View Full Version : Why do Barrels lose girth about 12'" down?
I'm a newb, so if it's obvious to most...forgive me. I notice on lite barrels, they lose girth for about an inch, 12 inches down the barrel, then they get thicker til the end of the barrel. Is this to allow for a mid-length gas block? Also are you able to transform a carbine to a mid-length just by relocating the gas block?
Lots of different barrels, lots of different profiles.
We'd need a picture of which "light" barrel you are talking about.
OH! I think I know what you are talking about now- you are looking at an M4 profile barrel I'd bet. Is this "skinny" part a few inches in front of the front sight tower?
http://us.st11.yimg.com/us.st.yimg.com/I/yhst-57056280658346_1900_247331
If that is the barrel you are talking about, it's stepped down to accept the clamp on the front end of an M203 grenade launcher and nothing more. It's not a "lightweight" barrel either. That is a standard or common profile, "government" I believe.
There are heavy Colt SOCOM M4 barrels where the portion between the receiver and the front sight is an even thickness and doesn't step down in front of the barrel nut and then back up just before the front sight base.
Since we've now explained that notch up front, it's NOT for a midlength gas block.
Barrels have a tiny hole in them under their front sight base to carry gas into the sight tower and then back down the gas tube.
They can only have one to function correctly ;)
That barrel I showed you (as 99.99% of any M4 profiled barrel) will use a carbine length gas system whether they are 14.5" or 16" (common civilian legal length.)
A carbine length gas system places the rear of the FSB (front sight base) almost exactly 7" from the front face of the upper receiver. These barrels/gas systems use "carbine" length handguards or rails. DD7.0, LaRue 7.0, etc. rails are for the carbine length system.
Midlength gas systems are two inches longer with the FSB being 2" farther forward than a carbine length gas system's FSB. They use 9" handguards, DD9.0 or LT9.0 rails if you wish.
Midlength gas systems are theoretically easier on the internals of the rifle due to slightly less operating pressure due to the extra distance for expansion in both the barrel and gas tube.
Now, one last thing- AR15s with either the midlength gas system and carbine length gas system can and both will be called a "carbine" because the rifle itself is a "carbine" because it is not a full size rifle. Just one more thing that tends to confuse beginners.
Wow I hope I assumed correctly and didn't just type all of that for no reason at all!
ETA: The shortest barrel you can have with a carbine length gas system is about 10.3" as used in the Mk18Mod0 Navy rifles. Common midlength systems usually have barrels that are 16" although some companies have used 14.5" barrels and permanently attached a FH of 1.5" or more to keep them the legal length.
Lots of different barrels, lots of different profiles.
We'd need a picture of which "light" barrel you are talking about.
OH! I think I know what you are talking about now- you are looking at an M4 profile barrel I'd bet. Is this "skinny" part a few inches in front of the front sight tower?
http://us.st11.yimg.com/us.st.yimg.com/I/yhst-57056280658346_1900_247331
If that is the barrel you are talking about, it's stepped down to accept the clamp on the front end of an M203 grenade launcher and nothing more. It's not a "lightweight" barrel either. That is a standard or common profile, "government" I believe.
There are heavy Colt SOCOM M4 barrels where the portion between the receiver and the front sight is an even thickness and doesn't step down in front of the barrel nut and then back up just before the front sight base.
Since we've now explained that notch up front, it's NOT for a midlength gas block.
Barrels have a tiny hole in them under their front sight base to carry gas into the sight tower and then back down the gas tube.
They can only have one to function correctly ;)
That barrel I showed you (as 99.99% of any M4 profiled barrel) will use a carbine length gas system whether they are 14.5" or 16" (common civilian legal length.)
A carbine length gas system places the rear of the FSB (front sight base) almost exactly 7" from the front face of the upper receiver. These barrels/gas systems use "carbine" length handguards or rails. DD7.0, LaRue 7.0, etc. rails are for the carbine length system.
Midlength gas systems are two inches longer with the FSB being 2" farther forward than a carbine length gas system's FSB. They use 9" handguards, DD9.0 or LT9.0 rails if you wish.
Midlength gas systems are theoretically easier on the internals of the rifle due to slightly less operating pressure due to the extra distance for expansion in both the barrel and gas tube.
Now, one last thing- AR15s with either the midlength gas system and carbine length gas system can and both will be called a "carbine" because the rifle itself is a "carbine" because it is not a full size rifle. Just one more thing that tends to confuse beginners.
Wow I hope I assumed correctly and didn't just type all of that for no reason at all!
ETA: The shortest barrel you can have with a carbine length gas system is about 10.3" as used in the Mk18Mod0 Navy rifles. Common midlength systems usually have barrels that are 16" although some companies have used 14.5" barrels and permanently attached a FH of 1.5" or more to keep them the legal length.
Yep, you assumed correctly. I was referring to the M4 profile. And you explained it perfectly too. I always wondered why it was stepped down...seemed pointless. So I'm assuming you can't convert a 7" to a 9" then, because gas would escape the hole? I was counting on that for versatility...but sounds like i'd have to buy a whole new barrel if I wanted to convert to 9" later. I have heard before that 9" is easier on the internals, but exactly what internals take the beating in a 7" system? Is it a big enough problem to buy a mid lenght system instead of a carbine length system? I'm getting ready to buy my first AR, so I'm trying to soak up as much info as I can that may effect which parts I buy. I appreciate the info, it was a big help;)
No it's definitely not a big enough problem or even a problem really...
Since many people are stuck with a 16" barrel, they end up getting a midlength for the improved sight radius anyway.
Aside from that, the midlength looks good IMHO. It has the correct "look" and amount of barrel in front of the FSB.
I think they look sweet with a free float rail.
No it's definitely not a big enough problem or even a problem really...
Since many people are stuck with a 16" barrel, they end up getting a midlength for the improved sight radius anyway.
Aside from that, the midlength looks good IMHO. It has the correct "look" and amount of barrel in front of the FSB.
I think they look sweet with a free float rail.
Yea no doubt they can look good. I prefer the 7" look a little more. Plus I may want to get my barrel shortened in the future.
14.5" carbines like the M4 are sexy as hell. I've got an 11.5" and CONSTANTLY kick around the idea of a true M4 clone with 14.5" barrel and A2. I just REALLY never liked the look of a 16" M4 barrel with any sort of FH that just adds to the length.
In fact, MM did a review of the Vltor flip up front sight base/gas block where he had it installed on an M4 barrel that was cut to about 12 or 12.5 inches. That was damn sexy for some reason!
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