The Firing Line Forums

Go Back   The Firing Line Forums > The Hide > The Art of the Rifle: General

Reply
 
Thread Tools
Old May 11, 2009, 09:51 PM   #1
Skyyr
Senior Member
 
Join Date: May 4, 2009
Posts: 265
AR15: Mid-length 18" vs Rifle-length 18"

I've been doing some research for awhile and have been comparing the 18" mid-length to the 18" rifle-length. We're not considering the intermediate-length gas system, so please don't open that can of worms :P

The general consensus in various posts across several boards seemed to be to use the longest-length gas system possible. Since a rifle-length gas system is, at minimum, feasible for a 17", most people opted for rifle-length gas systems.

http://www.ar15.com/forums/topic.html?b=3&f=66&t=266108

Most of you have probably seen the above charts. It's seems accepted that an average ~200ms dwell time is optimum for operation performance. Upon reviewing the charts halfway down the page, the 18" mid-length is closer to the optimum dwell time (~.234, or .034 off, or 15% from optimum), whereas the rifle-length 18" is around .131 (.69 off, or 35% off).

Since pressures are only about 1.5K higher in the mid-length (being negligible), doesn't this support the use of mid-lengths? Or am I missing something here?
Skyyr is offline  
Old May 12, 2009, 12:17 AM   #2
Zak Smith
Senior Member
 
Join Date: December 12, 1999
Location: Fort Collins, Colorado, USA
Posts: 2,675
A RLGS on a 18" will have a more smooth recoil impulse than a MLGS. My 3Gun rifles are 17" and run RLGS.

-z
__________________
Zak Smith . DEMIGOD LLC . THUNDER BEAST ARMS CORP . COLORADO MULTI-GUN
My PM inbox full? Send e-mail instead.
Zak Smith is offline  
Old May 12, 2009, 07:30 AM   #3
Bartholomew Roberts
Senior Member
 
Join Date: June 12, 2000
Location: Texas and Oklahoma area
Posts: 5,641
My thinking on this is that the original system is designed around the 20" barrel with rifle-length gas system. That is the most mature of the AR systems and that is the one that the bolt and other key internal parts that most ARs share were designed around.

That system has a .184ms dwell time, with an average port pressure of 16,062fps. On an 18" midlength, the dwell time will be .234ms (about .05ms too long). On an 18" rifle, the dwell time will be about .05ms too short. Given that neither one is idea, I'd go with the one that best matches the port pressure - which in this case is the rifle.

Having said that, it isn't a big issue in any case. Gas systems differences are going to be a minor influence compared to all the other things going on out there.
Bartholomew Roberts is offline  
Old May 12, 2009, 08:14 AM   #4
Skyyr
Senior Member
 
Join Date: May 4, 2009
Posts: 265
@Zak

How is that? According to the charts, I would find myself thinking that the MLGS would have smoother operation. Is it simply the slightly lesser pressure of the RLGS?

Regardless, is there an advantage to using the MLGS with it's closer-to-optimum dwell time?
Skyyr is offline  
Old May 12, 2009, 09:45 AM   #5
Technosavant
Senior Member
 
Join Date: May 29, 2007
Location: St. Louis, MO area
Posts: 3,851
Without going into the dwell time (that's being covered pretty well already), the rifle length gas system does have a wider range of handguards and rails available. With the popularity of the midlength system, that's quickly becoming moot, but rifle length still has the advantage. I'd also consider the longer sight radius a benefit as well.

And I can say that I bought a 18" SPR barrel a month or so ago, and it has a rifle length gas system.
Technosavant is offline  
Old May 12, 2009, 09:53 AM   #6
Skyyr
Senior Member
 
Join Date: May 4, 2009
Posts: 265
So from what I'm gathering, the MLGS has a slight advantage in dwell time, but not enough to make a major difference, correct?

Factor in that rifle-length is more abundant, and that it has a slightly lower pressure, and that makes the rifle more favorable?

I guess I'm looking for some sort of definite performance difference, as on paper, the mid-length looks marginally better than the rifle-length, though everyone swears by the rifle-length. The fact that another gas-system (the intermediate-length) was built for 18" makes me believe this even further, since the data seems to support that both mid and rifle-lengths are about equal on an 18" barrel. I'm simply trying to make up for the discrepancy.
Skyyr is offline  
Old May 12, 2009, 10:14 AM   #7
Bartholomew Roberts
Senior Member
 
Join Date: June 12, 2000
Location: Texas and Oklahoma area
Posts: 5,641
As a practical matter, there is no huge difference. The rifle length 18" will usually feel smoother when shooting and there is less barrel poking out of the handguards to brand yourself with if you are doing a lot of shooting.

Either one will work just fine. For the record, I once faced a similar decision. I needed a new barrel. I wanted a midlength; but didn't have the money for my new go-faster carbon fiber barrel and a new rail system. I ordered the carbine length and they shipped a midlength instead. Taking this as an omen, I kept itand bought a new rail. Now after several thousand rounds, I can say that it would have been better and smarter for me to have spentthe extra money on training rather than minor tweaks in gear.

Having said that, I love my midlengths
Bartholomew Roberts is offline  
Old May 12, 2009, 12:08 PM   #8
5whiskey
Senior Member
 
Join Date: October 23, 2005
Location: US
Posts: 1,544
My personal opinion is that there is little enough difference in ACTUAL operation that you will not notice it or it won't affect the actual function of the firearm. The mid-length would be a tad lighter with standard forearms, and much lighter with piccy rail forearms. Just my .02, I have a 16" bbl with a midlength gas system and it eats everything thrown at it, steel cased wolf and all.

It will all depend on what you want to do with it... if you wanted more of a carbine then I would def. stick with mid-length. If it's a varminter (18" varmint?) then rifle length if that's what you want. Really weight and use would be my determining factor more so than minor nuances with the gas system, the later of which will more than likely never be felt or noticed.
5whiskey is offline  
Old May 12, 2009, 12:50 PM   #9
Zak Smith
Senior Member
 
Join Date: December 12, 1999
Location: Fort Collins, Colorado, USA
Posts: 2,675
Skyyr,

See this tech note
http://www.armalite.com/images/Tech%...4%E2%80%A6.pdf

However, one can simply observe the behavior when comparing rifles.
__________________
Zak Smith . DEMIGOD LLC . THUNDER BEAST ARMS CORP . COLORADO MULTI-GUN
My PM inbox full? Send e-mail instead.
Zak Smith is offline  
Old May 12, 2009, 07:10 PM   #10
stubbicatt
Senior Member
 
Join Date: September 15, 2007
Posts: 1,061
I feel dizzy.

Glad I have a HK93~!
stubbicatt is offline  
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 10:48 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2014 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent: thefiringline.com
Contact Us
Page generated in 0.07629 seconds with 7 queries