Dwell Time - the amount of time the BCG is receiving gas pressure. It is also expressed by the length of barrel between the gas port and the muzzle.
In more simple terms: the length between the hole (gas port) in the top of the barrel where the gas vents out and the end (muzzle) of the barrel.
So, why is it important? If you don't have the right amount of Dwell Time, your gun won't reliably cycle, if it cycles at all. This is literally the thing that makes the gun automatically cycle, so that you don't have to run the action manually after each shot. If you don't have enough Dwell time, it won't cycle weaker loads and may lead to failure to extract or failure to load issues. If you have way too much, the gun may cycle too quickly and cause different feeding issues or extraction issues. Too much Dwell Time will also wear down the parts much faster because the action will cycle more violently, which also has a negative effect on your follow up shots. If the gun is cycling more violently, you won't be able to hold it on target as well - no matter how strong you are. Also, I want to make it clear: when speaking of recoil here, it is not a matter of 5.56 hurting your shoulder - it's a matter of lining up your next shot as fast as possible. I mean, it's 5.56...
How do you find out your Dwell Time? Reference the following list and subtract the length listed for your gas system from the length of barrel
So, a Mid-Length gas system on a 16" barrel gives approximately 7" of Dwell Time (16" - 9" = 7"). A Pistol gas system on a 12" barrel also gives 7" of Dwell Time. Your typical Colt LE6920 (16" barrel and Carbine gas system) gives about 9" of Dwell time. The AR platform, when originally adopted, was a 20" barrel with a Rifle gas length, so 7" Dwell Time. An M4 is a 14" barrel with a Carbine gas system, so 7".
So, how do you know if you have the right amount? I can tell you from personal experience, 7" to 9" will reliably cycle a 5.56-chambered AR with most ammo. I prefer 7" DT (Dwell Time) as it provides me the best recoil management and reliability for my rifles. It's 100% reliable and noticeably lower recoil than a 9" DT, like you'll find in the Colt LE6920. (Bear in mind, I use brass-cased ammo exclusively.) If you are going to use steel-case ammo, I would recommend the 9" DT for reliability. But, then you're using steel-cased ammo, so clearly reliability isn't your biggest concern... ZING! One way people combat muzzle rise and recoil, particularly on 9" DT rifles, is to use a Compensator. Compensators are Muzzle Devices intended to control the movement of the
rifle by using the gas exiting the barrel, but they are loud, tend to
do a poor job of hiding flash, and are less than kind to anyone sitting
next to you at a range. And, using one in your house for self-defense will more negatively impact your hearing. (And, your family's hearing!) They can help a lot, but they don't save the internals of your rifle from abuse and are worse for your hearing. Overall, I would prefer to just get the right DT and use a flash hider.
So, to put a bow on it, try to stay between 7" to 9" DT and you should be just fine, in terms of reliability, but 7" DT is preferred.