MK12 Mod 0 Chamber Reamer drawing/details

[Hohn]

Apologies for bringing up something that has likely been covered, but my search skills are not up to the task of finding it in the archives.

I'm seeing references to a special chamber design being used by Douglas on the MK12 SPR barrels. I'm assuming this isn't a Wylde chamber or it would likely just be referred to as such. Centurion makes a similarly cryptic reference to this chamber design for both its Recce and SPR barrels, just referring to it as "optimized for MK262."


While the Wylde chamber is probably the most popular "improved" chamber, there are other reamer designs floating around oriented towards particular uses. I know Frank at CLE has his CLE chamber design that is very highly regarded (and optimized for mag length ammo), but I can't see that working with MK262 without serious pressure issues (it's a short freebore chamber).


The well-worn graphic from ar15barrels.com showing the various reamer details (you know the one) shows nothing peculiar to MK262 or MK12 SPR.

Who came up with this chamber design or made the reamer? Douglas? Crane? What is its geometry? I'd love to see how the chamber was tweaked relative to standard 5.56 or Wylde.

[pointblank4445]

Good luck...I've tried multiple times to get down in the weeds and figure this out and not come out with any answers. Someone will be along shortly to post the PTG reamer dimension differences that you noted.

[Hohn]

Well, it's been known for awhile how to get the 77 SMK to shoot. The HP match shooters had that figured out awhile ago. Frank's CLE chamber is amazing at allowing mag length ammo to shoot tiny groups. The real question IMO is freebore. Frank's chamber is pretty short and I suspect (haven't tested) it might be overpressure with box spec MK262. I know several guns running MK262 in Wylde chambers that will show mild pressure, and the Wylde has substantially more freebore (and less pressure) than the CLE chamber.

I have a contact out at Crane from the HP matches, maybe he can dig this up for me.

[pointblank4445]

Oh no doubt, it's not hard at all to get 77gr SMK to shoot. The hard part for me at least was finding one that could maintain consistent standards in adverse conditions and/or have similar ability to run "clone" (Fed GMM 77gr or IMI) when BH/Mk262 was not available. The Noveske Mod 0 and the Centurion Mk12 chamber specs would be what I would like to have if given the choice but having that chamber reamed to a custom blank was not in the cards.

That being said, I quit chasing this dragon long ago.

[lysander]

It's metric, but I think this is what you are looking for:

https://i.imgur.com/IoG5sPT.png?1

[Hohn]

Awesome thank you!