14.5" Recce/"Do-all" build conundrum

AR/M4 AR General Discussion
1

I am torn between a 14.5" carbine vs. midlength build, for use with a suppressor about 75% of the time.

Daniel Defense is the build, it’s the “configurator” for an upper that I will be using.

EVERYTHING can/would be IDENTICAL except the barrel/gas system. So rail, etc. won’t enter into this discussion.

Reasons I like the idea of the midlength:

-Shoots smoother
-Less wear and tear

Reasons I like the idea of the Carbine system:

-Can run a heavier buffer (H2 likely) for more mass/more reliability
-More likely to cycle cheap ammo for training
-The military has used 14.5/Carbine gas in 1 or 2 rifles on 1 or 2 missions and it works.

Unknowns:

Will the smaller carbine port but longer dwell-time equate to better, or worse over-gassing when suppressed? Which will matter more, dwell time, or port-size? The suppressor effectively over-gasses either by adding dwell time, but which will be overgassed more?
Will the 14.5" middy run Wolf, MFS, Silver-Bear, and other “cheap” training ammo just fine?

Knowns:

-Daniel Defense builds their M4A1 rifle with a carbine-length gas system.

Things to keep in mind:

This is NOT a 16" middy. This is a 14.5" NFA build. Yes, the lower is already registered, and has a 10.3" 300BLK upper, so no lecture from the “Go as short as you can!” crowd. BTDT. The suppressor is a 556 SOCOM Mini Surefire.

The rest of the build as planned:

-Kahles K16i in LT104 mount (I hate LaRue, but the levers are out of the way/minimal footprint, the split-rings are a win, and the mount has good reviews)
-Surefire M600 Ultra at 1:30, unsure of the mount
-Surefire SR series tape-switch mounted at 12*
-Folding BUIS, unsure which one’s yet (If I go Unity Tactical/VTAC, then that handles the light mount and BUIS, just unsure at this time which way)
-BCM Gunfighter CH
-Registered DDM4 complete SBR’ed lower
-B5 SOPMOD
-SF FH/SOCOM Mount
-BCM Shorty VFG
-Geissele Super-V trigger

2

I cannot speak on the effect the suppressor will have on the system, but I have a 14.5 middy and if I were to buy another 14.5 I would get a carbine gassed barrel to have better reliability with cheap ammo.

3

Can you share more details (brand of barrel/ammo it isn’t liking/what buffer, etc.?)

4

Might want to post your question in the AR forum, although there’s already a ton of carbine vs middy threads.

https://www.m4carbine.net/showthread.php?p=1015576

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5

If you’re running a suppressor, I’d choose the midlength gas on a 14.5". If not suppressed, I’d get a lightweight barrel with carbine gas.

6

This is really a thread for the technical section, as you have some very specific questions. I would PM a mod and ask it to be moved.

7

The can WILL add some backpressure, so middy is the answer there. If it’s part-time, the A5 can make it run adequately unsuppressed as a middy, and still brilliantly with a can.

For my part, light offset sights fit my do-all recce requirements quite well, it helps that the KAC ones are ridiculously light as well. Same idea with the weaponlight - 3V SF scout type setups save even more weight on the nose end of the rifle, and it’s only matter of time before SF has 3V Vampire heads, for now it’s a nonissue since I haven’t needed IR illumination for that yet. If cost is a concern, the VTAC L4-3V is a very affordable light, and any aftermarket mount will do.

My honest answer on the ammo front is that if your final setup runs junky ammo, cool, but don’t build around it. For it to be an optic worthy of $2k worth of glass, make it function well with proper brass ammunition, you can always configure a different rifle to digest junk. A lighter buffer on hand can make it run unsuppressed with cheap stuff a ton better, so if you have to make that work it can be an alternate configuration, but the focus should be to make it run the best possible with prime ammunition. A do-all recce is a challenging enough setup to make work, don’t hurt it’s longer range capability or shot recovery with building the rifle around underpressured stuff.

As much as you’re going to hear the ‘make it as short as you can’ nonsense, there is still a case for seeing what you can pull off with a 12.5" in the same weight range. 1-6x seems like a lot of optic, but with a can on there you’re back to looking at a really stout carbine that is more maneuverable, and as a side effect of the carbine gas paired with that, you’ll be able to run anything through most likely.

8

I have a 10.3 300blk and 10.3 5.56 already. A 12.5 5.56 would be giving up more velocity than I want for this build without much benefit and be a pita unsuppressed. I’m set on 14.5 for better or worse. With my suppressor I am looking at an effective barrel length of 18.75 to 19.25". About like a 16" gun with a vortex, or close enough.

My angle with carbine gas was that I could run an H2 or H3 buffer vs. An h. This would give more inertia and reliability with a dirty gun wouldn’t it, or am I all wet there?

The m600u is only 1.5oz more than the m300u. Thoughts? I thought the 300 extra lumens justified it but maybe not. Did you know it was so near the same weight or does this change your opinion?

Yes, 6x is a lot but it’s the lightest true 1x variable with daylight visible reticle illumination that I have found. It should be point and click 0 - 300m with my browntip 70gr stuff.

9

If you could live with a 1-5x , Kahles also has their new K15i with G4b reticle for approx $360 less. http://www.sportoptics.com/kahles-1-5-10548.aspx

10

If you already have the M600, or are picking it up used (the 6V tend to be cheaper), that 1.5oz is miniscule, but if you’re going from scratch, it’s less weight on the nose, which is nice.
The carbine gas can work, it’s more of a case where I’m really impressed in my experience with the 14.5 middy units when paired with the A5, which are truly their own animal for being able to run a wide spectrum of gas port impulse profiles really well - the A5H2 can really do a ton of things well.

6x is the sweet spot, I’m with you on that. My do-all recce is set up with the Leupy Mk6 - cheaper, but not without minor issues on the illumination, so I’m with you that a 1-6x optic is the answer for the range of a 5.56 rifle. 14.5" will do that full range just fine, I guess the next step down for usefulness would be a lighter/handier SBR ala 12.5" with a light 1-4x that’s set up for tighter spaces (urban recce), so if you’re basically running this as the leggy sibling of your Mk18, then go with exactly that. If the rail you’re using, or a factory upper is already set up that way, run carbine without hesitation (especially if you see a DD RIS-II equipped one for a deal). Otherwise, the middy will fit that role a touch better in my opinion, honestly finding anything with the barrel profile you want should be the priority, gas port location secondary.

11

I plan on sticking with a mil-spec re/buffer type setup.

12

With a can on, you can run a h3 with springco blue on a 14.5" middy and run everything. Now if you take off the suppressor… Likely it won’t work so well with weak ammo.

13

That’s about what I would have guessed - I’m running an H2/Springco Blue on a 16" Middy, but that’s a reflex can (M4-S) that probably isn’t providing that much backpressure. If mil RE/buffer is the requirement, it’s not going to be super smooth and still run crap, at that point if you still need it to run Tula/Silver Bear/Wolf without changing the buffer/spring, then carbine/H buffer is probably the closest thing to a good answer. Otherwise, I’d say run a sprinco Blue, H2 or H3 as your primary setup, and keep a standard spring and a lighter buffer for when you need to run it with lower pressure stuff sans can, since the latter are cheap parts (in that in one afternoon, the savings in ammo cost can basically pay for the CAR/H buffer and stock spring).

14

A 16" middlength gas system has the same length of barrel past the gas-port as does a 14.5" carbine system. Lots less data to pull from once you throw that out there, as 16" and 14.5" barrels behave differently with the same length gas systems on them.

15

You have been a member for quite a while-

The custom build area is a place to show off/discuss and ask questions and get feedback about your completed custom build (this means a completed AR that is functional and already completed). If you are going to post pictures of your build, please post a complete picture not just your parts, partial lowers, etc…

Also, buying an upper separate and then a lower and throwing them together does not constitute a custom build. It would be nice if you took time to explain how you came to your decision and explain your set up. Thanks.

If you do not follow the guidelines and your post disappears it’s probably because it was moved to the trash or deleted.

16

Thank-you. Understood.

How did I come to my decision? Specifically regarding my choice of barrel length and optics, or…?

17

It seems silly to me that you’re already having an NFA upper and you want a “do it all” rifle but you’re not looking at the 12.5"

18

^^this is what I’d be doing if i were in his shoes. 12.5 with a variable optic. Then again he did touch on the 12.5 in post #8 and how it trades off more velocity than he would like.
WS6, i know you said youre set on a 14.5 for better or worse but I remember reading a post about constructor’s 12.5 1/8 5R barrel yielding higher velocities than a 14.5.

19

Understood, but the extra 2" of powder burn is going to make it much nicer to shoot unsuppressed, as well as kinder to my suppressor (I refuse to run a brake, see the first half of this sentence). It’s not just velocity I’m looking at, but powder-burn, concussion, blast, etc.

I really just have no use for a shorty anymore after getting some actual trigger-time in and around vehicles. I actually think I prefer a 14.5"-16" gun in a car at this point.

This is my reason for the 14.5" barrel.

-Concussion (for my sake)
-Powder-burn (for the sake of the suppressor)
-Velocity (goes hand-in-hand with the above 2, and while a polygonal rifled barrel would fix this, it does nothing for the other 2 aspects).

Another aspect is that I am stuck on the DDM4 rail. It’s about the same weight as a slick Troy Alpha of the same weight (including barrel nut), and I like at least 10" of rail on the small side, especially with how I want to run the SR tapeswitch. The DDM4 rail comes in 9" and 12" lengths, respectively.

The reason I have not considered the KMR and other rails is that I perceive (I have not shot one) that they heat up faster, and I have gotten my DD rails plenty hot as-is, to the point that I am glad I wear gloves and cover them in Index-clips. Also, the VFG’s currently on the market interface “smoother” with the 1913 rail when you butt index clips up against them. I use my VFG as a hand-stop, as well as a “tool” for stand-off and on my ankle, and other things when doing 1-handed drills (A’la VTAC), which is why a simple hand-stop won’t work for me. The BCM Shorty VFG is PERFECT for me. I just like the 1913 interface, at this time, is all I’m saying. The lightest 1913 rail I know of is the DDM4 (12oz for a 12" rail, including barrel nut, I believe), thus, another reason I am looking at the 14.5" barrel. A 12" rail won’t work on a 12.5" barrel, esp. considering the DDM4 rail is 12.44", “technically”.

20

Have you ever shot a 12.5"? Blast with a FH is similar to a 14.5". If there is more concussion, it isn’t noticeable. And I’ve fired a bunch of rounds through my 12.5" with both an M42000 and 762SD with no issues on the first baffle.

The velocity loss is 100fps or so. Is that really going to matter?

Its your rifle, and you can do with it as you wish. I’ve had 14.5" and 16" “do it alls”, but without a doubt my favorite is the 12.5"

It shoots to 565 yards just as well as the 14.5" with no trade offs.