Considering a Mk12 Mod 0…twist and .223 q’s

[carbon]

I’ve broken this down into two parts, twist and chamber.

So, this rifle I like comes in 1:7 and 1:8 variants, has an 18” barrel, and of course is chambered in 5.56.

PRI 18" Mk12 Mod 0 SPR Gen III Rifle w/B5 Stock 5.56 1:7 in FDE

A full Mk12 rifle from PRI - Mod 0, Gen 3 with B5 Bravo stock - all in FDE

charliescustomclones.com

(I’d buy it directly from PRI, but that’s the link I had handy.)

TWIST QUESTION
I want to shoot factory ammo like 77gr Black Hills TMK 5.56 down to 40gr Vmax .223 (like Fiocchi). Not a reloader.

Will 1:8 be enough for 77gr TMK? It appears “marginally stable” at 15°F/1000ft, just running through the Berger online gyro stability calculator (I’m in MN). It turns to “stable” at 80°F at the same altitude. (.420 bc, 1.072” long)

Will a 1:7 be too much for 40gr Vmax? (.200 bc, 0.686” long)

If I found the right bullet specs and did my calc correctly, 1:7 at the “box speed” of Fiocchi’s 40gr vmax (3650fps) = 375,429 rpm, and a 1:8 gives me 328,500 rpm. Both of which seem too high.

However, I have an 18” 1:8 Armalite AR15 that seems to love the Fiocchi 40gr vmax. Doesn’t blow them up at 100yds at least.

CHAMBER QUESTION
Will the Fiocchi 40gr .223 work well in a 5.56 chamber? My Armalite has a Wylde chamber, so I cannot deduce anything from that, exactly.

 

[natdscott]

Just go with a quality barrel in 7.0 twist, and don't overthink it too much. As tight as 6.5 inch twist will still shoot match bullets under half minute. Got the fake walnut to prove it.

As far as chamber is concerned, just go with a standard 5.56 NATO, but if you want to "match it up" do not go tighter than a Wylde chamber, if you want versatility.

 

[natdscott]

Oh, and if you want to really see what an upper can do, you should probably have at least the receiver and barrel done by a match shop like White Oak. They can build the whole thing or you can put the rest of the pieces on yourself, but if you could at least have them set up the barrel, and glue it into the receiver, that would impress you at the end of the day, I promise.

 

[carbon]

Just go with a quality barrel in 7.0 twist, and don't overthink it too much. As tight as 6.5 inch twist will still shoot match bullets under half minute. Got the fake walnut to prove it.

As far as chamber is concerned, just go with a standard 5.56 NATO, but if you want to "match it up" do not go tighter than a Wylde chamber, if you want versatility.

Yeah, I’m inclined to go 1:7 to better stabilize the 77gr and let the chips fall as they may with the 40gr.

You can’t select options on the PRI build…it’s sorta a cloner’s rifle and even though I’m not a cloner, I am lazy lol

 

[natdscott]

Oh, you can be as lazy as you want with White Oak, Compass Lake, or Keystone Precision. They'll put the whole thing together, as much or as little as you want.

They just use better barrels. Barrel quality is EVERYthing, except 3.8% correct assembly.

 

[Ryan66]

@carbon I’m also in MN and I’m in the process of putting together a WOA upper in 1:8 to shoot the 77 smk along with other varmint bullets. I’m a reloader but plan on shooting the broad variety of bullets you mentioned - Sierra’s website shows the 77 smk and tmk requiring a 1:8 and many others I know have had success with that twist rate and a wylde chamber. I personally am a believer that 1:8 can work for about everything loaded to AR mag length and velocities that an 18in barrel can realistically produce.

I think it’s a rabbit hole that is easy to get stuck on … 1:7 or 1:8 … worst case the 40’s get over spun and you have to shoot 50’s for varmint bullets but I doubt it. Not much of a worst case though as the 50’s do well at splattering varmints and there are lots of factory ammo options for that.

 

[Shutstr8r]

My Mk12 rifle has a stainless fluted Wilson Combat 18” 223 Wylde chambered 1:8 barrel and consistently slings the Sierra 77gr mk’s @2765fps into solid 1” groups at 100m. The gun uses an adjustable gas block and an AB Raptor 8 5.56 can w/3” reflex. It will shoot 5-7” groups at 500m. I’m happy with it. It will do what I need it to do when and if I ever need it to. I’d get a 1:8 twist if you are wanting to shoot lighter stuff. It will be more versatile in the long run. YMMV

—Darrell

 

[cmillard]

My mk12 that I put together is a correct Mk12 minus a couple things.....I have a jewell trigger, my barrel is a compass lake (they did the barrels for the mk12) but has the CLE match chamber instead of wilde and my suppressor is an authorized clone done by otter creek. The 7" twist has shot 77TMK handloads well out to 1431 yards. I havent shot anything lighter than 62 grains though.

 

[SecondsCount]

I have multiple 1:8 twist, AR and bolt based 223 rifles. They all shoot the 77 grain bullets from the major manufacturers just fine. In the bolt gun, I shoot the 80 ELD and 85 RDF out to 1K.

A friend was shooting 40 grain bullets from his 1:7 twist 16" Colt 6920, and they came apart in flight.

The Mk12 clones are a ton of fun!

 

[cmillard]

Why not have another upper so you can shoot the 40 grain stuff if you really want to run that ammo?

 

[carbon]

Why not have another upper so you can shoot the 40 grain stuff if you really want to run that ammo?

A decent idea.

 

[cmillard]

You can have an infinite number of uppers with 1 lower. The beauty of the ar15.

 

[Bamban]

Get the barrel from WOA that is already machined up front to accept the mounting system for the AEM5 5.56 can

 

[RegionRat]

Another vote for using a WOA Varmint upper for those 40 - 53 VMax and mounting an appropriate optic for varmint work, and then the 1:8 rig you like for those 77 loads with the appropriate optic for that sort of rig.
YMMV

 

[cmillard]

I would still do a 7 over an 8 twist! Honestly, on my next, i may do 6.5 twist

 

[RV-Customs]

My Mk12 has a 20" 1-7tw, its a 1/4MOA gun all day with 77SMKs! Go 1-7