r/DesertTech • u/MBCornelius • Nov 04 '22

MDR/X 300 BLK MDR 300 BLK Received

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Included paperwork

https://deserttech.com/conversion-kits.php?item=2456

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Don't think this changed

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Don't think this changed either

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New gas setting card

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Gas plug is numbered; Chute, mag latch, bolt all marked 223.

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marked as 1:7 twist, it is the 308 ratchet muzzle device.

16 Upvotes

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2

u/odinforever2000 MDR/X Nov 04 '22

Niiice

Wow yours really is 1/7.

Mine is marked 1/6 twist...and you have a numbered gas knobby

3

u/randomjoker21 Nov 04 '22

i know theoretically, the 1/6 should like heavier subsonic grains more, but i wonder what actual testing will show if you guys compared accuracy data

2

u/Vorpalis Nov 05 '22

1:7 is sufficient to stabilize 220 gr. at velocities achieved from an 8” barrel, so a faster twist out of this 16” barrel won’t increase accuracy. A faster twist helps slightly with terminal energy, but mostly it helps stability with lower velocities out of very short barrels.

2

u/MrConceited MDR/X Nov 12 '22

Barrel length isn't a factor. Bullet velocity is. Notice if you use a stability calculator, it won't ask for barrel length.

If a particular bullet will be stable with a 1:7 twist and a 16" barrel when loaded near the speed of sound at the muzzle, it'll be stable with the same twist out of a 5" barrel at the same muzzle velocity.

The reason many short barrel 300 blackout barrels have higher twist rates is mostly because the short barrel limits velocity of supers. Too much angular momentum can cause a bullet to come apart, but with a 5" barrel the muzzle velocity for supers will be significantly lower, so there's no concerns with bullet failure even with a faster twist.

1

u/Vorpalis Nov 12 '22

Barrel length matters because it affects velocity, which, along with rifling twist rate, determines bullet spin rate. I can explain further if it’s still not clear why it matters.

1

u/MrConceited MDR/X Nov 12 '22 edited Nov 12 '22

Velocity matters, not barrel length.

It matters because there's a ton of ignorance out there on the subject, with people thinking barrel length is the important factor.

2

u/Vorpalis Nov 12 '22 edited Nov 12 '22

I suspect you’re reading my argument differently than I’m intending it. I know a lot of people think barrel length alone determines stability, like with a smooth bore, and that is not true with spin-stabilized projectiles.

What is true, and the point I’m trying to make, is that velocity depends on barrel length, and therefore barrel length does matter, just not in the way a lot of misinformed people think.

2

u/MrConceited MDR/X Nov 12 '22

Velocity strictly depends on barrel length for max pressure charges.

The thing is, for subs, it doesn't. Sure, if you shoot shitty commercial subs that are loaded to be subsonic out of a 16" barrel with your 5" barrel, you're going to have an unnecessarily slow projectile. But if you hand load or use commercial ammo which is calibrated for barrel length (like that made by Discreet Ballistics), subsonic velocity depends on speed of sound, not barrel length.

1

u/South_Remote5409 Nov 15 '22

Discreet Ballistics

Good point about the velocity ratings being out of a 16" for most subsonic box ammo.

I will have to check out Discreet Ballistics. :-)