AR optics- HEIGHT OVER BORE issue

Kevp · Jan 22, 2014

Okay, we can simplify so new guys can easily understand. If you are using factory XM193 55gr ammunition from a manufacturer like Federal, you can expect to see impacts between 5-6" high at 100 if you zeroed at 25 with a 16" barrel. If you zeroed at 50, expect impacts between 1-2" high at 100. I do not recommend zeroing at less than 25 with iron sights and RDOs.

IMBLITZVT · Jan 22, 2014

Ok, if I can get slightly OT and give my specific example related to the issue.

I am getting a M16 and will add a Shrike upper to it. This gun should compare and function fairly closely to the US M249 SAW. On the Shrike I will be shooting with a 12" barrel with and without suppressor and a 16" barrel. I plan to shoot whatever ammo is cheapest.
From their website: Trijicon.com TA11MGO-M249

The TA11MGO-M249 3.5x35 ACOG is a Machine Gun Optic (MGO) designed specifically for the M249 weapon system. The MGO provides the shooter enhanced target identification and increased hit probability out to 1000 meters utilizing the Bullet Drop Compensator.

Optic Features:
500m Field Zero and 10m BZO Zeroing Instructions Laser Etched on Left and Right Side of Optic

Reticle Features:
Dual Illuminated Green Horseshoe Dot Reticle for Close-in, Reflexive Fire
BDC Designed for the M249 and M855 Ammunition to incorporate Spin Drift of the Found at Extended Distances
Horizontal Target Reference System (50 Mils)
Integrated 10m Z Qualification Reticle
Ranging Reticle from 100m (as shown in reticle image) to 1000m

So considering this optic was designed Specifically for a SAW, I thought it would be a good fit. However I will have different barrel lengths for sure. I think I will often be shooting 55gr ammo too.

So I am beginning to wonder if its going to be a good fit considering I am possibly changing two major issues used in the Bullet Drop Compensator. While I am not a target shooter, If I sometime have the need or desire to take a carefully aimed shot at something, is this optic going to keep me on target? Or have I changed enough that I should look for something else? I figure most often, I will be shooting at 50, 100 and 200 yards. I want something good for any of those ranges.

The other idea was just to go with a more simple ACOG.

Kevp · Jan 22, 2014

IMBLITZVT said:
Ok, if I can get slightly OT and give my specific example related to the issue.

I am getting a M16 and will add a Shrike upper to it. This gun should compare and function fairly closely to the US M249 SAW. On the Shrike I will be shooting with a 12" barrel with and without suppressor and a 16" barrel. I plan to shoot whatever ammo is cheapest.
From their website: Trijicon.com TA11MGO-M249

So considering this optic was designed Specifically for a SAW, I thought it would be a good fit. However I will have different barrel lengths for sure. I think I will often be shooting 55gr ammo too.

So I am beginning to wonder if its going to be a good fit considering I am possibly changing two major issues used in the Bullet Drop Compensator. While I am not a target shooter, If I sometime have the need or desire to take a carefully aimed shot at something, is this optic going to keep me on target? Or have I changed enough that I should look for something else? I figure most often, I will be shooting at 50, 100 and 200 yards. I want something good for any of those ranges.

The other idea was just to go with a more simple ACOG.

I have quite a bit of experience working ammo/ ACOG BDC reticle issues. In this case, I think you will be okay shooting 55 and 62 gr. You will have to play around with the 12" barrel to see what the impacts are, but the 16" shouldn't be an issue. Anytime you add a suppressor into the equation, you will see a delta. I don't care what the manufacturers say about POI shift. You have to shoot it to verify. Is this going on a registered lower? If so, don't worry so much about it. Remember that a belt fed MG is going to produce a cone of fire and beaten zone. The optic will help get you in the ballpark. Although you may be able to switch from semi to full auto belt fed with it, there is only one MG I know of that can really do a precise semi shot followed by full auto belt fed fire, and that is the HK 21E. The Shrike is not a HK 21E

.

IMBLITZVT · Jan 22, 2014

Kevp said:
I have quite a bit of experience working ammo/ ACOG BDC reticle issues. In this case, I think you will be okay shooting 55 and 62 gr. You will have to play around with the 12" barrel to see what the impacts are, but the 16" shouldn't be an issue. Anytime you add a suppressor into the equation, you will see a delta. I don't care what the manufacturers say about POI shift. You have to shoot it to verify. Is this going on a registered lower? If so, don't worry so much about it. Remember that a belt fed MG is going to produce a cone of fire and beaten zone. The optic will help get you in the ballpark. Although you may be able to switch from semi to full auto belt fed with it, there is only one MG I know of that can really do a precise semi shot followed by full auto belt fed fire, and that is the HK 21E. The Shrike is not a HK 21E .

Yes Registered Lower. Just got the form 3 back to go to my SOT. So I am probably damn near a year out. Yeah an HK21E is 308 right? Yes, different Ball Park. I have a 8mm MG08 with optics ranging out to 2000 yards for large rifle calibers.

It got a slow enough cycle rate, with a fast trigger pull, semi auto is easy.

Plus the HK21 does not have a 75lb mount holding still!

Anyway, just trying to figure out optics for it. Its not an easy thing. However seeing something custom made for the SAW had me thinking it could be a good option for my setup. Sounds like the difference should not make that much of a difference for my purpose....

SCARCQB · Jan 22, 2014

I have been working on the same project. I have a RR m16 lower fitted with an ARES MCR beltfed upper. Im using thr 16" barrel and a 20" heavy barrel. With a high magnification optic, good ammo and if you do your part, the 20" barrel will give you sub MOA groups.

While it is not an HK21.. It is more accurate than the FN m249 in semi automatic mode.

Im also going to be using a low magnification optic on it. However, i am concerned that most optics with BDC on them are specific to a platform due to height over bore issues. an ACOG that is specific to a flat top m16 / ammo, may not work with an MCR-Shrike or even a selective fire m14. The difference in mounting height can booger up the BDC.

Please keep us updated on this. i really dont waant to spend a good chunk of change in a optical sighting system that is not compatible with thr host firearm.

Kevp · Jan 22, 2014

SCARCQB said:
I have been working on the same project. I have a RR m16 lower fitted with an ARES MCR beltfed upper. Im using thr 16" barrel and a 20" heavy barrel. With a high magnification optic, good ammo and if you do your part, the 20" barrel will give you sub MOA groups.

While it is not an HK21.. It is more accurate than the FN m249 in semi automatic mode.

Im also going to be using a low magnification optic on it. However, i am concerned that most optics with BDC on them are specific to a platform due to height over bore issues. an ACOG that is specific to a flat top m16 / ammo, may not work with an MCR-Shrike or even a selective fire m14. The difference in mounting height can booger up the BDC.

Please keep us updated on this. i really dont waant to spend a good chunk of change in a optical sighting system that is not compatible with thr host firearm.

SCAR,

The M249 is not select fire (semi/FA), so comparing it is apples to oranges. The ACOG that was referenced is specified for the M249, not a flattop M4. Again, 4x magnified optics used on machineguns are not meant to provide a high level of precision/accuracy. They are meant to help you see better, and get you in the ballpark. Obviously height over bore is an issue with any optic, but it is not the driver when talking used on MGs. Example, many do not know that the Elcan M145 Machine Gun Optic (MGO) uses a hybrid BDC. It is meant to be used on both the M249 (5.56) and M240 (7.62) machine guns. That is not to say belt fed machine guns have not been used in the single shot mode. Both the M2 and M240 are known to be very accurate. There are documented instances of the M2 being used with a scope for target interdiction in Vietnam. I am unaware of any with the M240. That said, the Shrike is not meant to be a precision weapon system and the over-used sub-MOA standard is not appropriate.

SCARCQB · Jan 22, 2014

kevp, my buddy has an M249 in semi auto. I am familiar with the military SAW in full auto only. i was also in the Army a few years ago.

Im just comparing the accuracy ofthe ARES MCR/ Shrike ( closed bolt) vs an open bolt SAW.

My MCR shoots 0.75" at 100 yards consistently.. Is it appropriate to use the MOA term for that?
As per the MCR not being able to deliver sub moa groups..ill have to disagree with you.. I have two of these. You can also read the write up from defense review. hey have the same results.

Here is a nice write up.from shooting illustrated.. 0.56" group at 100 from the MCR
http://www.shootingillustrated.com/mobile/article.php?id=21431

Kevp · Jan 22, 2014

SCAR,

Semi-auto belt fed guns are collector novelties. As I stated, many machine guns are capable of very good accuracy when fired single shot, but that is not what they are meant to do with the exception of the HK21E.

Using the MOA measurement is fine, but it's not really appropriate when talking belt fed machine guns because they are area weapon systems. The vibration causes the barrel to move in a circular motion that creates a cone of fire, and beaten zone. It definitely isn't appropriate when talking 4x BDC optics purpose-built for machine guns because that is not what they are meant to do. Just making sure folks are well informed and have appropriate expectations.

Again, I think the ACOG is a fine choice for this civilian application. It all depends on how you are going to use it. If you are going to shoot it like a carbine in semi, it might disappoint. If shooting primarily from the prone in full auto off a bipod, it will be more than acceptable.

Pinecone · Jan 22, 2014

That is why I specifically put in MV data.

Barrel length WILL change the ballistics. So you need to know what YOUR load in YOUR rifle is doing.

I used the Hornady 55 gr FMJ-BT for my 55 grain data (JBM has this bullet) and the Sierra 77 gr Match King (which is the bullet used in M262). And I found a G7 BC (0.151) for the SS109 bullet in M855 ammo and used that for that data.

And as I mentioned before, one of the design criteria for the M855 ammo was similiar ballistics to M193 ammo, so as to not confuse people.

I read the article that a link was supplied. He makes some good points and say some things that don't make a lot of sense.

But I do take exception with his scenario of a perp holding a family member hostage and you trying to take a head shot. Under the adrenaline rush you will be having, you will be lucky to hit your house, for the inside.

SCARCQB · Jan 22, 2014

Pinecone, unless you can see the future..I do not think you can predict how people are going to react under stress/ adrenalin. Everyone is different, you cant generalize.

We are talking equipment, not about the people that operate the equipment. Is the MCR capable of sub moa groups? In a selective fire version, is it equivalent to an M249 or maybe an m27 IAR? Will the BDC on ACOGs work on this platform? Since HOB is significantly different?

Unless Kevp did a government funded T/E on this system, we are all still in the learning stage.

Kevp · Jan 22, 2014

Pinecone said:
That is why I specifically put in MV data. Barrel length WILL change the ballistics. So you need to know what YOUR load in YOUR rifle is doing.

I used the Hornady 55 gr FMJ-BT for my 55 grain data (JBM has this bullet) and the Sierra 77 gr Match King (which is the bullet used in M262). And I found a G7 BC (0.151) for the SS109 bullet in M855 ammo and used that for that data.

And as I mentioned before, one of the design criteria for the M855 ammo was similiar ballistics to M193 ammo, so as to not confuse people.

I read the article that a link was supplied. He makes some good points and say some things that don't make a lot of sense.

But I do take exception with his scenario of a perp holding a family member hostage and you trying to take a head shot. Under the adrenaline rush you will be having, you will be lucky to hit your house, for the inside.

Pinecone,

Kenan Flasowski is a former Tier 1 Special Operations sniper, and very respected trainer. He knows what he is talking about. As I stated in an earlier post, many guys with that background are proponents of the 100m zero. I'm not sure what you think doesn't make sense? As for the scenario he uses, that is why you train and do so under as stressful of circumstances as possible (competition, timers, scenario-based training, sims). You don't know what kind of scenario you would face in something like a home invasion or any armed encounter. The alternative, to give up, is not a good one.

Kevp · Jan 22, 2014

SCARCQB said:
Pinecone, unless you can see the future..I do not think you can predict how people are going to react under stress/ adrenalin. Everyone is different, you cant generalize.

We are talking equipment, not about the people that operate the equipment. Is the MCR capable of sub moa groups? In a selective fire version, is it equivalent to an M249 or maybe an m27 IAR? Will the BDC on ACOGs work on this platform? Since HOB is significantly different?

Unless Kevp did a government funded T/E on this system, we are all still in the learning stage.

SCAR,
I see a lot of things in the course of my work. I don't comment on them specifically on the Internet, but I'm not just talking. Again, an ACOG that is meant for a 5.56 machine gun will work fine if this platform is used as a machine gun.

BDC reticles are not the same as using a scope with tried and true dope. An ACOG is good for minute of man out to a reasonable range (that will vary based on shooter and conditions). That said, it isn't an absolute plug and play system. You need to practice, and verify what it/you are capable of at the various distances. Remember, ammunition varies from lot to lot. No BDC is going to be perfect.

Example: I've used an ACOG in combat and was very comfortable with it, and and I've shot steel E-type targets out past 500yds with success. I've also tried to shoot ground hogs at 100+ yards with it, and was not successful- I equate it to trying to shoot a moving loaf of bread at 100yds. It's not easy, and is different than shooting a man sized target. I want a sub-MOA platform for shooting ground hogs, and anything with an ACOG ain't it.

I hope the question was answered. This was not supposed to be a discussion of the merits of the MCR or its accuracy in a semi auto mode.

SCARCQB · Jan 22, 2014

Kevp,

To be honest with you. im perfectly happy with The MCR with standard iron sights. Running the upper with a RR M16 lower, its is so much easier to just walk the rounds towards the target. i did use a scope on it to figure out accuracy. But you are right. My intent is to have a civilian legal SAW, similar to an M249, Negev or ultimax 100. It just so happens that the darn thing was very accurate in semi.

Im also not too confident about mounting an optic on the top over. Its only a matter of time until that thing loosens. Mounting an ACOG on an already raised rail will put the HOB at about 4". That is just a bit too high. Im certain that the ACOGs BDC will not line up at longer ranges.

SCARCQB · Jan 22, 2014

Kevp said:
Okay, we can simplify so new guys can easily understand. If you are using factory XM193 55gr ammunition from a manufacturer like Federal, you can expect to see impacts between 5-6" high at 100 if you zeroed at 25 with a 16" barrel. If you zeroed at 50, expect impacts between 1-2" high at 100. I do not recommend zeroing at less than 25 with iron sights and RDOs.

Thats sums it up very well.

:thumbsup:

toncapone1 · Jan 22, 2014

Awesome info in this thread, I just got an Aimpoint PRO (finally) and still need to zero it, this will certainly help so thanks to all who contributed

Biggfoot44 · Jan 23, 2014

Once again Pist #21 sums up the gist of the origional discusion.

My philosophical aproach values closer POI/POA to close to medimum distance ( meaning 10 or 15yd out to 100-150yd ). I figure closer shot will potentially be quicker shots , and I want to aim and ignore hold over/ under. At long range will require more reflexion on distance , evvironmental conditions, and holdover anyway , and less difference if the holdover is slightly more.

For typical rifle generally using for deer sized targets ( works for defensive too ) , I prefer to limit my maximim POI above POA to about 1.5 in , no more than 2in . In the AR world , this come reasonably close to 50yd zero.

If your focous was plinking golf balls , or head shooting chimpmonks at 100yds , then a 100yd zero would be useful.

SCARCQB · Jan 23, 2014

How about old school guys that have carry handle mounted optics?

Zeroed at 25, it was almost 11" high at 100

Pinecone · Jan 23, 2014

Kevp said:
Pinecone,

Kenan Flasowski is a former Tier 1 Special Operations sniper, and very respected trainer. He knows what he is talking about. As I stated in an earlier post, many guys with that background are proponents of the 100m zero. I'm not sure what you think doesn't make sense? As for the scenario he uses, that is why you train and do so under as stressful of circumstances as possible (competition, timers, scenario-based training, sims). You don't know what kind of scenario you would face in something like a home invasion or any armed encounter. The alternative, to give up, is not a good one.

My thing is, for HD, you are not going to be taking shots at longer ranges. And the difference in the various zeros, out to 50 yards, is fairly small.

And if you are going to take a longer shot, why not set up for a maximum point blank range that will work from muzzle to 280 yards?

If you set the vital zone to a +/- 2.6", a 45 yard zero works out to give you a hit in that zone out to 280 yards.

Here is the impact point to point of aim at 0 - 50 yards for 55 grain FMJ at 31500 fps MV.

Range 45 yard zero 100 zero

0 -2.5 -2.5
10 -1.9 -2.1
20 -1.3 -1.7
30 -0.7 -1.3
40 -0.2 -1.0
50 +0.2 -0.8

(forum formatting sucks)

I am not saying it is wrong, but saying I can see the case for other options. I sort of see his point out the hold over staying the same, but again, at HD distances, you are talking about differences of a a few tenths of an inch.

And I am a BIG fan of doing training or competitions to add stress to the situation to learn to work with that stress.

Half-cocked · Jan 23, 2014

I was thinking about creating a graph, showing the various trajectories obtained by zeroing standard 5.56 loads in an AR-15 at various distances, but then I found that someone else had done this, in great detail:

http://www.ar15.com/archive/topic.html?b=3&f=118&t=495607

erwos · Jan 23, 2014

I need those graphs for 7.62x39, 7.62x51, and 9mm.

AR optics- HEIGHT OVER BORE issue

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