I may SBR an AR of mine, and of course, I have questions.

[kristop64089]

In a couple days my "new" CMMG 7.5" pistol will be in. (CMMG lower, with a MEGA upper). The lower is legally registered as a pistol(here come the questions)
- Must it always remain a pistol?
- or can I put a carbine length barrel and buttstock on a "pistol" lower
- If I SBR when of my other lowers, must it always remain a dedicated SBR lower?
- Or can it serve as a multipurpose lower?
- Can I SBR the lower, and can the upper, on the same lower?
Right now the lower I would SBR, also is home for my 14.5" upper.

I understand the pistol lower must always remain a pistol lower, but I want to SBR the other upper, to put the 7.5" upper on it.

I have a few AR's and this is kind of confusing to me. One of my AR's will always remain as is(cost too much to screw with)
The others are "frankenguns", so I'm more Apt to swap uppers around.


Now that we are all thuroughly confused, Hopefully someone can point me in the right direction.

[Hkmp5sd]

Pistol first. ATF has been all over place on this subject. The most recent is that if you attach a stock and long barrel to your pistol lower, it becomes a rilfe for all times. This conflicts with their similar ruling that allows Glock receivers to have the carbine attachment added to them and then revert to a pistol when the attachment is removed.

If you are going to do this to your pistol, write to ATF and let them know your intentions and get a written response.

If your SBR one of your AR's, it can then have any length barrel attached. It does not have to be restricted to short barrels only.

You are on the edge of creating a real problem. ATF has ruled that for every pistol receiver and/or SBR receiver you possess, you may possess only one short barreled upper if you possess any other AR rifles. If you have more short barreled uppers than you have pistols/SBR receivers and own one or more AR receiver(s) and/or AR rifle, ATF says you are automatically in possession of an unregistered SBR.

Right now the lower I would SBR, also is home for my 14.5" upper.

If that is the case, you already have an unregistered SBR.

[kristop64089]

I should have clarified, it's a 14.5" with perm attached hider.

SO, if I have a complete, registered AR pistol.
AND 1 registered SBR lower, attatched to a 14.5" upper with perm. hider, LEGALLY, I could shelf the 14"upper, and drop the 7.5" upper, on the SBR lower.

One of the reasons I bought the pistol was for the hopes of SBR'ing a rifle(the other was it had an impossible to find, MEGA upper).

[Hkmp5sd]

Yep.

[kristop64089]

Whole new nightmare, to throw a can in the works?
Or is it easier than it sounds? I'd like the 7.5", to someday have a can on it. Right now, it's cost prohibitive, but that's the dream.

[Hkmp5sd]

Just pay another $200 for the suppressor and you can put it on any of the uppers you have.

I have a M16A1 and a .223 suppressor for it. Slap in my Ciener .22LR conversion and I can shoot .22LRs on full auto. Fast, quiet and cheap.

[kristop64089]

Cool deal HK...I knew this was simpler than they ATF makes it look.

[2ndchance]

Whoa! Before you put a can (suppressor) on a 7.5" AR15 barrel you may need to rethink that. Are you planning on shooting .223 out of that 7.5" barrel?

Advanced Armament told me that using ANY suppressor on a 223/5.56 with a barrel length of less than 11.5" will void my warranty, as the short length may not properly spin the bullet and cause it to instantly tumble while exiting the barrel (and into the suppressor). This can cause catastrophic damage to the suppressor and not so safe for the shooter, too.

Also, there is the issue of the "Gem-Tax". Before, most MFG can destroy your old/damaged/worn out suppressor and put your serial number on a new can. That's how most of them honored their lifetime warranties.

Now, they can't do that and if the tube is damaged beyond repair they will have to issue a new serial number and you have to pay the tax stamp again. I'm going through that headache right now.

[Hkmp5sd]

Advanced Armament told me that using ANY suppressor on a 223/5.56 with a barrel length of less than 11.5" will void my warranty, as the short length may not properly spin the bullet and cause it to instantly tumble while exiting the barrel (and into the suppressor). This can cause catastrophic damage to the suppressor and not so safe for the shooter, too.

That's odd. Their advertisement for the M4-2000 says it is good down to 10" on full auto.

Seems to me that the deciding factor in barrel length would be overpressure in the tube instead of a bullet tumbling.

[PTK]

Advanced Armament told me that using ANY suppressor on a 223/5.56 with a barrel length of less than 11.5" will void my warranty, as the short length may not properly spin the bullet and cause it to instantly tumble while exiting the barrel (and into the suppressor). This can cause catastrophic damage to the suppressor and not so safe for the shooter, too.

I really doubt AAC told you that a barrel shorter than 11.5" may not properly "spin" (I think you mean stabilize...) a bullet and cause it to tumble instantly. Anyone with an ounce of brainmeat wouldn't say such BS, and certainly not anyone at AAC I've talked to.

Note of interest, the manual for my M4-1000 silencer from AAC says it's waranteed down to 10.5" barrel length, but I've been using it without issue on my PLR-16. The PLR-16 only has a 9" barrel - no baffle strikes, no keyholing (tumbling) bullets, nothing.

You must have REALLY long barreled pistols, too.... Shoot, if 11.5" is the shortest anything can stabilize in, how in the heck are all these 38 Special K-frame pistols shooting cloverleaf groups at 25 yards?

[aroundlsu]

The muzzle blast coming out of a short barrel creates more wear and tear on the silencer. We just had this discussion with AAC on SilencerTalk. You can expect a life of 10,000 rounds or less with a sub 10" barrel. If your silencer becomes unusable due to violating the warranty AAC will replace it for half the cost of retail.

With that said, I'm running my M4-2000 on a 9.25" upper.

[ws6_keith]

- Must it always remain a pistol?
The pistol lower may have any length upper on it, but NEVER a stock designed to be shouldered.
- If I SBR when of my other lowers, must it always remain a dedicated SBR lower?
Once an SBR lower, always an SBR lower
- Or can it serve as a multipurpose lower?
You can then put any length upper on it.
- Can I SBR the lower, and can the upper, on the same lower?
A can is registered separately and can go on anything.

[Hkmp5sd]

Once an SBR lower, always an SBR lower

You can place a >16" upper on your SBR lower and then have it removed from the registry if you desire. It becomes a common rifle at that point.

[agtman]

Unless you're set on a can, I found that Noveske's KX3 significantly reduced noise & concussive blast on my SBR (through re-direction downrange). Picture attached, if you haven't seen one.

[aroundlsu]

+1 on the KX3! I used one until my M4-2000 came in and it made my SBR sound/feel the same as everyone elses 16" ARs. With a standard flash hider the noise would literally shut the range down when I was shooting.

[agtman]

Maybe a better pic of the KX3 on the SBR:

[FireCop203]

You are on the edge of creating a real problem. ATF has ruled that for every pistol receiver and/or SBR receiver you possess, you may possess only one short barreled upper if you possess any other AR rifles. If you have more short barreled uppers than you have pistols/SBR receivers and own one or more AR receiver(s) and/or AR rifle, ATF says you are automatically in possession of an unregistered SBR.

Where did you find this ruling? This is a new one. Do you have a link or source? I'd like to have a copy of it to post on some of the other sites I visit.

[Hkmp5sd]

Replace "machinegun" with "SBR".

Not exactly new. They have been stating it for around 10 years now.

4. Can you have several short barrel uppers (less than 16 inches) for the registered AR and still own semi-auto AR's?

The definition of a firearm in section 5845 of the NFA includes a rifle having a barrel or barrels of less than 16 inches in length. An individual possessing more than one short (less than 16 inches) barreled upper receiver for a registered AR15 machinegun along with one or more semiautomatic AR15 rifles would have under their possession of control an unregistered short barreled rifle, a violation of the NFA.

http://www-2.cs.cmu.edu/afs/cs/user/...f_letter90.txt

[2ndchance]

I called BATFE today regarding the whole SBR to non-SBR issue. I was told that if you take an existing rifle and SBR it, and may decide to revert back to a standard rifle, it can be done. The easiest way is to SBR is, but instead of marking the receiver, you can physically mark the short barrel. By marking the short barrel itself, you will need 3 pieces of info.

1. Your Name (as listed on the Form-1)
2. City/State (as listed on the Form-1)
3. Serial Number of the SBR receiver that will house this Short Barrel.

This way, if you revert back to a regular rifle, you can put back the original upper or barrel and sell it (after notifying BATFE in writing of your intention and receiving a confirmation letter).

Granted, you could do the same if you marked the receiver, but then you risk having your name on a receiver floating around out there.

The bonus to marking just the actual shortened barrel is that since it's noted with the serial number of the host receiver, you can have more than one because its showing ownership to that host receiver. Just have each length barrel marked (that is under 16")

[2ndchance]

I really doubt AAC told you that a barrel shorter than 11.5" may not properly "spin" (I think you mean stabilize...) a bullet and cause it to tumble instantly. Anyone with an ounce of brainmeat wouldn't say such BS, and certainly not anyone at AAC I've talked to.

PTK, with all due respect, what's the difference? A bullet travels down the length of the barrel and is forced to "spin" because of the rifling. If spun properly it will "stabilize" the bullet and allow it to fly straight. Spin is the cause; stability is the effect.

The original post was talking about a 7.5" barrel. Most AR-15 barrels have a 1:9" twistrate. It needs 9" of barrel for a full rotation. For a bullet to stabilize it needs to rotate/spin 1-1/2 complete rotations (minimal). That's why AAC will definately NOT warranty anything under a 10.5" barrel. Freddy Thompson, Jr from AAC told me that they may not warranty it shorter than 11.5" if the damage was caused by an unstable bullet striking the baffles.

May not and WILL NOT is two different things... but for this post, it's irrelevant. A barrel of less that 7.5" will NOT spin the bullet properly to stabilize it in flight and will most likely cause a baffle strike and can cause a catastrophic failure.

Here's more on this subject from SilencerTalk.Com. This site is crawling with AAC people and is a great place to learn "all things suppressors".

http://www.silencertalk.com/forum/vi...suppressor+sbr

http://www.silencertalk.com/forum/vi...suppressor+sbr

[PTK]

A barrel of less that 7.5" will NOT spin the bullet properly to stabilize it in flight

This I didn't know - the classes in ballistics I've taken say otherwise... such as, handguns still stabilizing bullets, short barreled rifles still stabilizing bullets, etc. I'll have to read the threads over on ST, I'm PTK there same as here.


EDIT: I read those threads (and participated in one a while back) and neither says anything about the twist rate vs the length vs the velocity effecting bullet stabilization. I don't see how a shorter barrel won't stabilize a bullet (other than VLD bullets, that's easy to understand) for things like 55gr... I mean, my PLR-16 will stabilize 69gr bullets quite happily, no baffle strikes. But then again, it's a 9" barrel.

[Hkmp5sd]

Most AR-15 barrels have a 1:9" twistrate. It needs 9" of barrel for a full rotation. For a bullet to stabilize it needs to rotate/spin 1-1/2 complete rotations (minimal).

It's not barrel length so much as muzzle velocity. For example, a Glock 17 has a twist rate of 1 per 9.84 inches. That does not mean the Glock needs a barrel 14.76" long for the bullet to stabilize.

For a bullet to stabilize, it must be spinning at a given RPM when it exits the barrel. RPM is determined by multiplying muzzle velocity by (720/twist rate). The minimum RPM for stabilization is based on bullet diameter and bullet mass (weight).

So a 55-grain .223 using a barrel with a 1:9 twist rate will have some minimum muzzle velocity to stabilize and it is that muzzle velocity which depends on barrel length.

[PTK]

See, that's what I thought. Thanks for the input.

[2ndchance]

It's not barrel length so much as muzzle velocity. For example, a Glock 17 has a twist rate of 1 per 9.84 inches. That does not mean the Glock needs a barrel 14.76" long for the bullet to stabilize.

For the sake of argument, this discussion is on the AR15... not the glock or any other handgun firing standard handgun rounds. You are correct that barrel length is NOT the only factor. A glock shoots a heavier bullet than the 55gr found in the common .223rem. Since the bullet fired out of the glock has more mass, surface area and moves slower, it can stabilize with ease out of a shorter barrel.

Still, this post is regarding a 7.5" barrel for an AR15. Whether it's a 1:9 twist shooting a 55gr or a 1:7twist shooting a 62-65gr bullet, the fact is a 7.5" barrel will be hard pressed to stabilize.

I mean, my PLR-16 will stabilize 69gr bullets quite happily, no baffle strikes. But then again, it's a 9" barrel.

PTK - This falls under the same principle. The 69gr bullet is probably traveling at a slower speed. The barrel is probably a 1:9twist, so the heavier bullet is compensating for the short length. I would bet that it would not do as well with a 55gr bullet. Try it. Do a 50yard test... see what happens.

[Hkmp5sd]

The deciding factor is muzzle velocity. If just happens that muzzle velocity is proportional to barrel length for a given cartridge.

A 7.5" barrel may just happen to be too short to achieve minimum muzzle velocity for stabilization for a commercial .223 cartridge with a 55-grain bullet using a 1:9. Jack up the powder load, get more muzzle velocity and a 7.5" barrel may work.

If you could get a 55-grain bullet going fast enough, a 5" 1:9 barrel would work. Speed is key.