The Firing Line Forums - View Single Post - AR15: Mid-length 18" vs Rifle-length 18"

Skyyr · May 11, 2009, 09:51 PM

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?

May 11, 2009, 09:51 PM	#1
Skyyr Senior Member Join Date: May 4, 2009 Posts: 266	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?