Mark 12 Mod 0 in 300 Blk

[Jeff in Texas]

New to your forum. I have a buddy who has a .223 in a Mark 12 Mod 0 AR-15 with an Allen Engineering (formerly Ops Inc) suppressor. Got to say I love this rifle and how it handles. I really want one but I'm done with the .223. Ready to pull the trigger on a the same rifle in 300 Blk with the AE suppressor. This means an 18" barrel which doesn't concern me.

Primary use of the rifle will be paper punching, deer and hogs at the ranch. I like the fact we there's subsonic ammo available for it as well, though not sure how well it would do on hogs but it could be fun.

Has anyone built this rifle in 300 Blk. Also, what mags are y'all using?

[BlogSarge]

Welcome to the madhouse, Jeff,

There isn't any reason that what you plan couldn't work really well, although the 18" barrel is a bit of overkill. If you like the balance and feel of it, though, go for it!

I can't recall the gas system length on the Mk12, but if it is too long you would have some difficulty with subs out of it. I think a pistol gas length is always best in 300blk due to the lower pressures compared to the 223/5.56.

There is a whole primer on subsonic hunting in our hunting section posted by members with a crap-load of sub hunting experience. Bottom line, it is very doable but think of it like you'd think of handgun hunting.

With the suppressor on it, you will inevitably try subs and get hooked on them!

Welcome again!

[John A.]

I'm not sure of an 18 inch blackout barrel, but there are plenty of 16 inchers.

Especially if you're going to try to use a midlength gas port location with it, which is how the true SPR 18 inchers are made.

You can suppress supersonic blackouts, but obviously aren't as good as subsonics, but you obviously wouldn't get the useful range the 18 inch .223 is capable of.

18 inch doesn't really seem to be a solution for anything. Blackout was designed with shorter lengths in mind and longer lengths don't really do much to help anything. While I don't mind that you want the long barrel, I'm trying to wrap my head around what you want it to do.

subs

supers

range/distance

[dellet]

18" barrels are simply a choice. They offer a few advantages and a few disadvantages.

You could most likely run a mid length gas tube, if you gave up the idea of subs. With a carbine length system subs are absolutely not a problem for hand loading. Factory loads might go super because of the added barrel length, but they will function in a properly set up system.

I picked up 40-50 fps with supers over the same load in a 16" barrel.

With a .110" or less, carbine gas port suppressed, it will be noticeably quieter with subs than a 16".

The longer the gas system, the less felt recoil you will have and the less blow back of both noise and gas in your face suppressed from the ejection port.

If you are worried about handling capabilities, go with a quality 7" suppressor. Overall length will be the same as a 16" with a 9" as will be suppression quality to the shooters ears.

I prefer Lancer mags, most any will work. Bullet choice is more of a determining factor than magazine choice.

As for disadvantages.

Depending on your choice of stocks and muzzle device, you might not be able to fit it in a 36" rifle bag.

The lightest subsonic bullet I can run without the suppressor, is 168 grains. With a suppressor I can go lower but reliability suffers due to filth from unburned powder that causes issues in the action.

[DoubleJ]

Pretty sure the Navy's SPR runs a rifle length gas system, at least that's what I copied when I built my version of one. I gotta say though, the SPR is a reacher-outer, takes the 5.56 out to about the end of it's usability, kinda the opposite of what the 300blk is for. If I was gonna build a big, heavy, long AR (well, another one) I'd be looking at 6.5 Grendel or maybe the new 7mm Valkyrie. This is America though, so go for it, keep us in the loop and let's see how it turns out.

[Jeff in Texas]

Help me understand the original purpose of the 300 blk. My neighbor has one in a 16" barrel and uses it for hogs and paper punching, not to mention I love shooting it. As I said I'm done with the .223 and was looking for something else to use around the ranch for hogs and plinking as well as the occasional deer. I also like the fact that there's plenty of sub-sonic ammo available. I thought the 300 blk was just the ticket. I was also looking to have a MK 12 built and thought swapping out the .223 for 300 Blk would be a great idea!

Sorry, I never really researched the original purpose of the round. Help me understand as I'm all ears!

[DoubleJ]

I don't hunt, so I can't speak of it's effectiveness there, but I hear good things. Wait, I shot a pdog in the neck with a 190 Nosler CC recently, it died, so it's 100% effective for me.

Mine is for HD. 8.5" barrel, 14" reflex can, EOTech, and a big light. I met a retired cop once, he got into a gunfight in a hallway with a 357 magnum and now he's legally deaf. I've already lost hearing in the range of my child's voice, I'd like to limit the ringing if I can. Worst case happens, I can deliver a quarter ton of energy at 120db and won't have to yell at everyone for the rest of my life.

I hear people are playing 300blk out to 500 and even 1000 yds, so it can be done, but it shines with subs and a good can. The supersonic guys should be along shortly to extol it's virtues with the light stuff.

[Dr.Phil]

I did a MK12 MOD-0 Build in 300 BLK with a 10" Noveske Barrel and 30P-1 suppressor.
Overall length is the same as the MK12 MOD-0 without the suppressor installed.

Rifle looks fantastic and turns out to be a fantastic semi-auto sniper platform for urban operations where anticipated engagement distances are less than 300 yards.
Send me a PM with your E-mail address and I'll send you a high res image.

[Whole Bunches]

Help me understand the original purpose of the 300 blk. My neighbor has one in a 16" barrel and uses it for hogs and paper punching, not to mention I love shooting it. As I said I'm done with the .223 and was looking for something else to use around the ranch for hogs and plinking as well as the occasional deer. I also like the fact that there's plenty of sub-sonic ammo available. I thought the 300 blk was just the ticket. I was also looking to have a MK 12 built and thought swapping out the .223 for 300 Blk would be a great idea!

Sorry, I never really researched the original purpose of the round. Help me understand as I'm all ears!

I can let others address the original purpose. As for me, I love the cartridge. I have several Blackout rifles. I enjoy reloading it, plinking, shooting steel, shooting suppressed, and hunting with mine. Mild recoil. I've taken 17 whitetail deer (farthest was 187yd, next farthest was 186yd), 15 hogs (largest 272 pounds), 4 coyotes, and 7 armadillos with my Blackouts. Based upon my experience, I'd not hesitate to hunt deer and hogs with the Blackout, but it's not a long range cartridge for hunting over 200yd in my opinion due to speed needed to expand bullets and its trajectory. I could look in my records for an exact number, but I've shot somewhere around 18,000 rounds of Blackout in my various rifles.

I also monitor all my barrels for wear as a hobby. Short version: Do not agonize over the bbl steel or treatment, you'll get nice bbl life from even plain 4140 steel with no treatment, but 4150, 41V50, and CMV steel will last longer, and even longer if chrome lined or nitride treated. Oh, my 416R stainless bbl is lasting a little bit longer so far than my plain 4140 barrels; specifically its calculated bbl life based upon wear so far is 720 rounds more than plain 4140.

Enjoy your Blackout. I think you're on the right track. As others have said, you will get a bit more velocity from an 18" bbl than a 16" one which will help when hunting. If we can help, please ask.

[jwb47]

If you want an 18 inch spr in 300 blackout then build one . I see no reason not to hunt deer or hogs with one . I would like to see a photo when your done .