I've written previously that clothing for armed professionals comes in three categories: uniformed, professional, and blending. Uniforms are obvious, and in them you are advertising to the world that you are armed. Professional clothing is the term I use to refer to the professional-looking plain-clothes such as made by 5.11. EOTAC, Woolrich and others. This type of clothing doesn't hide who you are and that you're armed, but it does present a neat, professional appearance to people like detectives, security personnel, etc., and with all the pockets and features it has, it makes a great choice for these professions. Blending clothing refers to garb that allows you to blend in with your environment, and not stand out at all. For undercover cops, this might mean street clothes, biker garb... or whatever the people in your target group wear. For off-duty cops and armed citizens, this means whatever blends with your local scene, whether it's jeans, Khakis, a suit, or swimming trunks. The term for this kind of clothing is "gray", as in you want to be the "gray man" (or woman). No one notices you.
The same set of categories applies to weapons carry, too. The terms I use here are: obvious, discreet, and disguised. Obvious weapons carry means carrying your long gun in an obvious rifle case (hard or soft), or carrying a pistol in an obvious pistol case (again, hard or soft.) Discreet weapons carry means that you are carrying your weapon in something that's not obviously a weapons case to most people, but that's still pretty clear to someone looking for someone carrying a weapon. Most rectangular, black (or Coyote Tan, or OD green, and certainly camo) rifle cases fit this bill. Many MOLLE'ed-up tactical-looking sling-packs that conceal pistols do, too. These low-key or discreet carry packs have their place, but they don't really conceal your weapons status from both good and bad guys who recognize them. If you don't want to be made, you need disguised carry, not discreet carry; you want no-key bags, not low-key bags.
That means, as a rule, that they shouldn't be black or tan or OD (or camo, obviously), nor should they have obvious PALS or MOLLE webbing on them, nor should they look tactical. You want them to look like something an ordinary citizen might carry; to look like something bought at a sporting goods store, not at your local cop shop. If they are black or tan or OD, or have webbing or loop Velcro areas on them, they should look like outdoor gear, not tactical gear.
Now any bag can technically carry any weapon that will fit in it - but that doesn't make it useful. The bag should protect your weapon and carry it positioned for as-rapid-as-possible access. It shouldn't let the weapon slosh around inside it, and it should carry - properly positioned and packaged - the other stuff you want to have handy with your weapon: magazines, med kits, active shooter gear, dope book, etc. Not just any Nike or Under Armor bag will do. Now, there are a number of bags on the market meeting this description that conceal a rifle very well - in fact you'd never guess they had a rifle in them because they're so small. They accomplish this, however, by breaking the weapon down. That's OK for no-key transport, but not for carry. Our disguised carry bags should carry our weapons ready to use. Also, it might be tempting to go for the most unlikely colors for a no-key bag: hot pink, chartreuse, etc. But this defeats our purpose; it draws attention to us - there are precious few places that a man can stroll around with a hot pink bag and not attract second glances.
Finally, some people like to carry a rifle in a soft guitar case or even a trombone case (!), or a pistol in a padded camera bag. These are clever ideas, but a guitar or camera bag invites theft, and trombone cases (and other unlikely cases) are sure to make people look at you.