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Posted: 4/12/2026 7:48:23 PM EDT
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Apologies if this has been covered, but I ran a search and didn't find anything. Anybody know what thickness I need to raise my TA31 up over an MBUS Pro rear sight using a Larue LT100 mount? I am confused because I saw on another board a dude said a 5.5 would do it but Weapon Outfitters advertises on their site a 7.0 for MBUS Pro. I'm trying not to go any higher than I need to because I have a piggyback RMR on the ACOG. Chin... errr cheek weld is already getting dicey. Pics would be appreciated! |
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Originally Posted By Raoul_Duke70: Already have a backup sight in the RMR. Lose the irons, they're an unnecessary meme at this point and set the ACOG at the best position and height for both eye relief and red dot use. Now let's say I'm asking for a friend that doesn't have a piggyback RMR? 
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Originally Posted By Raoul_Duke70: That friend should just save for the rmr. If you're dead set on using the BUIS I'd get the 7.0. It's really not that much higher and it'll definitely fit over the pros. Soooo, I just tried your suggestion and effing A, cotton - you're right. I took the Pro off and moved the ACOG way back and I actually had to move it up two slots. That's definitely the way to go. I suppose the world's most rugged prism with etched reticle and arguably the world's most rugged RDS is probably good enough. |
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Happy to help. Brother, if your ACOG and RMR take a shit on the same day you needed a rifle to protect your life, the BUIS wasn’t gonna help you on a day that shitty. Jokes aside, always prioritize your primary sighting system over trying to shoehorn redundant sights on it that make the rifle harder to use or heavier and more cluttered, while getting a less than ideal primary optic setup. |
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Originally Posted By LipschitzWrath: Alright, might have to build a new gun, haha. This gun has an FSB. It's begging for a rear BUIS... And before you suggest chopping, it's got Centurion C4 mid-cutout rail. When I was still in the infantry and had an M4A1, I moved my ACOG back as far as it would go and then put the matech rear sight in front of the ACOG. |
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I find these threads interesting because the only time I have even a slight issue with TA31 ACOG eye relief is when combined with the length of the A2 stock. I have rear BUIS on all my rifles and I only mount my ACOGs as far back as the rear BUIS will allow (no KRAM spacers either). I can only imagine it's due to shooting preferences and individual anatomy. I do notice a lot of people don't shoot "nose to charging handle" like I tend to do pretty naturally (though it is a slight stretch with A2 stocks). I'd figure that collapsible stocks would make the issue moot for most, but it's interesting that that doesn't seem to be the case. |
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| For me NTCH kinda sucks. I’m tall with long arms and use carbine stocks all the way out, unless it’s a longer stock or an A5 then it’s one notch in from full extension. Combine that with a herniated disc in my neck and also suppressor use and I’m gonna hard pass on NTCH. Plus in over 20 years I’ve never once used or needed BUIS. So I’ve never needed to worry about making room for them. Plus it’s just a lot of extra money on unnecessary superfluous stuff when you have a bunch of rifles. Not to mention that with optics other than red dots using mounts that are cowitness height, you need a QD to use them. Just makes the mounts heavier and requires finding optic and mount combinations that will clear a folded rear sight. Just easier to ignore a folding piece of overpriced metal that’s more likely to break or lose zero than the primary optic. |
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Originally Posted By 03RN: Fwiw I would use the 7.0 kram. As great as acogs are im still mounting irons. Ill take the 2 oz hit. I also just like shooting irons. With the right kram there is no negative Doesn't using a 7mm kram throw the bdc off? How does using it impact the rmr top dot height? Too high or just right? Thanks |
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Keep that in mind when reading my comments:-)
Keep that in mind when reading my comments:-)
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Originally Posted By carholio: Doesn't using a 7mm kram throw the bdc off? How does using it impact the rmr top dot height? Too high or just right? Thanks https://www.ar15.com/forums/general/How-bad-does-raising-an-ACOG-effect-the-BDC-/5-2686818/? As for the RMR height, I'll let you know when mine gets delivered tomorrow. I ordered the 7.0 KRAM. I'm excited to install it and see how well it works. |
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Originally Posted By carholio: Doesn't using a 7mm kram throw the bdc off? How does using it impact the rmr top dot height? Too high or just right? Thanks Not enough to matter at the ranges the ACOG is useful for. Besides, ammo weight and velocity, temperature, height above sea level and other things impact the accuracy of the BDC. Not to mention it’s a BDC, it’s not super precise anyway. It’s meant to help you get hits on torso size targets quickly without having to do any real thinking. |
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Originally Posted By carholio: Doesn't using a 7mm kram throw the bdc off? How does using it impact the rmr top dot height? Too high or just right? Thanks For a piggyback RMR on an ACOG, rather than lifting your head up to look through the RMR, you can just cant the rifle 45 degrees and look through the RMR with your left eye. I thought it was stupid at first but tried it a few times and it works surprisingly well. If you're one of those people that can look through an ACOG with both eyes open (I'm not), it works even better. Just thought I'd share because when used in that way, I'm not sure the KRAM spacer will really have any effect. But again, I'll let you know tomorrow. |
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Originally Posted By carholio: Doesn't using a 7mm kram throw the bdc off? How does using it impact the rmr top dot height? Too high or just right? Thanks Originally Posted By carholio: Originally Posted By 03RN: Fwiw I would use the 7.0 kram. As great as acogs are im still mounting irons. Ill take the 2 oz hit. I also just like shooting irons. With the right kram there is no negative Doesn't using a 7mm kram throw the bdc off? How does using it impact the rmr top dot height? Too high or just right? Thanks Considering im using 75gr bullets at 2800fps I need to verify poi regardless. |
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Alright got the KRAM today and installed it. It truly does help a ton. Reminder, I did the 7.0mm spacer. Plenty of room for a BUIS and the ACOG is positioned perfectly. I could even go one more slot to the rear with the ACOG and still have room for a BUIS but it's totally unnecessary. Cheek weld is repeatable and places your eye perfectly to really make the most of the TA31's immense FOV. I love it. The added height of the spacer is pretty much unnoticeable. In fact, I almost feel like the head position is more comfortable and my head naturally falls in behind the scope. Canting the rifle 45 degrees as previously mentioned places the piggyback RMR perfectly in front of my left eye. FWIW, the packaging from Weapon Outfitters clearly indicates the 5.5 spacer will clear MBUS Pro rear sights, the 7.0 says it will clear the KAC rear, and the 9.0 states it will clear MBUS polymer rear sights. Overall, it's highly recommended and hard to come up with any downsides to this setup. |
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