AR-15 Dwell Time

[FPrice]

Interesting video by Caleb Savant. Not sure (yet) how critical it is to the DIYer but some good information.

Quick Tip: What Is AR-15 "Dwell Time"? | Top Rated Supplier of Firearm Reloading Equipment, Supplies, and Tools - Colt

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[jayhitek]

Funny, I watched this video yesterday and was thinking.. holy cow.. how would I know how to time any of this.

 

[cathouse01]

I really didn’t pay any attention to this aspect of the AR type until I was doing my .50 Beowulf build. Not really understanding the details, I was thinking with the hard hitting .50 caliber, you’d need a heavy buffer spring and buffer. It wasn’t until I read Alexander Arms F.A.Q. that I realized that the buffer spring and buffer you should use are completely dependent on the chamber pressure of the cartridge and the dwell time. What you realize with the .50 Beowulf is that the chamber pressure of 33,000 psi is considerably less than the 5.56 NATO chamber pressure of 58,000 psi, but it kind of makes up for that lower pressure with a longer dwell time (since that heavy built takes longer to accelerate down the barrel). The combination of the lower pressure, using the correct sized gas port and the longer dwell time means that the .50 Beowulf uses a standard strength carbine spring along with an H2 buffer (or a standard rifle spring with 5.2 oz rifle buffer for a rifle length stock).

As Alexander Arms says:

The AR-15 buffer and buffer spring are used only for the cycling of ammunition inside of the rifle. Recoil in an AR-15 rifle is almost entirely unrelated to the buffer components, especially with .50 Beowulf®. You can thank the 1,300 MPH projectile exiting the other end for the recoil.

And while the dwell time will be measure in milliseconds, you can get a good idea how well your particular rifle is timed based on its ejection pattern. 80% Arms gives you one of the many ejection pattern charts:


Unless you have an adjustable gas block, most people won’t want to mess with the barrel’s gas port or gas block, so you’ll be messing with your buffer spring and buffer weight. Note that this doesn’t fully correct for incorrect gassing, it just mitigates it.

And here is another good article on dwell time (though the author does use the simplified version of its definition):
what-is-dwell-time
The author points out that the “average" commercial AR is actually overgassed so that they will run with the lowest quality 5.56/.223 ammo, but that the overgassing will really only effect them after several 1000 rounds where as the “average” AR won’t see near that many rounds. That’s might be why some of the high end ARs run poorly with the cheap ammo. They actually are properly gassed to only work with high quality ammo.

 

[KMM696]

I believe that if you're sticking with the standard configurations, dwell time is not something you need to worry about much. If you're looking at putting together a 5.56/223 with a 14.5" barrel carbine length gas, or 16" midlength or carbine gas, or 20" rifle length it's going to work fine. Any fine tuning can usually be covered by buffer weights. Where dwell time becomes something to think about is when you're well off the beaten path. In cathouse01's example a major change in cartridge pressure going to 50 Beowulf gave the system a much lower pressure to work with, and increased the dwell time to compensate. I never even thought about dwell time until I put together a Dissipator style upper in 5.56, using a 20" rifle gassed barrel cut down to 16.5". This results in a dwell time that's roughly half what the AR system likes. While my chamber pressures were what the system is designed for, they were acting on the bolt for half as long as it needed. Since I couldn't change the barrel length, that left me with drilling out the gas port (several times - tough to put metal back) to allow more gas volume in that shortened operation time. It's still not as reliable as it should be - low powered ammo is a disaster in it, but I got it to where it works with M193 or M855 consistently.

Today's Dissipator style barrels have solved the dwell time problem by installing a gas block at the midlength position on the barrel and leaving a front sight base that isn't carrying gas at the rifle position. Dissipators are a curiosity at this point anyway. Good optics and flat top receivers have made them a dinosaur.

 

[cathouse01]

I believe that if you're sticking with the standard configurations, dwell time is not something you need to worry about much.

This ^^^^^ is absolutely true. Particularly if you aren’t building your upper from scratch. Even in a .50 Beowulf build, I really don’t think you have to worry about the dwell time since I don’t think you could find a .50 Beowulf barrel that didn’t follow the Alexander design for gas port size and placement. And Alexander specifically took into account the lower pressure and slower bullet so that the rifle would cycle properly with pretty much standard buffer springs and buffers. It was more of a learning experience for me, since I thought there would need to be a difference. Not to mention you don’t have to worry about cycling with “cheap” underpowered .50 Beowulf ammo since there is no such thing.

If you do your builds based on a complete upper, then the upper manufacturer will have taken care of all this for you. If it’s something out of the ordinary (like the BCA .22 WMR upper) they will either tell you what you need or, as in BCA’s case, actually provide you with the correct buffer spring and buffer.

 

[mac1911]

Dwell time gets fun when your playing around with different powders and such ; cast loads, reduced loads , its all fun and generally theres always a solution to the problems: