SBR 5.56 performance?

[Rikakiah]

I have a 16" Daniel Defense M4v7 (1:7 twist). I was going to get a .300BLK SBR upper to satisfy my SBR dreams (and basically use the 5.56 to proved brass to reload to .300). I was planning to suppress both of them (probably with the same .30 suppressor).

However, I've estimated the timeframe for that to be at least 2 years out (gathering finances, dealing with the backorder crisis from most manufacturers, ATF wait times). Also, putting a ~6" suppressor on my current rifle will make it decidedly long, so was thinking of just trimming it (professionally) and putting a .300BLK option on indefinite hold.

My question is, how will performance be affected if I trim it down to 12 or 14 inches (as much as I can without interfering with the gas block)?

[Rob62]

I don't know but I am going to tag this thread. I should be going out to the range this week sometime to shoot my .223/5.56, 10.5" barrel SBR'd AR15 and can compare it to my AR15 M4gery with 16" barrel.

When I do the chronograph check I'll post the results here.

Somebody has probably already done this however and will beat me to the punch.

Note - without question I expect MV will be less with the shorter barrel. How much I have no idea. My best guess is about 75-125 FPS per inch of barrel.

Regards,

Rob

[Rikakiah]

I'm mostly concerned about accuracy, probably out to about 150ish yards (maybe 200 max?). I'm figuring it'll still be lethal to coyote out to those ranges, so long as I can still get accurate hits.

[Theohazard]

I think Rob62 is about right, I think it's about 100 FPS per inch or so. Check out www.balliisticsbytheinch.com, they have a breakdown of .223 velocities there.

The other issue you're going to have if you start cutting your barrel down is gas pressure. The length of barrel in front of the gas port is referred to as "dwell time"; that's the length of time that the bullet has passed the gas block and gas pressure builds up in the gas tube before the pressure drops off when the bullet exits the barrel. The longer the dwell time the higher the gas pressure. Conversely, the shorter the dwell time the lower the gas pressure.

If I remember correctly, the DDM4 V7 has a mid-length gas system. The general rule of thumb is that the ideal gas system for a 14.5" barrel is carbine-length and for 16" it's mid-length. If there is too much gas because you have a 16" barrel and carbine-length gas system it will probably run just fine; most ARs with this configuration are a little over-gassed and still run fine. And it's easy to fix a rifle that is malfunctioning due to being over-gassed.

But a 16" barrel with a mid-length gas system generally isn't over-gassed, so if you start cutting the barrel down it won't take long until your dwell time is too short and your rifle is under-gassed. And an under-gassed rifle is usually much worse than an over-gassed rifle; at best an under-gassed rifle will be ammo-sensitive, will have lackluster ejection, and might not lock open on the last shot. At worst it will short-stroke and not work at all.

If you have a mid-length gas system and you start cutting your barrel down to 14" or even 12", you're going to have under-gassing issues. You might be able to fix this with a lighter buffer and/or spring weight, but at that point your rifle will be more ammo sensitive than it was before.

[Theohazard]

Originally Posted by Rikakiah

I'm mostly concerned about accuracy, probably out to about 150ish yards (maybe 200 max?). I'm figuring it'll still be lethal to coyote out to those ranges, so long as I can still get accurate hits.

All other things being equal, a shorter barrel isn't any less accurate than a longer one. In fact, often the opposite is true: A shorter barrel is often inherently MORE accurate than a longer barrel because it tends to flex less.

The reason people think a longer barrel is more accurate is because longer barrels often give you longer sight radius with open sights, allowing the shooter to be more precise. Also, a longer barrel generally gives you more velocity, meaning it can often be easier to shoot accurately at range due to less bullet drop and a lower effect of wind on your shot.

[Rob62]

As far as accuracy goes I am expecting about 3 - 4 inch, 5 shot groups at 100 yards with my SBR. I have an Aimpoint Comp ML2 on it, not the best for accuracy but still OK. Maybe I am being over cautious here but to me that is not good enough for a relatively small target such as a 'yote's vitals at 150 yards.

Edited to add - Reference gas pressure. I test fired the SBR already - basic function test and rough accuracy test only. With conventional FMJ loaded brass case MILSPEC ammo no problem it runs flawlessly.

However with steel case Russian Silver Bear brand ammo it hick ups like crazy. With steel case Russian Wolf brand ammo it runs OK also. The gas port / hole is the standard one used on longer, 16" barreled AR's.

Rob

Here 's a pic of the guns that I will be shooting. (And in case anyone cares, that is a Form 1 Tax Stamp SBR)

[Deja vu]

This link should give you an idea.

http://www.ballisticsbytheinch.com/223rifle.html

[Rikakiah]

Originally Posted by Theohazard

If I remember correctly, the DDM4 V7 has a mid-length gas system.

Yep, I believe you're correct. I'll double check to be sure, but not hopeful. Oh well, guess I'm back to my original plan.

Yep, mid-length. Would really need a pistol-length for my ideal goal. :-(

[Rikakiah]

Actually, wouldn't adding a suppressor increase the pressure and give it the necessary dwell time? Most suppressors I'm looking at are about 6 inches, and the gas block is about 4.5-5 inches from the end of the barrel so I could only trim probably about 4 inches off, giving a total length with suppressor of about 18 inches.

[Theohazard]

Yes, a suppressor will noticeably increase the gas pressure. Also, if you use a VLTOR A5 buffer system your rifle will be more forgiving of lower gas pressure.

[Rikakiah]

One last question that my Smith would probably be able to answer just as easily, but when I get it cut and rethreaded, is the barrel thick enough I can have it threaded directly for the standard .30 cal can instead of 1/2x28? Looking to get 1 can for most of my future needs and if I can get a more useable length AND save needing a thread adapter, I would be overjoyed.

[Theohazard]

It should be thick enough to thread in 5/8x24, but you should still thread it to 1/2x28. Unless you plan to put it on your rifle and not remove it, I wouldn't recommend a direct-thread can. Most people use quick-attach cans for ARs, that way they go on and off easily and you don't need to have bare threads exposed on your rifle, you just need that company's proprietary muzzle device (I wouldn't recommend the cans that attach to the generic flash hiders).

I have an AAC SDN-6 (a 7.62 can) and I mostly use it for 5.56. You can get mounts for either caliber and several different thread patterns. So I have the AAC 51t flash hider for a 5.56 that attaches to my 1/2x28 barrel and it will fit most of AAC's current production rifle suppressors.

[Rikakiah]

Well, it will probably only go between this one and a .300BLK upper. That one will definitely have the muzzle inside the rails (thinking this one will, too, if just), so a quick detach won't really work. I don't really see swapping often and won't have the 2nd upper for likely at least a year after I get the suppressor.

[Brit]

All I could think, that shorty is going to be loud!

[Theohazard]

Well, in that case you should see if you can thread it in 5/8x24. Direct-thread cans are generally not as popular, though. My SDN-6 is a full-auto rated 7.62 can that is optimized for subsonic 300 Blackout: The subs are closer to a pistol round in terms of velocity and pressure, so a 9mm can usually is noticeably quieter than a normal 7.62 can. But the SDN-6 is one of the quietest 7.62 cans on the market when it comes to subs, almost as quiet as if you used a 9mm can.

The AAC Hunter is the direct-thread version of the SDN-6. AAC has been promising for almost two years, and supposedly it's going to finally come out in the next few months.

[Rikakiah]

Originally Posted by Brit

All I could think, that shorty is going to be loud!

Yeah, I don't expect it to be too quiet, even with a suppressor on the current 16" length. Hopefully, it'll tame it down to hearing-safe, though.

[Brit]

Rikakiah,

Hearing safe, that is the purpose of going to all that expense, is it not?

Be nice to see all these regulations pulled, yes? Having a device that protects your hearing, what is needed to see that as being a good idea, I can see some big committee set up in some Government Offices.

"What do silencers/suppressors do Mr. Chairman?" "Why it takes the big bang away from a gunshot, basically protects peoples hearing"

"Why is this a bad thing?"

"People who would want to commit a crime whilst having one of these devices, could do it with less noise."

"Is it not illegal to commit crimes anyway? With or without a suppresser."