Are the smaller holographic sights like the Burris Fastfire, JP or Leupold DeltaPoint good for a primary sight along with the BUIS.
Without going to the EOTech's larger type of sight

Check out the Primary Arms MicroDot. I have two of them on my ARs.

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*Aim small, miss small...

its all preference, you could use whatever you like best. but i believe they made the micros originally so they could fit on the rail next to riflescopes, canted to the side or ontop.

In my opinion your best bet with something small would be the Aimpoint Micro. You get all the qualities of a good Aimpoint sight and in a much smaller package. They are quite expensive, but you get what you pay for. Stay away from the knock offs. They are cheap for a reason, and its not just because they lack a brand name. Most of them are produced overseas which mean QC measures are questionable at best. Do yourself a favor decide what you want, save the money for it, and purchase your optic one time. You will save money by not buying a bunch of interim junk and having to upgrade. Just my two cents. Good luck to you.

If the weapon in question will be relied upon as a duty weapon or a defensive weapon then I concur with the advice offered above. If your AR15 is for recreational shooting only then Primary Arms branded optics are worth consideration. I use an EOtech 512 on my AR - it's a defensive weapon and so I stick to combat proven hardware only on this weapon. I have three Primary Arms scopes on recreational firearms and one of their micro red dot sights on another. I gave a Primary Arms red dot sight to a friend more than a year ago. All five of these optics have performed flawlessly, some through ~1,000 rounds moderate to heavy recoil, all have held zero. The guy who runs Primary Arms is clear about the intended use of their Chinese manufactued optics: recreational/training use, not service/duty/combat.

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M&P15T M&P9 M&P9c SW1911 4506

To answer the OP question: yes, a Burris Fastfire type reflex sight will work as a primary sight on an AR. You'll need to get a riser mount though as it will sit too low using the standard mount. I have no experience with them so I can't comment on how much abuse they can take. Many of the reflex sights have a larger dot than, say, an Aimpoint. Just something to be aware of.

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M&P15T M&P9 M&P9c SW1911 4506

My Burris FastFire II just finished a 4 day carbine course. It is the secondary optic on my M&P15. It worked every time I needed it. I had to smile a bit when my buddy's Doctor reflex shot itself loose half way through the course.

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Stu

You mean the mount came loose because he didn't tighten it enough, or the reticule came loose and rendered the optic unusable? I have a couple of Primary Arms Microdots that have "survived" several tactical carbine matches with no adverse effects. In the first match I did with one, I had neglected to tighten the mount down enough, and the recoil loosened the mount, but it was easily addressed while on the run.

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*Aim small, miss small...