come apart and damage the suppressor. I am considering the purchase of a Sig Sauer MCX chambered in .300 Blackout, which has a 6.75”, 1:5 twist, barrel. Should I avoid using 110gr Vmax with a suppressor on this firearm? I saw the warning on this page. https://www.stillwoodammo.com/produc…-110-gr-v-max/ "

Response:

“With a 1-5 twist barrel this would put these rounds at 331,000 RPM. I would be reluctant to run this type of bullet through this barrel with a suppressor with anything above 300,000 RPM. This barrel design is going to be exclusive to either subsonic loads or monolithic solid bullets. Thanks”

ETA 2:

So, here’s a new wrinkle:

Someone on another forum just shared with me a formula for calculating bullet RPM. 12 / twist rate * velocity *60

Based on what little chrono data I’ve been able to find on 110gr .300blk, it looks like 1950fps muzzle velocity would be very optimistic on a 6.75" bbl, with something in the 1900-1850 range more likely. According to the formula shared with me (12/5195060), that works out to 280,800 RPM, worst case. The Hornady guy was only worried about RPM above 300,000.

Was the Hornady guy calculating based on muzzle velocity for a 16" barrel? It looks like it. I’ve seen 2300 FPS listed as the MV for 110gr V-max. 12/5x2300x60=331,200 That’s almost exactly the figure the guy quoted.

It’s looking more and more like the two incidents I’ve found documented on the internet were caused by first, a specific 125gr all-copper “slotted” HP bullet and second, a frangible bullet (which I’ve always read should not be used with a suppressor, in any caliber).

2

Has anyone had any issues when using the MCX Virtus 9 inch barrel pistol model with Hornady or Barnes 110 gr blackout ammo ?

3

OP updated.

Just to clarify, it doesn’t look like barrel length is a factor. It seems to be Sig’s fast 1:5 twist rate overspinning the bullets and causing them to come apart. Both the 6.75" and 9" barrels use that twist rate, as does the new 5.5" barrel on the Rattler.

4

Ok, I just got a quick reply from Hornady about running 110gr V-max through the MCX’ 1:5 barrel with a suppressor.

My message:

“I’ve seen warnings that your .300 Blackout 110gr Vmax cartridge should not be used with a suppressor in barrels with twist rates faster than 1:7 as the bullet can come apart and damage the suppressor. I am considering the purchase of a Sig Sauer MCX chambered in .300 Blackout, which has a 6.75”, 1:5 twist, barrel. Should I avoid using 110gr Vmax with a suppressor on this firearm? I saw the warning on this page. https://www.stillwoodammo.com/product/300-blk-110-gr-v-max/ "

Response:

“With a 1-5 twist barrel this would put these rounds at 331,000 RPM. I would be reluctant to run this type of bullet through this barrel with a suppressor with anything above 300,000 RPM. This barrel design is going to be exclusive to either subsonic loads or monolithic solid bullets. Thanks”

I guess that answers my question. Damn, I hope Sig releases a .300 Blackout barrel with a 1:7 twist rate.

5

Is such a fast twist really required to stabilize the bullets? I’ve never heard of a twist that fast.

6

While I can not speak specifically to the 300 I have noticed that barrels shorter than the twist rate in other calibers (rifles…not pistols) tend to offer a bit less accuracy. IE: a 8" barrel with a 1 in 12 twist for example. In some barrel that are shorter than the twist rate you will often see key holing.

Maybe…and I am very much taking a SWAG here, Sig decided to make the twist rate that fast due to the short barrel in an effort to ensure to stabilize the bullet and provide acceptable accuracy. It would seem to be one way to deal with the issue.

7

It’s real. Both the 9" and 6.75" barrels on the MCX have 1:5 twist rates.

Why was it done? My guess is that it has to do with the MCX being specifically designed for the LVAW thing for SOCOM. The focus from them seems to be on subsonics as a more accurate, longer ranged, subsonic bullet launcher to replace the MP5SD. The faster twist might be to help insure stability with 220gr subsonics from a 6.75" or 5" barrel with a suppressor attached.

After looking at this I started thinking about how cool it would be to have 6.75" barrel firearm that won’t eat a suppressor in 1000 rounds and whose rounds will still fragment out to 50 meters. Screw the subsonics, useful supersonic ammo from a suppressed 6.75" barrel is cool.

8

I am going to contact Sig, Barnes and Hornady tomorrow on the following specific 110 gr ammo out of the 1:5 twist rate barrels. I know that the jackets on some bullets are more frangible and lightly constructed than others of the same weight. I will also ask Sig about the the possibility of slower twist rate short barrels (9" inches and under) for the sig mcx virtus 300 blackout pistols in the future. I think it was kind of misleading that they never mentioned possible ammo compatability issues with supersonic ammo out of their 9 inch and under 1:5 twist rate barrels. Has anyone on the forum had expierience or heard of any issues when using the following ammo out of the new 9 inch and under 1:5 twist rate barrels ?

Hornady FULL BOAR 110GR Gilding metal jacket bullet ammo

Barnes VOR-TX 110GR Tipped TAC-TX flat base bullet ammo

MAGTECH 123GR Full Metal Jacket ammo

9

Would you consider throwing in a request about the 110gr Hornady V-max as well? I’d like to see if we get a second opinion.

10

Seems as if they are designing it for subs only…

11

So, here’s a new wrinkle:

Someone on another forum just shared with me a formula for calculating bullet RPM. 12 / twist rate * velocity *60

Based on what little chrono data I’ve been able to find on 110gr .300blk, it looks like 1950fps muzzle velocity would be very optimistic on a 6.75" bbl, with something in the 1900-1850 range more likely. According to the formula shared with me (12/5195060), that works out to 280,800 RPM, worst case. The Hornady guy was only worried about RPM above 300,000.

Was the Hornady guy calculating based on muzzle velocity for a 16" barrel? It looks like it. I’ve seen 2300 FPS listed as the MV for 110gr V-max. 12/5x2300x60=331,200 That’s almost exactly the figure the guy quoted.

It’s looking more and more like the two incidents I’ve found documented on the internet were caused by first, a specific 125gr all-copper “slotted” HP bullet and second, a frangible bullet (which I’ve always read should not be used with a suppressor, in any caliber).

12

Here is a link to the Hornady 110gr Vmax 300 black out ammo. 2375 FPS out of a 16" inch barrel. https://www.hornady.com/ammunition/rifle/300-blackout-110-gr-v-max-black
What kind of RPM would I be getting out of my 9 inch 1:5 twist blackout barrel ?

13

110gr Vmax from a 9" bbl is around 2190 FPS.

12 / 5 * 2190 * 60 = 315,360 RPM

2100 FPS gets you down to 302,400.

14

No…at least with the Rattler the twist rate is a factor of the barrel length. You need at least one full rotation of the bullet before it leaves the barrel to achieve stability. Hence the 5 twist in a 5.5 inch barrel.

The RPM of the bullet is not just a factor of the twist rate, velocity is the second part of the equation. The faster the bullet travels down the barrel, the faster the rpm.

With the Rattlers shorter barrel I’d be surprised if it could reach the RPMs required to spin apart most conventional bullets. Maybe some weird pre fragmented designs might have problems.

It’s a pretty simple test. Take your suppressor off and shoot a couple of mags of your ammo at a big piece of white poster board.

You’ll know if your bullets are slinging apart.

15

You’re dead wrong with that statement. Please do some research before you make statements that come across as facts.

For example, standard twist rate for S&W .38SPL is 1:18.75. J frames with 1.875" barrels stabilize…

16

FWIW, some chrono data below from a 1:5 7.5’’ barrel and 110s:

[video=youtube_share;4VNl7QXykUY]https://youtu.be/4VNl7QXykUY[/video]

17

Q uses 1:5 twist on all of their 300blk guns, and Kevin Brittingham knows that caliber more than almost anyone else.

18

As he worked at Sig between AAC and Q I think he implemented the 1:5 at Sig. Of note is the honey badger is very much geared towards subs. I have a Honey Badger barrel I plan to only run suppressed and with subs.

19

I have a legacy MCX with the same twist, I have not ran subs through it at all yet, only supers.

20

The ultra short 300 barrels sometimes show excess precession. This is the reason for faster twists.

Vmax is not a reliable shape for defensive use. Because the OAL is so short it can cause a magazine to jam. Stick to full length rounds like black tip to avoid this. Also for terminal performance and not worrying about the twist rate.