SBR Carbine Gas 12.5" barrel Dwell

[mhearn]

I'm in the process of building my first SBR. Beyond other possible uses, I hope to use the rifle to compete at local Tactical Matches when the distance is generally going to be under 300 yards.

This project stems from the fact that the now out of office governor of the great state of Alabama repealed the ban on SBRs late last year. Seems like a good time to submit the paper work to the ATF and add one to the inventory.

As far as the games of shooting go, I've always run a rifle length buffer with a JP low mass buffer, low mass bolt carrier, reduced power recoil spring. Like it. I also started this game with a 20" barrel and rifle length gas system. Moved to 18". Now to Sabre 16" mid length gas. Like it. Watching friends out at the range hitting stuff at 600 with a 14.5 Noveske convinced me that it was possible to play this game with a shorter barrel.

I know the conventional wisdom is 18" barrel with rifle length gas, maybe mid. I just want to try the short barrel. I believe I can make it work. I intend to use it at the local level at first. After that we will see. USPSA minor is an issue upon occasion. I think I can find a weight and velocity that will work. Maybe not with 55 grs. My 16" makes it fine with XM 193. I doubt the 12.5" will make it with enough cushion to calm my nerves. But some of the heavier stuff should work. Outlaw matches? Who cares. 55 on the close and 77 on the distance. I run an Elcan Spectre Dr with a ballistic reticle. I think I can figure the dope. Hell, I like figuring out the dope on varying distances.

Long intro with TMI to my question.

I've been reading as much as I can about dwell. The following post talks about the ideal dwell being around .20 ms

There has been a lot of discussion lately about dwell time and how it relates to the function of certain barrels, particularly 18" rifle-gassed, 18" mid-length and 14.5" mid-lengths.

I put together a new graphic to illustrate DWELL TIME.

To use this graphic, find the gas port location above the pressure trace (Orange marks) and then locate the barrel lengths below the pressure trace (Blue marks).

The DWELL TIME is the time between these two marks (indicated on the X axis of the graphic as well as in the data below the graphic.

Optimum dwell time is right around 0.200 ms when you use the two most common gas system/barrel length configurations of 20" rifles and 14.5" carbines.

AR Dwell

I don't get the dwell thing when it comes to the carbine gas system. This post shows the 14.5" barrel with a carbine gas system as having a dwell around .206 ms. ???

With the mid and the rifle length gas, I get that we want some time past the gas port before the bullet exits the barrel. Makes sense. Give the gas time to get back to the BCG. It makes sense that if the gas got there too soon while the system is still under a lot of pressure and started pulling on the BCG that things could get rough. That makes sense. How does that work with the 14.5" barrel? The gas is going to be pulling on the BCG for a while. Yet the post indicates that this setup (gas port at 7.8, barrel 14.5) is an ideal "dwell". What am I missing?

My plan is a 12.5" Rainier Ultra Match barrel. Wylde chamber. 12.5 should give me a bit more velocity over the 10.5. It should also reduce the muzzle blast a bit over the shorter barrels. PRI Fat boy gas tube (to slow down the gas pulse). Regular buffer. Regular recoil spring. Maybe even a Light BCG. PRI muzzle brake.

I know the conventional wisdom with shorter barrels (10.5) is to add weight behind the Bolt. More weight. Heavier buffer, extra strength spring, D ring. Reciprocating mass. I would rather not if possible. I hope to also use a JP adjustable gas block to take some of the pressure off the parts.

I would appreciate feedback on this crazy idea.

[J.Schmitt]

I can't answer the math on the dwell time. What I can attest too is that I used to have a short m4 and some of my co-workers still do. At 200 yards on a 8 and 10 inch MGM flashers a low hit does not carry enough energy to activate the targert. High hits in the upper 1/3 would usually get it to move enough to call a hit but you really had to be looking close.

jay

[mhearn]

At 200 yards on a 8 and 10 inch MGM flashers a low hit does not carry enough energy to activate the targert.

jay

Thanks. That part I understand. This question is about gas system function.

[HRider]

I have a 10.5" gun that has a standard weight BCG, CAR buffer and spring, and CAR gas tube. It runs fine, it will run Wolf without any problem. I had heard of reliability problems with short guns because of the lack of dwell time, but it hasn't shown up in mine yet. It is fun to shoot, I have wanted to run the House stage at Tapp's match with it ever since I built it.

Hurley

[OPENB]

I have an 11.5" SBR with a Noveske Pig brake, regular AR buffer and carrier. I have no reliability issues. I believe the Pig increases back pressure for increased reliability in SBR's. Bolt speed can be too fast, ripping case heads off before pressure can drop enough for the brass to extract easily. Therefore I run an o-ring on the extractor to increase tension. When I picked my barrel length for my SBR, the word was 11.5" was the magic minimum for a reliable gun, with standard DI gas parts. A buddy tried a 10.75" DI gas gun, and could not get it to be reliable. He did not mess with the gas port size.

[mhearn]

I have wanted to run the House stage at Tapp's match with it ever since I built it.

Hurley

Hurley

My plan exactly. Thanks. Hope to see you there.

MHearn

[mhearn]

Pig brake.

11.5" was the magic minimum for a reliable gun

I like the idea of the pig brake. Go Hogs.

I've also been looking at the 11.5 but I'm leaning toward the 12.5.

[caspian guy]

Pig brake.

11.5" was the magic minimum for a reliable gun

I like the idea of the pig brake. Go Hogs.

I've also been looking at the 11.5 but I'm leaning toward the 12.5.

Hearn,

Give me a call...

[mhearn]

Pete.

Sunday. Will do. Wouldn't do the Pig thing without talking to you first.

Hearn

[mpeltier]

With the mid and the rifle length gas, I get that we want some time past the gas port before the bullet exits the barrel. Makes sense. Give the gas time to get back to the BCG. It makes sense that if the gas got there too soon while the system is still under a lot of pressure and started pulling on the BCG that things could get rough. That makes sense. How does that work with the 14.5" barrel? The gas is going to be pulling on the BCG for a while. Yet the post indicates that this setup (gas port at 7.8, barrel 14.5) is an ideal "dwell". What am I missing?

Mark

You are not really missing anything. Although I do not think it was explained very well the importance of the distance from the chamber to the gas port, as it relates to the peak pressure in the chamber at extraction. This is the secret to the whole balancing act. It boiles down to this, the closer to the gas port the chamber is the higher the pressure in the chamber will be (at the moment of extraction), so, it is neccessary to slow down the bolt carriers rearward movement to allow pressure to drop. This is typically done by using more mass (heavier buffer). Depending on the size of the gas port in the bbl you are using, and the ammo, will dictate what weight buffer you need to have it run reliably. Sometimes it takes a little trial and error. If you look at the pressures in the chart, its clear that their is a huge differance in peak pressures between the differant gas systems, and all have less to do with dwell time than chamber to gas port distance. Hope this is helpful.

[mhearn]

less to do with dwell time than chamber to gas port distance.

Mark

Thanks. That does seem to the 400 pound gorilla that can't be ignored. That is part of the reason I'm interested in the "Fat Boy Gas Tube".

This is an new setup for me. I'm trying to avoid buying a lot of stuff that doesn't work. I feel like a parts store. So much stuff laying around that I'm not using or didn't work out.

The consensus (Adams, Lund, etc) seems to be: start at the default condition and start adding as needed.

Gumble Gumble. Yes Mother.

MHearn

[tk4]

With the mid and the rifle length gas, I get that we want some time past the gas port before the bullet exits the barrel. Makes sense. Give the gas time to get back to the BCG. It makes sense that if the gas got there too soon while the system is still under a lot of pressure and started pulling on the BCG that things could get rough. That makes sense. How does that work with the 14.5" barrel? The gas is going to be pulling on the BCG for a while. Yet the post indicates that this setup (gas port at 7.8, barrel 14.5) is an ideal "dwell". What am I missing?

Mark

You are not really missing anything. Although I do not think it was explained very well the importance of the distance from the chamber to the gas port, as it relates to the peak pressure in the chamber at extraction. This is the secret to the whole balancing act. It boiles down to this, the closer to the gas port the chamber is the higher the pressure in the chamber will be (at the moment of extraction), so, it is neccessary to slow down the bolt carriers rearward movement to allow pressure to drop. This is typically done by using more mass (heavier buffer). Depending on the size of the gas port in the bbl you are using, and the ammo, will dictate what weight buffer you need to have it run reliably. Sometimes it takes a little trial and error. If you look at the pressures in the chart, its clear that their is a huge differance in peak pressures between the differant gas systems, and all have less to do with dwell time than chamber to gas port distance. Hope this is helpful.

So, does this mean that mid length gas should be "more ideal" for a 14.5" barrel as opposed to carbine length, as stated above?

[mpeltier]

With the mid and the rifle length gas, I get that we want some time past the gas port before the bullet exits the barrel. Makes sense. Give the gas time to get back to the BCG. It makes sense that if the gas got there too soon while the system is still under a lot of pressure and started pulling on the BCG that things could get rough. That makes sense. How does that work with the 14.5" barrel? The gas is going to be pulling on the BCG for a while. Yet the post indicates that this setup (gas port at 7.8, barrel 14.5) is an ideal "dwell". What am I missing?

Mark

You are not really missing anything. Although I do not think it was explained very well the importance of the distance from the chamber to the gas port, as it relates to the peak pressure in the chamber at extraction. This is the secret to the whole balancing act. It boiles down to this, the closer to the gas port the chamber is the higher the pressure in the chamber will be (at the moment of extraction), so, it is neccessary to slow down the bolt carriers rearward movement to allow pressure to drop. This is typically done by using more mass (heavier buffer). Depending on the size of the gas port in the bbl you are using, and the ammo, will dictate what weight buffer you need to have it run reliably. Sometimes it takes a little trial and error. If you look at the pressures in the chart, its clear that their is a huge differance in peak pressures between the differant gas systems, and all have less to do with dwell time than chamber to gas port distance. Hope this is helpful.

So, does this mean that mid length gas should be "more ideal" for a 14.5" barrel as opposed to carbine length, as stated above?

Most likely.

[HRider]

I have wanted to run the House stage at Tapp's match with it ever since I built it.

Hurley

Hurley

My plan exactly. Thanks. Hope to see you there.

MHearn

I'm in his match. Supposed to have a crew from Mobile comming with me. See you there.

Hurley