Picked up an unused ach from au and probably gonna chop it. My understanding is there is standard high cut and fast style. Most of the ones I see on YouTube and such use the standard high cut. Is there a difference between the two and will the fast style fit an ach chop? It looks more vertical on the back.

High cut works better with earpro.

Where do u send it to get cut?

This. I'm thinking about getting my ACH cut so I can add rails and ear pro.

Well you can do it yourself as it's not really difficult. KustomACH seems popular though.

Surely someone knows the difference between the profiles of FAST vs Standard. Everybody here has a helmet right?

Ops-Core has a FAST cut and super high cut. Is that what you're referencing?

Edit: Double tap sorry..

Super high cut gives you a little more room depending on headset used and shape of your head. My comms always touched the bottom of the ARC rail on the OG FAST helmet.

I guess after further research it appears to be between fast (og) and fast sf (super high cut). Will a chopped ach fit a super high cut rail and is that better than the standard fast? What comms fit and don't fit?

That definitely clears it up. I'll get the rails that people who have cut achs recommend.

You get ACH ARC rails or Ops Core ARC rails. ACH has a particular shape, different from standard Ops Core. They don't interchange, even though Ops Core makes an ACH ARC rail version.

Ops Core makes ACH ARC rails for other brand ACH helmets and OpsCore standard ARC rails for their own helmets. Two different things. The Ops Core ACH ARC rails are, or at least were, one size only. ARC is a spec for the way the accessories fit in the slots, not the rest of the rail or shape.

Galvion, formerly Revision, makes ARC rails that fit per each of the 4 ACH/Viper helet sizes. They are 100% ARC spec but slightly different than OpsCore ACH ARC rails and Galvion pays a royalty to OpsCore to use their ARC name/spec.

Galvion/Revision used to sell both stand alone ACH/Viper ARC rails and their integrated ACH/Viper ARC rails that tie the rails into their proprietary 1 hole shroud. Revision/Galvion was the #1 manufacturer of ACH helmets for the military toward end of ACH production and Viper was the exact same ACH helmet sold for LE use with NIJ rating

I have both types. I have an older Revision branded 3 hole ACH/Viper helmet I paired with Revision stand alone ARC rails and a Wilcox shroud. I also have a Galvion branded one hole ACH/Viper helmet that is paired with the integrated ARC rails/shroud. If you have a one hole ACH the Galvion integrated rail/shroud setup is the way to go because it integrates the two front side rail bolts into the shroud to give it three points of contact. Looks kinda funny but works REALLY well. The Galvion brand shroud uses two rubber bumpers that take out slack/slop like a Norotos shroud and fits better than my Wilcox shrouds that don't.

I know that at least for LE pricing, Galvion has the best price when you combine the cost of shroud and rails together and don't consider a Norotos "fell off a truck" shroud off of Ebay and buy everything from legit vendors.

Unfortunately, Galvion made their stand alone ACH/Viper ARC rails LE purchase only and was discontinuing them as of just over a year ago when I spoke to them. They also were going 99.9% to one hole helmet production and 3 hole was SUPER custom order only for their Viper helmets with very limited availability of stand alone rails. The Revision/Galvion ACH rails fit better on ACH sizes other than medium compared to the OpsCore ACH ARC rails that are sized to fit medium or large helmets IMO. You just will not find them easily. If you do find a set cheap, or even used, buy with confidence.

Also, FYI, the Premium (not regular) Galvion ACH/Viper helmet cover
Sized in two sizes (XS/S and L/XL) is also by far the best fitting/quality ACH helmet cover I have found as well. Though you have to install the cover then install the rails and shroud on top and make little cuts for the side bolt holes, which is kind of a pain if the helmet is already assembled. It also comes with a single hole up front so perfect for a single hole helmets, but looks like you have a .22lr bullet hole if you use a 3 hole setup and you cut more holes to mount a 3 hole shroud. I use that cover on both of my ACH high cut helmets. On the upside, that cover will wrap around and cover up the edging of your fullcut chopped to high cut ACH, so it will stay protected and always look nice. If you are having someone else cut the helmet, I would consider buying your rails and cover first and sending them with the helmet to ensure the cut fits both the cover and the rails.

ACH high cut chops are usually just going to roughly follow the contours of the ear bumps and chop them before they flare. Having the cover and rails ensures they don't leave too much material making the cover fit poorly, but also gives a guide to cut away as much as possible to follow the bottom rail shape.

I already drilled mine and mounted a wilcox 3 hole shroud. Works good. I'll look into the other stuff.