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Posted: 7/15/2026 1:21:25 PM EDT
[Last Edit: awd_xtc][Edited]
| I am in the market for a high(er) end upper. I was going to just do a mk18 but having used them in the army I’m not sure if that is the best modern day solution to an sbr. They tend to be over gassed. So I started to consider the Geissele super duty but I can’t seem to decide between the 11.5 and the 12.5. In theory the 12.5 should be smoother with a smaller gas port but I see a lot of reviews that say it is not really noticeable. The gun will be shot with an arc fire frt occasionally. I will not have the need to shoot beyond a few hundred yards, 300 at most and that would be rare. What would you choose? Should I just go mk18? |
| I think 12.5's are the best all around rifle for out to 500 yards and I like an LPVO on them. However, I recently switched my HD/under 2-300 yard rifle from my 12.5 to a 10.3. It's just so handy and I am ok with the velocity loss. 11.5 with a K can would be nice too, but I would go as short as you are comfortable with for a 300 yard and in gun. |
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10.3 Super Duty with a Hoffman SS. July 15, 2026 |
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Originally Posted By awd_xtc: I am in the market for a high(er) end upper. I was going to just do a mk18 but having used them in the army I’m not sure if that is the best modern day solution to an sbr. They tend to be over gassed. So I started to consider the Geissele super duty but I can’t seem to decide between the 11.5 and the 12.5. In theory the 12.5 should be smoother with a smaller gas port but I see a lot of reviews that say it is not really noticeable. The gun will be shot with an arc fire frt occasionally. I will not have the need to shoot beyond a few hundred yards, 300 at most and that would be rare. What would you choose? Should I just go mk18? Why do you say they're overgassed? |
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Originally Posted By bluedog82: I’d do the 11.5, personally, but all 3 would be fine (I’ve got all of them, length wise). They’re all gassed approximately. |
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Originally Posted By awd_xtc: So you don’t think the mk18 is more heavily gassed compared to a super duty 11.5? No, unless you are buying just the SD barrel. That is my only complaint with Geissele, their stripped barrels are gassed differently than their uppers/rifles. OP I have a complete super duty in 10.3 and a 12.5 upper. Both are awesome, but I tend to reach for the 10.3 SD. |
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Originally Posted By BULLDAWG_556: No, unless you are buying just the SD barrel. That is my only complaint with Geissele, their stripped barrels are gassed differently than their uppers/rifles. OP I have a complete super duty in 10.3 and a 12.5 upper. Both are awesome, but I tend to reach for the 10.3 SD. Sorry I was talking about a Daniel Defense mk18 |
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Originally Posted By awd_xtc: Sorry I was talking about a Daniel Defense mk18 For what it’s worth I just bought a DD MK18 barrel from Primary Arms and I pinned the gas port when it arrived, it is .070. I rebarreled my CQBR Block II clone after I shot out the Colt Socom chop that I had in it. The SD is also .070… |
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All 3 are GTG but there's zero doubt that an 11.5 Super Duty is a more pleasant shooting experience, suppressed, over the DD 10.3 bbl. 11.5 vs 12.5. Meh, unless you need the rail space the velocity difference is negligible. I've had numerous of both, prefer the feel/ balance of the 11.5.  " />
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Originally Posted By awd_xtc: I am in the market for a high(er) end upper. I was going to just do a mk18 but having used them in the army I’m not sure if that is the best modern day solution to an sbr. They tend to be over gassed. So I started to consider the Geissele super duty but I can’t seem to decide between the 11.5 and the 12.5. In theory the 12.5 should be smoother with a smaller gas port but I see a lot of reviews that say it is not really noticeable. The gun will be shot with an arc fire frt occasionally. I will not have the need to shoot beyond a few hundred yards, 300 at most and that would be rare. What would you choose? Should I just go mk18? I run and really like the 12.5”. I thought it had a pretty good fps boost for an inch running 77gr rounds. Nice handling gun. |
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Originally Posted By bluedog82: I’d do the 11.5, personally, but all 3 would be fine (I’ve got all of them, length wise). at Blue dog and everyone else who says they'll do 11 and 1/2. I'll be picking one up and have kind of been leaning towards the 12.5. Little more velocity, it also likely takes away a little bit of the concussion, and with a k-can, probably at a similar length to a 16-in with a bird cage muzzle device. What is your rationale on the 11.5 over the 12.5? To me? 10 in. Might be getting a little short and 13.9 Not enough difference from a 16 |
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My 12.5 Attached File Attached File I hit 900y with my 1-6 on it last week |
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Originally Posted By hoody2shoez: My 12.5 https://www.ar15.com/media/mediaFiles/470854/197_jpg-3800148.JPG https://www.ar15.com/media/mediaFiles/470854/202_jpg-3800147.JPG I hit 900y with my 1-6 on it last week Holy cats that is screaming out of a 12.5" barrel. |
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I have two 11.5 suppressed, perfect length suppressed, gassing, felt recoil, handling, etc. No 12.5 experience. I have Rhino X on one and it's sooooo nice to shoot. The other has a Turbo 3 on it, really thinking of a other Rhino X for it or Rhodie, not sure. Attached File |
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Originally Posted By hoody2shoez: My 12.5 https://www.ar15.com/media/mediaFiles/470854/197_jpg-3800148.JPG https://www.ar15.com/media/mediaFiles/470854/202_jpg-3800147.JPG I hit 900y with my 1-6 on it last week Are you reloading your NAS3 or are those loadings from the few companies putting them out? I have been doing reloads with NAS3 using mostly benchmark, staball match and just picked up some tac to do some barnes bullets. I have seen some awesome numbers so far using 5-10% over published max data. The most notable is 12.5 and then the 14.5. With high pressure ammo I am loving the shorter barrels with suppressors attached. I have gotten similar velocity to what you published with the best being criterion core barrels are faster than all my other barrels from other brands. |
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Originally Posted By Kgknight1037: Are you reloading your NAS3 or are those loadings from the few companies putting them out? I have been doing reloads with NAS3 using mostly benchmark, staball match and just picked up some tac to do some barnes bullets. I have seen some awesome numbers so far using 5-10% over published max data. The most notable is 12.5 and then the 14.5. With high pressure ammo I am loving the shorter barrels with suppressors attached. I have gotten similar velocity to what you published with the best being criterion core barrels are faster than all my other barrels from other brands. Badlands ammo. That data is from my criterion core funny enough |
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Originally Posted By awd_xtc: I am in the market for a high(er) end upper. I was going to just do a mk18 but having used them in the army I’m not sure if that is the best modern day solution to an sbr. They tend to be over gassed. So I started to consider the Geissele super duty but I can’t seem to decide between the 11.5 and the 12.5. In theory the 12.5 should be smoother with a smaller gas port but I see a lot of reviews that say it is not really noticeable. The gun will be shot with an arc fire frt occasionally. I will not have the need to shoot beyond a few hundred yards, 300 at most and that would be rare. What would you choose? Should I just go mk18? Either a Geissele 11.5 or 12.5 will do the job. They are definitely not over gassed if you are running a flow through suppressor. I have not shot a 12.5 yet, but some people swear by them religiously over the 10.3 and 11.5. I have a 11.5 Geissele and it works great. Either one will get you out to 300. The 12.5” will give you just slightly more velocity, but not much. I can’t comment on the use of a FRT. |
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Originally Posted By hoody2shoez: My 12.5 https://www.ar15.com/media/mediaFiles/470854/197_jpg-3800148.JPG https://www.ar15.com/media/mediaFiles/470854/202_jpg-3800147.JPG I hit 900y with my 1-6 on it last week Originally Posted By hoody2shoez: My 12.5 https://www.ar15.com/media/mediaFiles/470854/197_jpg-3800148.JPG https://www.ar15.com/media/mediaFiles/470854/202_jpg-3800147.JPG I hit 900y with my 1-6 on it last week Originally Posted By hoody2shoez: My 12.5 https://www.ar15.com/media/mediaFiles/470854/197_jpg-3800148.JPG https://www.ar15.com/media/mediaFiles/470854/202_jpg-3800147.JPG I hit 900y with my 1-6 on it last week That is incredible out of a 12.5”. Hence the HICAR requirement :-) https://blackarcmunitions.com/product/nas3-262-alpha-50ct/ |
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Have both - they are VERY similar, with the weight/handling obviously going to the 11.5. And they should be similar- I’ve got 100% identical components and optics on both except for barrel length and handguard length. Same everything, I mean everything. I did these builds recently, on purpose. You couldn’t get a better comparison. Both adjustable gas, both dialed to perfection from a gas/smooth/low recoil perspective. One’s an inch longer and a slight bit heavier. The difference (length/weight/balance/pointability) becomes more pronounced with a can on the front. I can’t imagine the 12.5 really giving any sort of edge over the 11.5. Ammo is so good these days there’s not a lot of (as in no) penalties between these 2 lengths. I’ve also really grown to love my new 11.5 KAC KS-3 - awesome gun. YMMV I’ve gotta try that NAS3 stuff! Just recently started playing with the 70gr TSX hotter stuff. |
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12.5 is best .5 Suppressed 5.56 SBR Velocity Tests--41 Factory Loads from 10.5", 11.5", 12.5" & 14.5" ALL DONE! |
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I have both URGI and SD Geisseles in 11.5 Both are excellent. I use Surefire cans. The only advantage to a 12.5 in my eyes is the extra velocity you get from it. If your likely use scenario is inside 100y don’t worry about it. My SBRs are loaded with 75gr Speer Gold Dot which performs well. URGI with RC3 Attached File |
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Originally Posted By Luny421: 12.5 is best .5 Suppressed 5.56 SBR Velocity Tests--41 Factory Loads from 10.5", 11.5", 12.5" & 14.5" ALL DONE! Completely different barrel manus / types. Test is invalid. And so what - we all should hopefully know a longer equals higher velocity. Guess how fast a 20” barrel get things going. Shoot, let’s go 24” while we’re at it. |
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