Be very careful with a Mk12 barrel. We did some testing with a number of barrels (many name brands) which profess to be either Mk12 or SPR, but will not support an AEM-5 suppressor. If you don't care about running suppressed, then many barrels will aceept the Allen collar and brake.
If you are looking at something that like a Mod1, then you will want an 18" or 20" barrel. The 18" is a little hot, and will cause some early gas erosion, but the original Mk12 Mod1 was an 18" Douglas SS barrel, and those have held up well. I remember visiting Frank White in his shop at Compass Lake a number of years ago, and he commented on how he sees early gas port erosion on every barrel except the Douglas. He did not have a good explanation. We guessed maybe it was the steel Douglas uses.
If you do not care about historical accuracy, and weight is not an issue, I think 20" is worth considering. You get a more natural "dwell" with a rifle length gas system at 20".
Here is a list of
Mk12 barrels we have tested and offer as a kit- barrel and AEM collar / brake. There is a little bit of the case study we did, so a little more than a link to sell. Mk12 is one of our specialities, so we take it very seriously, and have worked hard with some of the better barrelsmiths and also with Allen Enginnering to find the best solutions to baffle strikes on the suppressor.
