AR15A4 16" rear sight on 20"?

I have a Colt AR15A4 w/20" barrel. It came without a rear sight but has a factory front sight fixed to the barrel.
I have a rear Troy battle sight off a 16" barrel. What do I have to do to make it work on this rifle?
I also have a Vortex Strikefire II red dot, will it co-witness with the front sight right?

With the Troy sight on it, the large hole seems too large on the 20" barrel.

Thanks for any info that anyone can give me.

Would I need to move the rear sight forward?

The Troy sight should work fine with the rifle front sight.

Troy says the sight is 1/2 moa per click, but doesn't specify if that is on the carbine or rifle length sight distance. Doesn't really matter though. One click will move POI a bit less on the rifle, than the carbine. That isn't going to be a problem though.

Doesn't your sight have a large and small aperture? Mine does. Flip up the small aperture. You shouldn't need to move the sight forward.

The field manual for M16A1 and M16A2 Rifle Marksmanship states, “The larger aperture, marked 0-2, is used for moving target engagement and during limited visibility. The unmarked aperture is used for normal firing situations, zeroing”.

Originally Posted by 79S

The field manual for M16A1 and M16A2 Rifle Marksmanship states, “The larger aperture, marked 0-2, is used for moving target engagement and during limited visibility. The unmarked aperture is used for normal firing situations, zeroing”.

J, a buddy of mine uses a flip up (I believe it's a Magpul) on one of his A4 bushmaster's with the A2 FSB and 20" barrel. I was asking him how the sights line up, since he's not using the carry handle. Similar situation as the op. He is a retired Airborne Ranger and seems to know his schidt regarding AR's and other military type weapons. He says the majority of flip ups are made to work with the A2 FSB. I'm assuming he is correct and that the op should have no issues. Personally, I'd keep the flip up more towards the rear of the receiver and nose into the charging handle. What say you? Also regarding aperture size, you are absolutely correct. In low light especially, it is much better to use the larger aperture. On a standard mr31 or similar sized black target at 100, the small aperture works well..

Originally Posted by bsa1917hunter

Originally Posted by 79S

The field manual for M16A1 and M16A2 Rifle Marksmanship states, “The larger aperture, marked 0-2, is used for moving target engagement and during limited visibility. The unmarked aperture is used for normal firing situations, zeroing”.

J, a buddy of mine uses a flip up (I believe it's a Magpul) on one of his A4 bushmaster's with the A2 FSB and 20" barrel. I was asking him how the sights line up, since he's not using the carry handle. Similar situation as the op. He is a retired Airborne Ranger and seems to know his schidt regarding AR's and other military type weapons. He says the majority of flip ups are made to work with the A2 FSB. I'm assuming he is correct and that the op should have no issues. Personally, I'd keep the flip up more towards the rear of the receiver and nose into the charging handle. What say you? Also regarding aperture size, you are absolutely correct. In low light especially, it is much better to use the larger aperture. On a standard mr31 or similar sized black target at 100, the small aperture works well..

They all should work just fine with FSB front fight. In the Army we ran this style of BUIS on the M4 carbine when equipped with M68’s or ACOG’s.

https://www.amazon.com/MaTech-Mil-Spec-Back-up-Sight-B-U-I-S/dp/B002E6SXN4

You can hit really well out to 100 easy and I can to 200 with that huge hole if you trust yourself.

I prefer smaller. The smaller holes are not really good enough for the MR31 target mentioned if shooting paper. There you need hooded and small. But no one shoots paper much anymore in matches like that with irons. I forget rear ap sizes a bit. thinking we were around 042 for the rear with a hood. Could be mistaken

Originally Posted by rost495

You can hit really well out to 100 easy and I can to 200 with that huge hole if you trust yourself.

I prefer smaller. The smaller holes are not really good enough for the MR31 target mentioned if shooting paper. There you need hooded and small. But no one shoots paper much anymore in matches like that with irons. I forget rear ap sizes a bit. thinking we were around 042 for the rear with a hood. Could be mistaken

An adjustable iris rear sight is better, something like the Merit that is on my Colt, but the small aperture works alright for most people. So Jeff, when you shot your targets in the black rifle challenge, you were using the big aperture for the MR31 target? The smaller aperture is more precise..

All I think I know about the irons is the A2 type rear sights had the small hole on a different plane than the big hole.

Carry handles for rifles adjusted to 800 meters, carbine carry handles only went to 600.

Troy, Daniel Defense and a few other BUIS’s used a “same plane” aperture. Meaning the only difference was for low light.

But ya, they will all work on rifles or carbines.

Thanks for all the great input.
Wanted to make sure before taking a day to go to the range for not.
Think I will use the Troy now that I understand it now.

Originally Posted by bsa1917hunter

Originally Posted by rost495

You can hit really well out to 100 easy and I can to 200 with that huge hole if you trust yourself.

I prefer smaller. The smaller holes are not really good enough for the MR31 target mentioned if shooting paper. There you need hooded and small. But no one shoots paper much anymore in matches like that with irons. I forget rear ap sizes a bit. thinking we were around 042 for the rear with a hood. Could be mistaken

An adjustable iris rear sight is better, something like the Merit that is on my Colt, but the small aperture works alright for most people. So Jeff, when you shot your targets in the black rifle challenge, you were using the big aperture for the MR31 target? The smaller aperture is more precise..

I shot only service rifle. No merit allowed.

We shot rear sights .25 moa both ways.

Screw in hooded rear sight apertures. I've never been able to see precisely enough with anything stock so to say, to shoot 600 yards really well.

Again pretty sure the hooded ones were 042 and 046 depending on the light. But those might not be the right numbers.

But again for smacking a pig or such, the big ghost aperture is plenty good. Just gotta hit vitals. Quick. Easy. Accurate enough. I don't think I saw this guy going to compete with that gun or my answer is different obviously.

Can you not shoot with the big ap? The only real thing that matters is front sight focus. The other 2 can be blurry enough and if you shoot a bit often the eye does all the centering of the ap and post. I mean you have to try, but the eye auto wants to center things up. Its like my wife that can't stand anything out of order it seems. LOL. She was and is a good shot too.