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

[Aries144]

I've been researching a 6.75" barrel for my MCX pistol, intending to run it suppressed with 110gr Hornady V-max, and have run across reports that indicate non monolithic (all copper, brass, etc) supersonic ammunition is not safe to fire suppressed with the 9" and 6.75" MCX barrels, since they use a 1:5 twist.

I contacted Hornady about the issue and got this response:

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"

I'm getting comments from some people that their MCXs don't have issues firing the supersonic loads suppressed. I'm checking here to try and get more input.

[dellet]

I've had more subsonic monolithic expanding bullets come apart than jacketed supers.

I've ran the Vmax and Varmageddons to over 330,000 rpm. and they made it to the target at 200 yards.

Maker's 85 grain solid copper to 340,000

I've had Lehigh's supers not make out the end of the barrel intact.

To give you an idea if the stress you're putting on the bullets at those rpms, it's 4800 fps in a normal 30 caliber with a 1/10 or 3800 fps in a 1/8.

You will be hard pressed to reach those speeds in the 6.75". In the 9" it could be a problem. You would need to limit your velocity to 2100 fps.

12/twist X velocity X 60 = RPM

12/5 X 2100 X 60 = 302,400

12/5 X 1050 X 60 = 151,200

I'm won't say it's safe to do, only that I have done it. I have sent a fair amount of bullets back to the drawing board with this cartridge.

It only takes one shot to ruin a can.

[Aries144]

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 you've shared with me, that works out to 280,800 RPM, worst case.

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.

Is this a Case Solved, Scoob? The Hornady guy would have gotten away with it too, if it wasn't for us meddlin' kids? Is 110gr V-max safe from a 6.75" bbl? It's kind of looking that way now, unless there's another wrinkle here. 9" bbl gets about 315,360 at 2190 FPS, so that's iffy if we're still taking the Hornady guy's word for it that over 300,000 RPM is bad juju.

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).

Dellet, thank you very much! Please share any additional thoughts if you think I've missed anything important.

[pruhdlr]

While I am definately not a "guru" on this subject, I have had some limited expierances with rotational velocities of some different bullet/barrel/twist rate combo's.
I would just add that the condition of your weapons inner barrel could help/hurt the stability of the bullet. If I even thought that I was gonna come close to bullet instability due to slow or fast velocities,I would make sure that my inner barrel was as smooth and free of tooling marks/imperfections as possible.
I would use JB Cleaner or Rem Clean on a tight fitting patched jag to make a hundred or so trips up and back thru my barrel.
I do this to most all of my new guns and some guns that belong to others who claim that the barrel is "shot out". This procedure smooths and cleans remarkably well. ---- pruhdlr

[dellet]

The problem is good hard data. This question comes up a lot in the varmint shooters crowd with smaller lighter bullets. On some of those forums you can find people who have a lot of experience with bullets not reaching the target.

As pruhdlr said there are other factors that may come into play that could compromise the jacket, the part that fails. Target shooting, hunting, load development barely gets the barrel warm. If youre at the bullets limit, a mag dump or full auto could change results in a hurry.

Varmint bullets, for the most part, have thinner jackets. That’s what makes the explode on impact at high speed. The numbers from Sierra that I have seen, show there Varminter bullet good to 250,000 rpm, while their Blitzking is good to 350,000.

Bottom line is that this lowly, under powered cartridge can place some serious stress on bullets. That’s part of the reason people have accuracy issues. They don’t look at the whole picture. Cast bullets need to be of very high quality. Any imbalance will cause a wobble.

With a 1/5 twist you just need to figure any bullet you shoot super will be spinning about 1.5 times faster than design. The barrel has twice the spin, the cartridge has 2/3 the velocity.

Most bullets don’t come apart in the barrel. Generally it’s down range. The shortest distance I have documented one coming apart is 5’. I’m guessing that one would have been a baffle strike. You just don’t know until it’s too late.

I have pushed the limits to the point of breaking things because I find it interesting, but any info I share is worth what I’m paid for it.

[pruhdlr]

The up and coming 224Valkyrie will be somewhat interesting. It's speeds will approach that of a 22-250 Remington but the bbl twist rate is far less than the older guns of 1:14.
I have noticed that Ruger makes the American with a 1:10 twist which would be fast enough (rotational). If the newer Valkyrie is even faster . . . well . . . hmmmmmmmm. --- pruhdlr

[Aries144]

A final followup:

I contacted Stillwater regarding the warning that they have on their 110 Vmax page, and they responded saying: "We have no known actual incidents involving the v-max projectile and issue the warning only because of the type of projectile that it is."

[strobro32]

I had a baffle strike with factory Hornady 110 grain ammo within 3 rounds on my Noveske 8.2" 1:7 and Liberty Chaotic. First words out of the Hornady rep on the call, "Wasn't out ammo." They blamed counterfeit/reloaded ammo. :rolleyes



Liberty Suppressors took care of me. Great company.

[Aries144]

Any idea what happened with the bullet? What was the state of the rest of the baffles?

I'm wondering whether the bullet came apart in your can or if some bullets in that weight range can actually be under stabilized by a 1:7 twist from an 8.2" barrel.

Maybe Sig did the 1:5 twist in their sub 16" .300blk barrels for more than just subs?

[strobro32]

viewtopic.php?t=87476

We suspected improper loading. When I disassembled the rest of the cartridges from the magazine, one of the cartridges had less than 2 grains of powder. One or more had loose bullets. Who really knows. Could have been the problem you mentioned.