AR-15 DMR/Precision Build- give me guidance

[Browning]

I've seen green tip chrono in the 2930's out of a 16" inch barrel. No idea what it would be in a 14.5", but I suspect it would be a bit lower.

 

[C-grunt]

Obviously every barrel is different but from my experience, what Ive seen, and military specification... M855 should be in the mid 2900s out of a 14.5 inch barrel.

I got this from a manager at FN who posted it on another forum.

 

[Browning]

Well, I think the key phrase in there is 'should be'. Some lots are going to be low, some lots higher. If you in good faith were testing a certain kind of ammo to document its performance and you bought several lots of it and then posted an average online and then some random individual person online said that they tested it and that it was higher are you going to pull that graph? I wouldn't.

I got this from Wes at MSTN who posted it online.

M855 VELOCITY DATA
__________________________

7.5" 2244 FPS ... 10.5" 2639 FPS
DIFFERENCE 395 FPS
DIFFERENCE/INCH 132 FPS

10.5" 2639 FPS ... 14.5" 2861 FPS
DIFFERENCE 222 FPS
DIFFERENCE/INCH 56 FPS

14.5" 2861 FPS ... 16" 2938 FPS
DIFFERENCE 77 FPS
DIFFERENCE/INCH 51 FPS

16" 2938 FPS ... 18" 3046 FPS
DIFFERENCE 108 FPS
DIFFERENCE/INCH 54 FPS

18" 3046 FPS ... 20" 3061 FPS
DIFFERENCE 15 FPS
DIFFERENCE/INCH 7.5 FPS

Which corresponds with what I've seen with both our chrono and random tests on the web.



The graph is saying 2700 which seems a bit low to me. Unsure how to account for that figure (other than to guess that those lots might have been low), the round-about 3000 FPS average for 20" inch AR's on the graph is what I've seen.

Edit : At any rate rather than chase our tails on a quest for a FPS at a speed that we can both agree on with an ammo I'm generally not using for anything but plinking lets just say the reason I went with a 20" inch AR is for the increased wounding performance.

Fragmentation is going to be more pronounced with a 20" barrel than with a 7", a 10.5", a 12" inch, a 14.5" inch, a 16" inch or an an 18" inch while still retaining something that's maneuverable. To me a 26" isn't maneuverable at all. So a 20" inch it is.

 

[Varminterror]

I don't have a fancy chart, but I've shot over 600 AR's with 27.3grn of Varget under a 50grn Vmax in the last ~18yrs. The gap between 14.5" to 24" tends to follow ~43fps per inch with this load, almost linear.

77smk's over H335 follows 45-50fps per inch in 14.5-24" as well.

For general purpose with a twist of long ranges, 18-20" has proven to me (and my customers) to be the most versatile lengths. Enough speed to keep the 69-77grn pills stable out to 700yards and beyond, but not as burdensome to carry as the long tube rifles. Just short enough to still be manageable with a full size suppressor. I've had a lot of folks want to live on a "two rifle" plan, one 16" carbine or SBR, and one long gun in the 22-24" ballpark, but a large majority of them end up coming back to get an 18-22" at some point, because they love how their carbine handles, and love the extra range of the long rifle, but don't like the length of the long gun, and want more range than their carbine - all in one rifle. I know everybody loves their particular rifle and their barrel length is the best, and everybody else loves saying the application dictates the design, but a DMR is a general purpose rifle with a moderate range expectation. It's NOT a "sniper rifle," it doesn't need to be a long range weapon which carries like a boat anchor.

If I'm honest, if I were building one away from the mantra of military/NATO compatibility, I'd most likely build one in 6x45 or 6-6.8 instead of 5.56 to give me a little more thump at range. I built my "DMR type" rifles for myself in Grendel and SPC, doing it again, after building a few 6mm AR's the last few years, I'm heavily favoring them.

For civilian application, I don't see much purpose in designing around M855.

 

[Jackal]

For civilian application, I don't see much purpose in designing around M855.

Agree. I've found M855 is inferior to M193 for most applications. M855 is generally less accurate, doesnt penetrate armor as well as M193 and doesnt fragment as well at range. For hunting coyotes, the 50gr varmint tip has dropped them all where they stood, regardless of barrel length (11.5"-20"). I'm not against 20" barrels, but much like the .308, the velocity difference between barrel length isnt nearly as pronounced as many of us have been taught over the years.

 

[Browning]

'Designing it around M855?'

More like a useful unit of measurement.

 

[C-grunt]

Edit : At any rate rather than chase our tails on a quest for a FPS at a speed that we can both agree on with an ammo I'm generally not using for anything but plinking lets just say the reason I went with a 20" inch AR is for the increased wounding performance.

Fragmentation is going to be more pronounced with a 20" barrel than with a 7", a 10.5", a 12" inch, a 14.5" inch, a 16" inch or an an 18" inch while still retaining something that's maneuverable. To me a 26" isn't maneuverable at all. So a 20" inch it is.

Very true. We are definitely nit picking here. Im with you on the 20 inch rifle though. I love them. Having actually used the M16A4 rifle my entire enlistment (even when I was a machine gunner I still had to qual on a rifle) I dont find them to be the hindrance that many people make them out to be. Im keeping my 20 inch with irons sights for it to be my "I dont have to worry about batteries or banging my scope off zero" gun. I can grab it and without worry Im good from 0 to 500 meters.

Though I have no qualms using M855 for defense. I dont. But if it was all I had I wouldnt think twice about it. A lot of people talk bad about the round, but Ive seen it put a lot of dudes in the dirt.

 

[Driftertank]

Built a 20" AR for my father-in-law using a Criterion 20" "Hybrid-contour" 1:8" .223 Wylde barrel, a Brownells phosphate BCG, Rock River NM Varmint trigger. Total cost about a grand.

First 5 rounds through the barrel were under an inch at 100yd, using some 55gr V-max handloads he had laying around. Same with my 75gr BTHP loads.

 

[Browning]

I'm with you on the 20 inch rifle though. I love them. Having actually used the M16A4 rifle my entire enlistment (even when I was a machine gunner I still had to qual on a rifle) I dont find them to be the hindrance that many people make them out to be. Im keeping my 20 inch with irons sights for it to be my "I dont have to worry about batteries or banging my scope off zero" gun. I can grab it and without worry Im good from 0 to 500 meters.[/B]

Me either.

The centerfire rifle that I started lugging around at 12 years old on the 3 day to week long shooting trips was the M1 Garand though. If I was able to carry a 9.5 lbs rifle through my teenage years I don't think it would be an issue to carry a 8.5 lbs rifle at 44. I kinda because accustomed to full length rifles.

I read awhile back that the the Marine Corps is going with the M4 across the board. Kinda thinking that's somewhat of a mistake. I can see the M4 being useful in urban areas (I.E. : Fallujah), but not so much in mountains or plains (I.E. : Afghanistan)

According to another article I read they're talking about going with suppressors across the board as well and supposedly they're canning the use of M855 and going with MK318 SOST

The gap between 14.5" to 24" tends to follow ~43fps per inch with this load, almost linear.

77smk's over H335 follows 45-50fps per inch in 14.5-24" as well.

For general purpose with a twist of long ranges, 18-20" has proven to me (and my customers) to be the most versatile lengths. Enough speed to keep the 69-77grn pills stable out to 700yards and beyond, but not as burdensome to carry as the long tube rifles. Just short enough to still be manageable with a full size suppressor. I've had a lot of folks want to live on a "two rifle" plan, one 16" carbine or SBR, and one long gun in the 22-24" ballpark, but a large majority of them end up coming back to get an 18-22" at some point, because they love how their carbine handles, and love the extra range of the long rifle, but don't like the length of the long gun, and want more range than their carbine - all in one rifle. I know everybody loves their particular rifle and their barrel length is the best, and everybody else loves saying the application dictates the design, but a DMR is a general purpose rifle with a moderate range expectation. It's NOT a "sniper rifle," it doesn't need to be a long range weapon which carries like a boat anchor.

If I'm honest, if I were building one away from the mantra of military/NATO compatibility, I'd most likely build one in 6x45 or 6-6.8 instead of 5.56 to give me a little more thump at range. I built my "DMR type" rifles for myself in Grendel and SPC, doing it again, after building a few 6mm AR's the last few years, I'm heavily favoring them.

Meant to reply to this (as I agree with far more that I disagree), didn't get around to it.

Agree with the above whole-heartedly.

I had a 6.8 SPC at one point. Great rifle, liked the caliber quite a bit. Only problem that I had was ammo availability after the last presidential election and exploitation of Sandy Hook. Even .223 was scarce and overpriced during this period, let alone 6.8. On pure ballistics though I liked the 6.8 SPC a lot and it was a great hunting cartridge.

The one beef that I had with it was ammunition availability and that's what led to me selling that AR and replacing it with an 18" PTR-91 .308 Win.

Though I have no qualms using M855 for defense. I dont. But if it was all I had I wouldnt think twice about it. A lot of people talk bad about the round, but Ive seen it put a lot of dudes in the dirt.

No, M855 works and it'll even fragment in some circumstances.

There's just better stuff out there for what I'm using a rifle for. Hopefully no need for me to ever try to penetrate Russian helmets at 600 meters in the states.

I'm sure you've seen this before ...
(Or knew all the info even though it didn't have pictures and whatnot)

[URL]https://www.gandrtactical.com/images/archive/5_56mm_military_info.pdf[/url]

I use M855 for plinking occasionally, but even there I use way more M193. That's cheaper, more accurate and on the game I've shot with M193 it usually both yaws and fragments.

 

[C-grunt]

I haven't bought M855 in years for the same reason. M193 is cheaper and tends to be more accurate.

 

[C-grunt]

I do believe the USMC switch to the M4 is a good idea. A smaller and lighter gun that is more useful in 90 percent of situations. As much as I like the 20 inch guns, the M4 is better for combat in my opinion. There really is not very much of a difference between the max ranges of the two guns. Either way if a unit is engaging enemy out past 500 meters the rifle isn't going to be doing most of the fighting.

 

[mavracer]

My MK12/DMR whatever inspired build is a White Oak 16" mid weight barrel, with a 12" carbon free float tube and a Geissele trigger with a Leupold Mk AR 3-9X mil dot the rest of the gun is M4 ish, Kind of a early Recce build.

It's fairly light at 8 lb 2 oz with a full mag and a bi pod and accurate as heck.

 

[1911 guy]

I'm a rifle guy and accuracy appeals to me. 20 inch rifles were issued for the majority of my time in the service. But two things have changed that make the switch to the M4 a good idea. First and most obviously, warfare has changed. MOUT is the norm, not the exception. Even in Afghanistan where the terrain is generally more open than other places we're involved, the rifle is taking a backseat to air and artillery. When used, it's a lot of squad or fire team volley while the element leader is on the horn with aforementioned CAS and Arty. Secondly, kevlar is MUCH more common than it used to be and the collapsible stock is great for it. Fixed A2, not so much. .