re: Official AR-15 thread

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Some rounds perform BETTER out of barrels shorter than 16", or an added 4" only returns 10fps.

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5.56/.223 is sensitive to barrel length. 7.62/.308 not so much. Civilian 10" AR SBR's are only built for the cool factor though some will attempt to cite their reasonable justifications. Less reliability, more wear and tear on parts, less energy and velocity. You lose around 500 fps on average with a 10.5" bbl vs a 20". From about 24" - 16" barrels each inch is worth about 25 fps. From 14" and less it is a much larger difference per inch. When you get down to a 7" - 10" SBR you may as well carry a pistol like a 10mm since that is about as much muzzle energy you have left and no SBR is as maneuverable as a handgun.

11.5" is about as far as you should go. The 7" pistols are just absolutely ridiculous in 5.56/.223, and are just a trend for now.

Here is a quick quote from BCM about their decision for 11.5" over the 10.5".

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Q: Why BCM chose the 11.5" SBR over the 10.5"

I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.

First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.

A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
The BCM 11.5" Runs Like a Dream.

Hope this info helps,
Paul

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Also here is a chart, make of it what you will.

Did you get it dialed in?

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Civilian 10" AR SBR's are only built for the cool factor

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That is just wrong.

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some will attempt to cite their reasonable justifications

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Well, if one does not believe carrying a light weight rifle with suppressor designed to protect your hearing is not a great idea then that person is just plain lost. So much fail in that statement.

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Less reliability, more wear and tear on parts,

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Wrong here to. The best SBR barrel lengths IMO are 10.5-12.5 with a normal carbine gas system. Properly built rifles (just like any rifle) will have no reliability issues. The wear and tear on 10.5-12.6" SBR's is so little it is insignificant.

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less energy and velocity.

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True. Suppressors actually increase velocity thoug, ok..like 30fps.. As said before, in 5.56 terminal performance of the bullet (such as fragmenting and such) occurs at above 2000fps. Therefor, in a 10.5 barrel, one has a rifle that will act similar to a 20" barrel within about 200 yds. No loss here, unless you are carrying in Montana and making 600 yard shoots with your farm rifle.

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From about 24" - 16" barrels each inch is worth about 25 fps.

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Close, but more factors play into this such as powder type and bullet weight.

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When you get down to a 7" - 10" SBR you may as well carry a pistol like a 10mm since that is about as much muzzle energy you have left

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Yes, as I said, 10.5 and up are my ideal SBR lengths. 7" SBR's are giving up too much and are where reliability and such becomes great issues. However, if someone using a 7" SBR as his 50yd rifle and he is confident with it, I don't mind.

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you may as well carry a pistol

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Rifles are easier for most folks to shoot accurately and have greater capacity. So depending on situation you may be wrong.

Most people understand that a rifle is used as a tool and is just a tool. Each setup has different strengths and weaknesses, if one understands this they have no issues.

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PA micro dots with 50k battery life soon.

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Id get the holosun variant. It has auto on and lifetime warranty

It also has auto off, so with 50k battery life I would just be keeping on all the time like I would an aimpoint. But with it being really untested in large numbers, can I really trust the stated battery life.

I have little faith in the auto motion on feature, but who really knows

It seems you are making your argument not based on facts but your personally opinions.

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Better, cheaper options available. Not lost, just experienced.

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Like what electronic ear protection? Perhaps being in the woods I'd like the ability to hear all around me. Once a person actually owns a sbr/suppressor they can understand the usefulness of such things.

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ou are going to sit there and state there is no increased wear and tear or adverse effects on reliability?

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You fail to copy my entire post correctly because you know you are wrong here. I said, with properly built rifles there are no reliability issues. My personal experience with 10.5-12.5 sbr's is where my fact comes from, along with many others in the firearms community. And I said the wear is very insignificant, so much so, after 5,000 rounds you will not see any noticeable wear issues over a comparable 20".

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No. Starting out 500 fps in the hole and nothing is going to make up the energy loss.

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You cannot be this dense. The fact is 5.56 is effective at fragmentation anywhere over 2000fps. This allows a 10.5 sbr to be effective with proper ammunition out to 200 yards and a bit more depending on other variables. This range is useful for a vast majority of folks who live in areas with wooded environments and lack the open ranges as seen in the mid west. To make it even easier to understand, a 500fps velocity decrease is not a deal breaker in ranges under 200yds with proper ammunition selection in 5.56.

Once again, each rifle variation excels at a particular use. Same goes for ammunition. I will continue to use a 10.5" AR as my ride along woods rifle. I simply understand that it, along with the suppressor, is the most comfortable (to carry and hear fire) option available at the moment. Largest fields on the property only allow for a long shot of 150 yards. My sbr is primarily for coyotes, wild dogs, and hogs while I am working/putting out corn/whatever and its value/usefulness cannot be understated. In fact, I took two coyotes in the last month with it, one at 30ish yards and one at just over 100yds.

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Like what electronic ear protection? Perhaps being in the woods I'd like the ability to hear all around me. Once a person actually owns a sbr/suppressor they can understand the usefulness of such things.

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Plenty of ear-pro options and with the clip-on suppressor options available for just about any rifle I'd say there are more cost effective options.

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You fail to copy my entire post correctly because you know you are wrong here.

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Wrong, I was posting from a cell phone and I only wanted to copy enough to show where I was responding. There are absolute reams of data showing that wear and tear are increased proportionally. My personal experience and training as an armorer as well as long discussions with engineers who have lab tested these rifles under every configuration you can imagine. Your personal experience is anecdotal at best. But please, pass it along to Aberdeen because you apparently know more than they do and the millions of dollars they have tied up in testing.

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You cannot be this dense. The fact is 5.56 is effective at fragmentation anywhere over 2000fps. This allows a 10.5 sbr to be effective with proper ammunition out to 200 yards and a bit more depending on other variables. This range is useful for a vast majority of folks who live in areas with wooded environments and lack the open ranges as seen in the mid west. To make it even easier to understand, a 500fps velocity decrease is not a deal breaker in ranges under 200yds with proper ammunition selection in 5.56.

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Oh where to begin...

Let's start off with that nonsense about bullet fragmentation. Apparently you aren't aware that it's not as much velocity but the roughly 300,000 RPMS a 1:7 twist barrel is going to put on a 62gr bullet that is mostly responsible for the bullet's fragmentation. You will never even approach that spin rate with a 10.5 SBR let alone the fact that that by 100 yards that bullet will be slowing down close to 2000 fps. It will be well under that by 200 yards and military tests have shown that reliable fragmentation usually occurs at 2200 - 2300+ fps (and that's in a barrel long enough to impart enough spin) which is essentially your muzzle velocity. Which is fine because a 10.5" SPR is a 75 - 100 yard gun at most in practical applications. With a 20" 1:7 twist barrel you may not get reliable fragmentation from the heavier OTM stuff at 200 yards and those usually have lower thresholds compared to 55gr loads that require even more speed. I know for a fact that 14.5" M4's on M855 is lucky to get to that threshold outside of 150m. Sorry, the facts don't not bear out the assertion that a 10.5" SBR can deliver ANY 5.56 at 200 yards and provide enough velocity to reliably fragment when much longer barrels cannot claim such.

I cannot conceive that anyone really thinks they are going to go room to room in their own house with one of these toys. Even if the worst were to happen I don't think I'd want something like that ending up in a police cage until everything is sorted out either. You've got $400 in tax stamps, hundreds more in the suppressor, $1000+ in an upper if you buy one complete, and few hundred in a registered lower. That's why I look at them as an enthusiasts toy, they are undoubtedly cool and most AR enthusiasts recognize the investment when they see them but I don't think anyone seriously thinks they are doing it entirely practical reasons.

Sent mine to ADCO.