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.
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