Barrel Comparison and Information Overload.

[Iraqgunz]

I see a lot of people asking questions about various barrels. I put this together to help paint a picture of the various types of barrels that are encountered and to help explain things like the barrel markings, etc... For some this will be old hat. For others, it may help to clarify some stuff. These pics are a small cross sample of barrels in my collection that represent different ones encountered.

Many barrels have markings. These markings can tell you part of the story, but ultimately you need to look at what the manufacturer tells you.
Here's a small cross sample of barrels in my collection or used for demo purposes.

Bushmaster 11.5" parkerized, 1/9, (UNK) with permanently attached muzzle device to make it compliant. Carbine gas system.
Ballistic Advantage 16" Melonite 1/8, 5.45x39 4150 CMV. Midlength gas system.
Advanced Armament 16" Nitrided 1/7, 5.56. 4150 CMV. Midlength gas system.
Sabre Defense 14.5", 5.56. 4150 CMV. Midlength gas system.
Colt 14.5", 5.56, 4150 CMV. Carbine gas system.




PIC 2. Sample of markings for various barrels.



PIC 3. Bushmaster v. Colt marking comparison. The Bushmaster is a 1/9 HBAR. The B MP is supposed to represent Bushmaster Magnetic Particle Inspected. It's also supposed to have a 5.56 chamber. Of interest is that the barrel extension is actually a rifle extension and not carbine.

Colt is marked C MP 5.56 NATO 1/7= This means Colt, Metallic Particle Inspected, chambered in 5.56 NATO with a 1/7 twist.



PIC 4. Pinned FSB v. Pinned low profile gas block v. dimpled barrel for low profile gas block.



PIC 5. Comparison of 14.5" and 16" OAL barrels. As you can see, the difference is less than 1" of the overall length between them. The juice isn't worth the squeeze in this case. 3/4 of an inch is not going to make you "more maneuverable" or more operator. It will however complicate things should you desire to change your rail/handguard configuration in addition to additional costs of making the barrel compliant.



There are (3) basic ways of securing a gas block. In picture (4) you will see a Colt 14.5" carbine with a pinned FSB. Each FSB is individually drilled and pinned to the barrel with taper pins. You can also have a gas block that is pinned or one that uses set screws (located at the bottom) and secured with pointed tip screws. Of prime importance is that the barrel be dimpled. The dimpling helps to insure that the gas block is pulled down onto the barrel, and provides a gas seal. It also ensures that the gas block is positioned correctly onto the barrel (centered and the correct stand off). If this isn't correct you will end up with short stroking.



Another consideration during this is the gas port sizing. All of these barrels will have different sizes. This is dependent on the manufacturer, and the type of gas system. Of note, is that there is NO STANDARD for a midlength gas system.

Ultimately, you will need to look at the specs listed for each company and manufacturer to see exactly what the specs are. Often times, you will not see gas port sizing published and you will need to ask for this information. In some cases, they may or may not disclose this.

[New Centurion]

Nice write up, IG. For OCD folks like me, some manufacturers put a "born on" date on the barrel as well (DD, that I know of off hand). Only really comes in to play if they have made changes to the barrel specs at some point, I suppose, or if a particular batch had issues that weren't initially apparent.

I have previously seen MPI described as "Magnetic Particular Inspection", are the terms generally interchangeable?

[Ryno12]

PIC 5. Comparison of 14.5" and 16" OAL barrels. As you can see, the difference is less than 1" of the overall length between them. The juice isn't worth the squeeze in this case. 3/4 of an inch is not going to make you "more maneuverable" or more operator. It will however complicate things should you desire to change your rail/handguard configuration in addition to additional costs of making the barrel compliant.

Not entirely accurate, or fair, comparison. When comparing the difference between a 14.5" & 16" OAL and using similar styles of comps (A2 vs A2x or BCM Mod 0 vs Mod 1), the net results are greater & in some cases, doubled. In an apples to apples comparison, a Colt 14.5" & Colt 16" with an A2x and A2, respectively, resulted in an exact difference of 1.5". Whether the "juice is worth the squeeze" is entirely subjective & the final decision of the end user.
In addition, the argument about losing or hindering any future customization of the handguard, while true, the same can be said about any AR with a fixed FSB.
That includes a dissy with a pinned FSB.

[Eurodriver]

Not entirely accurate, or fair, comparison. When comparing the difference between a 14.5" & 16" OAL and using similar styles of comps (A2 vs A2x or BCM Mod 0 vs Mod 1), the net results are greater & in some cases, doubled. In an apples to apples comparison, a Colt 14.5" & Colt 16" with an A2x and A2, respectively, resulted in an exact difference of 1.5". Whether the "juice is worth the squeeze" is entirely subjective & the final decision of the end user.
In addition, the argument about losing or hindering any future customization of the handguard, while true, the same can be said about any AR with a fixed FSB.
That includes a dissy with a pinned FSB.

I agree with your assessment of length, but I quickly found out that IG's point still stands (and I'm not being a kiss-ass.)

I have a BCM 14.5" Middy and I wanted to put a 9" KMR on it behind my FSB. I couldn't do this without cutting off the pinned $150 muzzle device, so I threw a Centurion on it instead. Not the end of the world, but a KMR wouldn't be just fabuloussstth.

[Ryno12]

I agree with your assessment of length, but I quickly found out that IG's point still stands (and I'm not being a kiss-ass.)

I have a BCM 14.5" Middy and I wanted to put a 9" KMR on it behind my FSB. I couldn't do this without cutting off the pinned $150 muzzle device, so I threw a Centurion on it instead. Not the end of the world, but a KMR wouldn't be just fabuloussstth.

You still would've had to unpin the FSB.

I'm not sure why people think that the pinned MD has to get trashed in the removal process. It's only drilling out a pin. Might have to remove some weld prior.

[Eurodriver]

You still would've had to unpin the FSB.

I'm not sure why people think that the pinned MD has to get trashed in the removal process. It's only drilling out a pin. Might have to remove some weld prior.

That's true, but it is so much easier to just unscrew it and put it back on!

I guess what I'm trying to say, is that 10 years after buying my first 14.5" AR15, I've seen the light and will stick to 16" from now on.

[Ryno12]

That's true, but it is so much easier to just unscrew it and put it back on!

I guess what I'm trying to say, is that 10 years after buying my first 14.5" AR15, I've seen the light and will stick to 16" from now on.

I know. I think it gets the brunt of criticism because it's first in line to be removed & thus the first "road block".

I just get a chuckle when I hear people complain about a pinned MD yet they'll grind down a FSB to free float a handguard or worse yet, build a dissy.

Just clarifying that it's not as bad as some make it out to be. Perming anything to a barrel will be cause for extra work down the road should the barrel nut have to come off. That includes any pinned gas blocks or FSB's.

[Eurodriver]

I know. I think it gets the brunt of criticism because it's first in line to be removed & thus the first "road block".

I just get a chuckle when I hear people complain about a pinned MD yet they'll grind down a FSB to free float a handguard or worse yet, build a dissy.

Just clarifying that it's not as bad as some make it out to be. Perming anything to a barrel will be cause for extra work down the road should the barrel nut have to come off. That includes any pinned gas blocks or FSB's.

Absolutely. In fact, to be truthful, I guess the issue isn't even that I had a pinned MB. It's just that I didn't want to go through the hassle of finding someone that could save the muzzle device while removing it. If it was an A2X or something cheap, I would have just hacked it off.

[GrumpyM4]

I have previously seen MPI described as "Magnetic Particular Inspection", are the terms generally interchangeable?

You are correct. It's probably a simple mistake in IG's part.

Magnetic Particle Inspection is a form of NDT that uses a powerful magnet and colored iron dust to find cracks in steel. I've been party to a lot of MP testing in a former life.

[Iraqgunz]

I only know MPI as Magnetic Particle Inspection. Though I am sure I could be wrong.
http://www.labtesting.com/services/n...le-inspection/

You are correct. It's probably a simple mistake in IG's part. Magnetic Particle Inspection is a form of NDT that uses a powerful magnet and colored iron dust to find cracks in steel.


I've been party to a lot of MP testing in a former life.