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-...affle-strikes/
http://www.300blktalk.com/forum/view...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 (12/5*1950*60), 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).
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