View Full Version : Serious Problem with DD Fixed Irons on BCM Rifle
Hey Folks, I promise I have searched deeply and can’t find anything to resolve this issue. Any admin that can view my activity will be able confirm my efforts. I only mention this because I’ve modded other forums where simple searches are seldom done for common questions. With that out of the way...
I recently purchased a 16” BCM Recce with a KMR A15 on which I mounted a pair of DD fixed irons. I only plan on running Irons for the foreseeable/time being but these are not working. My results are as follows. Using M193 ball ammo at 25 yds for initial zero, I was still approximately 1.5” below POA with the front sight post bottomed 100% completely out. After not finding a resolution here, I decided to contact BCM for advice, thinking I’d chosen improper sights for this rifle. They immediately requested I return the upper with sights installed, and to their credit sent me a prepaid 3day ups label. In short, they confirmed what I did, and obviously checked the upper for issues. They let me know today that with an A2 handle, and Troy flip front installed, they were able to complete a proper zero, no problem. About this, they stated that this setup met/matched proper MIL-SPEC height requirements. They did not state the DD sights were lacking in this regard but did say they suspect one of the sights was too tall or short. Very confusing. And they suggested I contact DD regarding the issue which I have done. I’m presently awaiting a reply from them, and shortly the return of my upper from BCM. I bought these sights from a reputable dealer and do not believe them to be counterfeit, and there’s nothing I can see that’s obviously wrong with them.
With the lack of availability on many AR parts, you can imagine the frustration I’m having. I really don’t want a carry handle, but do want to run irons now, and still when I do eventually acquire a good optic. I am new to ARs, but I’ve shot M1 and M1 Carbines extensively over the last 20 years so I am very partial to that traditional setup. If I’m an idiot for choosing the entirely wrong setup out of ignorance I don’t mind being called out on it. I’m just looking for advice and help from any of you here who no doubt know more than I. Thank you in advance for any assistance.
Thunderpigeon
10-17-20, 14:32
The general rule for moving iron sights is FORTS- "front opposite rear the same", which means you move the front sight down to make the point of impact go up, and vice versa for the rear sight. It sounds like you are moving the front sight post the wrong way.
The general rule for moving iron sights is FORTS- "front opposite rear the same", which means you move the front sight down to make the point of impact go up, and vice versa for the rear sight. It sounds like you are moving the front sight post the wrong way.
If that’s not issue (I assume BCM would’ve checked since they asked for the sights to be included), likely just a defective sight. There should be no problems with you chose. BTW, for fixed sights, Scalarworks are amazing. I’m sure DD will take care of you though, again assuming it’s not somehow user error.
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titsonritz
10-17-20, 15:01
BTW, for fixed sights, Scalarworks are amazing.
Ditto, they are my favorite fixed sights.
The general rule for moving iron sights is FORTS- "front opposite rear the same", which means you move the front sight down to make the point of impact go up, and vice versa for the rear sight. It sounds like you are moving the front sight post the wrong way.
Thanks for the reply. Yes, down on the front to make the POI come up. I’ve screwed the front post down as far as it will go. If the rear had elevation you’d have to raise that one up to raise the the POI. My apologies if that wasn’t clear in my first post.
I did see the Scalarworks sights, thought they looked amazing. Even thought they were expensive I wanted them but couldn’t locate a set anywhere.
Zero at 50 yards or 100 yards and problem is solved.
I had to replace the post on a DD fixed front sight once with a taller or shorter one. Don’t remember which. Sorry if that doesn’t help much. Try googling around for different height posts. I think maybe KNS makes one?
Thunderpigeon
10-17-20, 18:27
Do you have the tools to measure the height of the front post?
Do you have the tools to measure the height of the front post?
This would help.
Also, it would be nice to know what it does at 50 or 100 yards. You may be very close to zeroed. I think 1-1.25” low is normal for a zeroed AR at 25 yards.
Zero at 50 yards or 100 yards and problem is solved.
Not sure how that will work out if I’m this low at 25 with no adjustment left, but I’m willing to learn.
I had to replace the post on a DD fixed front sight once with a taller or shorter one. Don’t remember which. Sorry if that doesn’t help much. Try googling around for different height posts. I think maybe KNS makes one?
That’s interesting, and I will look around. I’ve come across a couple of early featured M1 Carbines with sight blades too tall, and had to deal with that. A whole lot would have to come off this one, so I will look into that. Perhaps DD will shed some light on this for me as well.
Thank you both...
Do you have the tools to measure the height of the front post?
Sorry I missed this. Yes, I have a good set of digital calipers, so I’m sure I could get this done.
Is it possible the front sight is intended for a rail gas block, not a handguard rail? How different are the front and rear sight heights measured from the rail? Not post and aperture, the entire sight.
By the way, a 25 yard zero crosses the sight line again at 400 yards. The height of the sights over the bore makes an AR entirely different from an M1/M1 Carbine.
Andy
Is it possible the front sight is intended for a rail gas block, not a handguard rail? How different are the front and rear sight heights measured from the rail? Not post and aperture, the entire sight.
Andy
I cannot presently give an answer either of those, but assume DD can regarding the front. Once I have the upper back in hand, I can measure and answer the latter.
If BCM can get it zero'ed at 25 with different sights, then I am sure DD will get the problem sorted. DD are great sights.
Andy
Just to clarify, are you trying to get point of aim to match point of impact at 25 yards? If so, that’s not typically how it is done.
Just to clarify, are you trying to get point of aim to match point of impact at 25 yards? If so, that’s not typically how it is done.
...it is and has been by the Army for decades.
...it is and has been by the Army for decades.
Correct, but for most civilians it isn’t desirable to be ~6” off at 100 yards.
Just to clarify, are you trying to get point of aim to match point of impact at 25 yards? If so, that’s not typically how it is done.
Literally everyday with M4s and M16s numbering on a scale that makes the entirety of this site look pathetic.
Not sure how that will work out if I’m this low at 25 with no adjustment left, but I’m willing to learn.
That’s interesting, and I will look around. I’ve come across a couple of early featured M1 Carbines with sight blades too tall, and had to deal with that. A whole lot would have to come off this one, so I will look into that. Perhaps DD will shed some light on this for me as well.
Thank you both...
You dont understand bullet trajectory. Shoot it a 100 and see your point of impact.
Not sure how that will work out if I’m this low at 25 with no adjustment left, but I’m willing to learn.
1.5" low at 25 yards is basically a 100 yard zero on an AR because of the sight height over bore. A 25 yard zero requires a pretty steep trajectory and the POI would cross the POA again at 400 yards.
All academic, I would be displeased if I had to bottom out the front sight to zero at 100 yards. Hopefully DD takes care of you.
Andy
Wonder where he bought the sights? A lot of fakes out there. https://danieldefense.com/counterfeit-information
1.5" low at 25 yards is basically a 100 yard zero on an AR because of the sight height over bore. A 25 yard zero requires a pretty steep trajectory and the POI would cross the POA again at 400 yards.
All academic, I would be displeased if I had to bottom out the front sight to zero at 100 yards. Hopefully DD takes care of you.
Andy
This is my concern since there is nothing left if I need it. And yes, it’s the front sight.
And to others no, I’m no expert and said I was new to this platform at the start, but I’m learning. I understand a little, and know there is offset as well, but something isn’t right here. Whatever the case, I’ll get it sorted. Thanks for your help all.
This is my concern since there is nothing left if I need it. And yes, it’s the front sight.
I see you caught my error before I did.
Andy
I see you caught my error before I did.
Andy
:secret:
Not sure how that will work out if I’m this low at 25 with no adjustment left, but I’m willing to learn.
That’s interesting, and I will look around. I’ve come across a couple of early featured M1 Carbines with sight blades too tall, and had to deal with that. A whole lot would have to come off this one, so I will look into that. Perhaps DD will shed some light on this for me as well.
Thank you both...
That low at 25 would probably still have you hitting high at 100 so I would would at 100 to check and it'll probably be good.
My standard procedure for zeroing same plane rear irons for POI=POA at 100 yards is to shoot a 25 yard target and adjust so the bullet is roughly 2 inches low at 25 and that will put nearly dead on elevation at 100.
I bought a used "DD" front that wouldn't zero at 50yards(hitting high with the post barely threaded in) so assumed I got a fake even though it seemed to match pictures of some DD sights through the years.
Go here and use 2.61" for the sight height.
https://www.federalpremium.com/ballistics-calculator
What you have seems to be on the ragged edge of being able to get either a 50 or 100 yard zero, but the maxed out part sounds like something is off.
Dr. Bullseye
10-27-20, 12:28
Are you married to DD sights? When you found a problem, you did not contact DD, you contacted BCM and they were just fine with other sights. DD rear does not even adjust for distance. OK, it is cool, sleek, A1 windage, slides on, but these are for up close work only. There is a cut down A2 from LMC and an even cooler design marketed by Palmento. These can be adjusted for distance. Or you could move into the 21st Century with Ultradynes. What is the fascination for DD?
Rifleman_04
11-17-20, 18:48
1.5" low at 25 yards is basically a 100 yard zero on an AR because of the sight height over bore. A 25 yard zero requires a pretty steep trajectory and the POI would cross the POA again at 400 yards.
All academic, I would be displeased if I had to bottom out the front sight to zero at 100 yards. Hopefully DD takes care of you.
Andy
Zeroing at 25M is a 300M zero. Army and Marines do this to get on paper. Confirm at distance.
Iron sights on a FF rail is stupid. I see this all the time.
What is happening to this forum?
Zeroing at 25M is a 300M zero. Army and Marines do this to get on paper. Confirm at distance.
Iron sights on a FF rail is stupid. I see this all the time.
What is happening to this forum?
I can't speak to the Marines, but I spend 20 years in the Army and never once confirmed at distance. Never fired at a KD range after basic training. Silhouettes only.
M193 @ 3200 fps is 4" high and M855 @ 3100 fps is 2" high at 300 M with a 25 M zero. From a ballistics calculator because why would I zero at 25 M now that I have a choice?
Andy
Literally everyday with M4s and M16s numbering on a scale that makes the entirety of this site look pathetic.
Just because it's the Army way doesn't mean it's the right way. In fact, that correlation coefficient is probably comically low overall.
Zero at 50yd if you're able to, 36yd if you need to, or calculate how low you need to be in order for you to get a usable 50yd zero.
What lets the 36yd zero work well is that you can line up a 300m BDC to that point, and you end up with something usable... you could do the same with a 400offset at 25yd, but 50/200 is still a preferable point because of the managable 115-125yd offset
I can't speak to the Marines, but I spend 20 years in the Army and never once confirmed at distance. Never fired at a KD range after basic training. Silhouettes only.
Andy
Fact.
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Iron sights on a FF rail is stupid. I see this all the time.
What is happening to this forum?
Why is that stupid?
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Rifleman_04
11-21-20, 07:51
I can't speak to the Marines, but I spend 20 years in the Army and never once confirmed at distance. Never fired at a KD range after basic training. Silhouettes only.
M193 @ 3200 fps is 4" high and M855 @ 3100 fps is 2" high at 300 M with a 25 M zero. From a ballistics calculator because why would I zero at 25 M now that I have a choice?
Andy
So your command sucked then. Unfortunate.
My Knights BulletFlight Military Version 4.6 for the M4 firing M855 with 25M zero is -0.3@25M and 0.5@300M.
Rifleman_04
11-21-20, 08:02
Why is that stupid?
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The subject of this thread? I’m solidly in the camp of iron sights mounted on FF rails just cause problems. As I said I’ve seen these problems enough to form my opinion.
Zeroing at 25M is a 300M zero. Army and Marines do this to get on paper. Confirm at distance.
Iron sights on a FF rail is stupid. I see this all the time.
What is happening to this forum?
I guess it’s like any forum. They are all susceptible to stupid people joining up, and asking dumb a** questions that others were born knowing the answers too. It’s a real problem I know from experience moderating for many years on one. Mostly because it puts a bad face on the forum, and by extension the activity/hobby when members berate and belittle instead of trying to help someone past “Ned and the first Reader” stage. I don’t know if this is how you walk up to someone on a range and start talking, but I wouldn’t be surprised if you did. I think back and remember it was definitely people like you that were one of the things that turned me off of the AR scene. People into the old stuff I was wild, and just learning about then loved sharing knowledge and helping out a new guy. Maybe even with parts, repairs and problems, or anything else they knew, but one thing for sure, they usually didn’t walk up and tell you how stupid you were for doing this or that. Especially if you were obviously new and green. Even if it’s a forum, I try hard to speak as if I’m talking to someone in person, at a range, or shop...whatever. Seems like that got lost on the internet transition. But enough on that.
I appreciate everyone else and sorry I haven’t replied but since this posted, I’ve had some important life and work things preventing me from getting this resolved. I did make contact with DD and can’t say enough good about them. When it’s all sorted, I’ll come back and finish this off. Again, I sincerely appreciate all the input, and I’m grateful for the help. Been trying to educate myself more and have taken it all into consideration. I’ll address you individually later on, but thanks again to all.
So your command sucked then. Unfortunate.
Even in SOF, KD ranges don’t make up the bulk of range time. I could probably count on my fingers the number of times I’ve shot paper past 100 while being paid. Steel and E-types instead. Its not about a command sucking, its that trying to reproduce a NRA High Power match is not the best use of range time. You’re either going cold to check targets hundreds of meters away, or half your dudes are in the pits instead of shooting. There are more efficient ways to train a bunch of goons than sitting on a KD range talking about the weather and what type of grass is best.
Even in SOF, KD ranges don’t make up the bulk of range time. I could probably count on my fingers the number of times I’ve shot paper past 100 while being paid. Steel and E-types instead. Its not about a command sucking, its that trying to reproduce a NRA High Power match is not the best use of range time. You’re either going cold to check targets hundreds of meters away, or half your dudes are in the pits instead of shooting. There are more efficient ways to train a bunch of goons than sitting on a KD range talking about the weather and what type of grass is best.
I’d enjoy learning more about that. I know just enough to be confounded about how they accepted weapons for adoption based on how they were judged and trialed, versus how people trained with them and used them in real world scenarios. All very strange if you ask me.
And hey Rifleman 04, I hope you come back and expand. I wonder if you say that because of something or things that have given me pause, and reason to wonder.
Rifleman_04
12-02-20, 20:43
I guess it’s like any forum. They are all susceptible to stupid people joining up, and asking dumb a** questions that others were born knowing the answers too. It’s a real problem I know from experience moderating for many years on one. Mostly because it puts a bad face on the forum, and by extension the activity/hobby when members berate and belittle instead of trying to help someone past “Ned and the first Reader” stage. I don’t know if this is how you walk up to someone on a range and start talking, but I wouldn’t be surprised if you did. I think back and remember it was definitely people like you that were one of the things that turned me off of the AR scene. People into the old stuff I was wild, and just learning about then loved sharing knowledge and helping out a new guy. Maybe even with parts, repairs and problems, or anything else they knew, but one thing for sure, they usually didn’t walk up and tell you how stupid you were for doing this or that. Especially if you were obviously new and green. Even if it’s a forum, I try hard to speak as if I’m talking to someone in person, at a range, or shop...whatever. Seems like that got lost on the internet transition.
Nice try with the lecture but I spent a decade in the Marine Corps and this is just how I speak. To everyone. I’m not an expert and I wasn’t born knowing all answers, I learned through experience. I’ll share my experiences and opinions but just like assholes everybody’s got one.
Also this is the internet and half of it is how you interpret my speak and tone. There is also an ignore button. Or grow thicker skin. Jeez.
Rifleman_04
12-02-20, 20:44
Even in SOF, KD ranges don’t make up the bulk of range time. I could probably count on my fingers the number of times I’ve shot paper past 100 while being paid. Steel and E-types instead. Its not about a command sucking, its that trying to reproduce a NRA High Power match is not the best use of range time. You’re either going cold to check targets hundreds of meters away, or half your dudes are in the pits instead of shooting. There are more efficient ways to train a bunch of goons than sitting on a KD range talking about the weather and what type of grass is best.
You’ve outlined the institutional differences between services...that’s why we shoot better.
Rifleman_04
12-02-20, 20:46
And hey Rifleman 04, I hope you come back and expand. I wonder if you say that because of something or things that have given me pause, and reason to wonder.
I’ll stay as long as you’ll have me 😉
You’ve outlined the institutional differences between services...that’s why we shoot better.
Lol. Ok.
So your command sucked then. Unfortunate.
My Knights BulletFlight Military Version 4.6 for the M4 firing M855 with 25M zero is -0.3@25M and 0.5@300M.
Perhaps you mean my branch of service not my command, but whatever, dude.
A trajectory that crosses POA at neither 25M or 300M means a 25M zero = 300M zero? I'm not following your logic.
Rifleman_04
12-03-20, 06:42
A trajectory that crosses POA at neither 25M or 300M means a 25M zero = 300M zero? I'm not following your logic.
Uh, that’s how it works out in the real world.
It will never be pinpoint. Day to day the variance in temperature/pressure/ammo will change the poi. Those were the numbers that particular day.
I’ll stay as long as you’ll have me 😉
Yea, I just meant this thread, and for you to explain your opinion on why it’s stupid. Are you saying that only optics should be run on FF Rail guns?
I'm not Rifleman, or speaking for him but...
Sights mounted to a free float handguard/rail simply cannot be as precise as sights attached to the barrel. If they are used as short range BUIS, the precision should be adequate and certainly better than point shooting.
Andy
Rifleman_04
12-03-20, 08:40
Yea, I just meant this thread, and for you to explain your opinion on why it’s stupid. Are you saying that only optics should be run on FF Rail guns?
I’m saying irons do belong on FF rail guns but solidly affixed to the barrel. I’m a fan of the Centurion cutout rails for that reason. I’ve seen point of impact shift issues with BCM, Centurion and Geissele rails running buis attached to the rails when the guns are used in other than flat range conditions.
Rifleman_04
12-03-20, 08:46
I'm not Rifleman, or speaking for him but...
Sights mounted to a free float handguard/rail simply cannot be as precise as sights attached to the barrel. If they are used as short range BUIS, the precision should be adequate and certainly better than point shooting.
Andy
I have observed less deflection in poi on guns with free float rails and fixed front sight base on the barrel when shot off of packs, barriers and bipods while using iron sights.
I have observed less deflection in poi on guns with free float rails and fixed front sight base on the barrel when shot off of packs, barriers and bipods while using iron sights.
Absolutely, FF rail and FSB is the best of both worlds.
Perhaps you mean my branch of service not my command, but whatever, dude.
A trajectory that crosses POA at neither 25M or 300M means a 25M zero = 300M zero? I'm not following your logic.
There's a bit of nuance about projectile weight and velocity that affects it - 36yd is preferable for 300m/yd if we're talking 20" barrel 62gr, and it's nice that we can abuse a BDC reticle that allows for quicker/easier 50M verification.
As far as barrel deflection versus FF handguard deflection, the answer is that barrel mounted will chase some of the imparted loads in various positions, it's as simple as that. Those deviations are all <2moa in the grand scheme of things, and if you can shoot ~5 MOA for an overall probability distribution, you are an Expert marksman by jarhead standards.
It's not ideal, but I haven't noticed a meaningful difference in spread between my FF railed DD fronts and Centurion FSP setups. The best end-run around that is primarily using an optic, which negates most of those issues.
The important way to think of it is that optics are just affixed to a receiver that doesn't chase those deflections, so any deviation between FF handguard deflection and what the relative barrel deflection would be at the FSP are still appropriately referenced against what an optic mounted to the upper would be.
So... short answer is that any differential is completely in the noise compared to how terrible human physiology as an artillery platform realistically is, even under ideal circumstances.
I use a DD rear with standard front sight/gas block. My zero is 36 yards and the front post is nearly bottomed out, works for me. The ears for the front sight do a good job of protecting the KNS pin head front I use.
I use a DD rear with standard front sight/gas block. My zero is 36 yards and the front post is nearly bottomed out, works for me. The ears for the front sight do a good job of protecting the KNS pin head front I use.
The KNS sight posts tend to be longer than standard, which explains why you're bottomed out. No harm, no foul though as everything works.
The KNS sight posts tend to be longer than standard, which explains why you're bottomed out. No harm, no foul though as everything works.
Not bottomed out but low I'd say. If it were bottomed out I would change something so that the front could come up a bit.
duroSIG556R
02-07-21, 06:04
It's possible you may have bought a front sight designed for a2 and not a same plain front sight. Also keep in mind the height over bore with an ar15. Holdover is always needed within close range. Try zeroing at 50 yards. One of my favorite front sight posts is from blitzkreig. The post has a built in offset.
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