View Full Version : BCG Weight & Dwell Time
Apologies if this topic has been covered before. I did my due diligence in searching but couldn’t find exactly what I’m looking for. Please point me in the right direction if I missed something.
Could someone school me on the relationship between bcg weight and dwell time? In theory, shouldn’t a lighter weight bcg require less dwell time?
For a specific example, let’s take a 15.5-16” barrel on rifle gas. Your dwell time is a bit lacking here. From my understanding, such a setup would require the port be opened up (.1ish?) to even begin to work unless you added additional backpressure to the equation.
Wouldn’t lightening the bcg alleviate this to an extent? Using say a 7.8oz bcg, could you technically reduce your port size to, say, .093ish? Let me know what you think. I’m sure I’m missing something. Thanks all in advance.
Reducing BCG weight is never a good idea. It's really the buffer weight that can be adjusted. I've seen a 16" rifle gas set up that ran with the standard BCG and rifle Buffer with an opened up port. I forget the exact size. But getting the gas right is always better than messing with bolt/buffer mass. You want a certain amount of reciprocating weight for reliable feeding.
titsonritz
02-09-22, 14:07
In the words of Jim Sullivan, "I know I've heard of people lightening the weight of the bolt carrier, that's crazy, you know."
6:20...
https://www.youtube.com/watch?v=gOUKXIrDE0I
Dwell time
Spring force
Bcg/buffer weight
Port size
are the variables
Reducing BCG weight is never a good idea. It's really the buffer weight that can be adjusted. I've seen a 16" rifle gas set up that ran with the standard BCG and rifle Buffer with an opened up port. I forget the exact size. But getting the gas right is always better than messing with bolt/buffer mass. You want a certain amount of reciprocating weight for reliable feeding.
Not without adjusting the gas from what I’ve seen. Most combinations usually come with some sort of adjustable gas block or key. I’m willing to bet that port size was around .110 with a standard bcg. My question though is would you theoretically be able to cycle using a lighter carrier with little to no adjustment to the port size? Say like X PSI required to cycle Y mass, but if you reduced Y mass, should it also require less X PSI? Reliable feeding is a whole new challenge I’d imagine.
In the words of Jim Sullivan, "I know I've heard of people lightening the weight of the bolt carrier, that's crazy, you know."
6:20...
https://www.youtube.com/watch?v=gOUKXIrDE0I
Crazy builds require crazy solutions. Those improvements were interesting to hear about.
Dwell time
Spring force
Bcg/buffer weight
Port size
are the variables
That’s about where I’m at currently. Full rifle dissipators eliminate all with the exception of Bcg/buffer weight and port size. If the port size were to remain somewhat consistent with rifle gas (.093ish?), your only option is then to adjust the carrier and buffer combo I’d imagine?
In the words of Jim Sullivan, "I know I've heard of people lightening the weight of the bolt carrier, that's crazy, you know."
6:20...
https://www.youtube.com/watch?v=gOUKXIrDE0I
Does anyone know what changes Mr. Sullivan made besides increasing the weight of the BCG?
Cagemonkey
02-09-22, 20:39
Does anyone know what changes Mr. Sullivan made besides increasing the weight of the BCG?
Check this out; https://www.m4carbine.net/showthread.php?161612-Jim-Sullivan-Armwest-LLC-M-4-Improvement-Program https://www.surefire.com/products/suppressors-division/suppressor-accessories/sf-obc-556-optimized-bolt-carrier-group/ https://defensereview.com/surefire-optimized-bolt-carrier-long-stroke-obc-ls-drop-in-bcg-and-h7s-buffer-system-for-better-weapon
https://defensereview.com/surefire-optimized-bolt-carrier-long-stroke-obc-ls-drop-in-bcg-and-h7s-buffer-system-for-better-weapon
Surefire already has their foot in the door with gov contracts. Wonder if DOD tested these and what the results were?
JP Rifles failures they blame on reloads.
https://www.jprifles.com/bulletin/propost_december_2015_01_01.html
https://www.jprifles.com/bulletin/propost_january_2016_02_01.html
JP Rifles failures they blame on reloads.
https://www.jprifles.com/bulletin/propost_december_2015_01_01.html
https://www.jprifles.com/bulletin/propost_january_2016_02_01.html
While perhaps not universally, doesn’t JP make stuff for the gamer crowd where light and fast is the objective?
When I shot competitively, reloading was a necessity, so you could load only hot enough to barely make or exceed the minimum power factor.
Why would anyone gaming be shooting hot loads?
I may be making the wrong assumptions here, but hot loads don’t make any sense to me in this context.
The pictures at the link you posted almost look like an out of battery detonation to me, but I’m certainly not an expert.
Dwell time
If, by dwell time, you mean the time the bullet is in the barrel from gosport to muzzle, no. What counts about the distance from port to muzzle is blow down time, the time it takes the gas to vent the bore and pressure drops. This is part of the gas drive. Ammo, gas port diameter and gas port location are the main components of the gas drive.
Spring force
Spring rate is part of it. Springs store energy from the BCG as it retracts and releases it to operate the action. The spring is the motor, so to speak.
Bcg/buffer weight
The reciprocating mass has to be just right- not too much, not too little. Too much and the motion of the reciprocating mass will push the AR around more than needed. Too little and the AR won't be reliable. (If the mass is far too much, the AR will malfunction.)
Port size
As noted above, the gas port is an important part of the gas drive.
the variables
I see the variables as gas drive (and gas drive starts with ammo), reciprocating mass and spring rate. All three have to be in balance with each other. Change reciprocating mass too much and the AR will be out of balance with the other two sides of the triangle.
If, by dwell time, you mean the time the bullet is in the barrel from gosport to muzzle, no. What counts about the distance from port to muzzle is blow down time, the time it takes the gas to vent the bore and pressure drops. This is part of the gas drive. Ammo, gas port diameter and gas port location are the main components of the gas drive.
Spring rate is part of it. Springs store energy from the BCG as it retracts and releases it to operate the action. The spring is the motor, so to speak.
The reciprocating mass has to be just right- not too much, not too little. Too much and the motion of the reciprocating mass will push the AR around more than needed. Too little and the AR won't be reliable. (If the mass is far too much, the AR will malfunction.)
As noted above, the gas port is an important part of the gas drive.
I see the variables as gas drive (and gas drive starts with ammo), reciprocating mass and spring rate. All three have to be in balance with each other. Change reciprocating mass too much and the AR will be out of balance with the other two sides of the triangle.
Appreciate the info. Given those variables, how would they fit within a rifle featuring a 16” barrel, rifle gas, rifle spring & buffer, and a lightened carrier? Assuming constants of quality ammo and a fixed port location, that would leave just your port size and reciprocating mass to adjust, right? Instead of adjusting the port size fully, would it be also possible to slightly lighten your bcg/buffer combo to achieve a similar effect? Reliable feeding may become an issue, but I also wonder if the increased momentum of a lightened carrier could solve this. Let me know if I missed something and thanks for the help so far
titsonritz
02-11-22, 00:46
https://www.pewpewtactical.com/what-is-an-ar-15-dissipator/
I think the solution might be to either increase the size of the gas port, or to add a non front venting suppressor. Reducing the weight of the BCG, or other such shenanigans, just turns it into a gamer gun.
Appreciate the info. Given those variables, how would they fit within a rifle featuring a 16” barrel, rifle gas, rifle spring & buffer, and a lightened carrier? Assuming constants of quality ammo and a fixed port location, that would leave just your port size and reciprocating mass to adjust, right? Instead of adjusting the port size fully, would it be also possible to slightly lighten your bcg/buffer combo to achieve a similar effect? Reliable feeding may become an issue, but I also wonder if the increased momentum of a lightened carrier could solve this. Let me know if I missed something and thanks for the help so far
What you have to understand is, you can adjust the gas drive, but the reciprocating mass (let's call it RM) and spring rate must match it. If you lighten the RM (in this case, the carrier) it means the RM will react faster to the gas pressurizing the expansion chamber resulting in higher carrier speeds. Higher carrier speeds means the RM hits the end of the receiver extension harder giving sharper feeling recoil.
The gas drive can be adjusted by ammo, gas port diameter & location. But it starts with the ammo. Gas port and location must be matched to the ammo. For example, the 300 BLK uses less powder than the 5.56 and thus generates less gas. Less gas in a larger bore means pressure drop is more rapid as the bullet travels down the barrel. Combine a quicker drop in pressure with less gas to begin with and the gas port has to be located closer to the chamber. That's why 300 BLK barrels use the pistol gas length system. If the port were located further away the AR, well, runs out of gas.
Gas drive, reciprocating mass and spring rate are like three legs of a stool. Shorten or lengthen one leg and the stool starts to wobble and when you get enough wobble, the stool becomes unstable. The other two legs will also have to be adjusted to stop the wobbling.
The constant with the 5.56 AR is that the reciprocating mass that gives the best span of operation using in spec ammo, is a standard carrier combined with buffers ranging from H to rifle and the correct springs. If you want to lighten the carrier, you have to use ammo generating less gas and adjust gas port diameter accordingly.
If, by dwell time, you mean the time the bullet is in the barrel from gosport to muzzle, no. What counts about the distance from port to muzzle is blow down time, the time it takes the gas to vent the bore and pressure drops. This is part of the gas drive. Ammo, gas port diameter and gas port location are the main components of the gas drive.
Spring rate is part of it. Springs store energy from the BCG as it retracts and releases it to operate the action. The spring is the motor, so to speak.
The reciprocating mass has to be just right- not too much, not too little. Too much and the motion of the reciprocating mass will push the AR around more than needed. Too little and the AR won't be reliable. (If the mass is far too much, the AR will malfunction.)
As noted above, the gas port is an important part of the gas drive.
I see the variables as gas drive (and gas drive starts with ammo), reciprocating mass and spring rate. All three have to be in balance with each other. Change reciprocating mass too much and the AR will be out of balance with the other two sides of the triangle.
I agree with that. I should have said distance between gas port and muzzle.
turnburglar
02-11-22, 14:24
When I shot competitively, reloading was a necessity, so you could load only hot enough to barely make or exceed the minimum power factor.
Why would anyone gaming be shooting hot loads?
As a response to your statement. Right now I have 3 gamer loads.
9mm is a 124gr at 1070 FPS. This load is just over Minor Power Factor.
556 is a 62gr at 2925 and a 77gr at 2700. None of these carbine loads are powderpuff and all of them are at max or slightly above book max and would be real 556 mil pressure.
Reason I dont make a "minor" load for 556 is that it isn't worth the trade off. All of my guns have proper gas ports so they are not over gassed. The recoil difference between a soft load and a full 556 load isn't enough to make any kind of reliability or wind/elevation trade offs. With a handgun I totally get it. With a rifle it just doesn't work out the same. Plus to be honest I use matches in replacement of 'training' so I like my carbine to work the way it will always work. My 62gr load is really close to 855.
556 is a 62gr at 2925 and a 77gr at 2700. None of these carbine loads are powderpuff and all of them are at max or slightly above book max and would be real 556 mil pressure.
Reason I dont make a "minor" load for 556 is that it isn't worth the trade off. All of my guns have proper gas ports so they are not over gassed. The recoil difference between a soft load and a full 556 load isn't enough to make any kind of reliability or wind/elevation trade offs. With a handgun I totally get it. With a rifle it just doesn't work out the same. Plus to be honest I use matches in replacement of 'training' so I like my carbine to work the way it will always work. My 62gr load is really close to 855.
That makes a lot of sense. Thanks for responding.
Makes the JP story more plausible than I initially thought.
What you have to understand is, you can adjust the gas drive, but the reciprocating mass (let's call it RM) and spring rate must match it. If you lighten the RM (in this case, the carrier) it means the RM will react faster to the gas pressurizing the expansion chamber resulting in higher carrier speeds. Higher carrier speeds means the RM hits the end of the receiver extension harder giving sharper feeling recoil.
The gas drive can be adjusted by ammo, gas port diameter & location. But it starts with the ammo. Gas port and location must be matched to the ammo. For example, the 300 BLK uses less powder than the 5.56 and thus generates less gas. Less gas in a larger bore means pressure drop is more rapid as the bullet travels down the barrel. Combine a quicker drop in pressure with less gas to begin with and the gas port has to be located closer to the chamber. That's why 300 BLK barrels use the pistol gas length system. If the port were located further away the AR, well, runs out of gas.
Gas drive, reciprocating mass and spring rate are like three legs of a stool. Shorten or lengthen one leg and the stool starts to wobble and when you get enough wobble, the stool becomes unstable. The other two legs will also have to be adjusted to stop the wobbling.
The constant with the 5.56 AR is that the reciprocating mass that gives the best span of operation using in spec ammo, is a standard carrier combined with buffers ranging from H to rifle and the correct springs. If you want to lighten the carrier, you have to use ammo generating less gas and adjust gas port diameter accordingly.
Thanks for the reply. The RM speed makes sense. I didn’t even consider the sharper recoil side of things. Now, if a lighter RM reacts faster to gas pressurization, wouldn’t it make sense to adjust for that through port size and possibly gas system length?
It seems kind of similar to the 300blk concept for gas drive, but more inefficient. With port location on a 16” barrel and a rifle gas system, you’re loosing that pressurization rapidly. Technically, the port location should be moved closer to the chamber (say, midlength). However, the workaround for this setup is majorly opening up the port. If you had a lighter RM that was more reactive to gas pressurization, would it then make sense to either leave the port size as is or open it up to a lesser degree than what is required for a standard RM? Stool starts wobbling here I imagine.
Adjusting spring rate is one leg of the stool that I struggle with. I’ve always been of the mindset RM should be adjusted first, but that may be AR9’s talking. I’d imagine you’d want a longer spring travel with a heavier buffer for this sort of build, so something similar to either a rifle spring & rifle/A5h2 buffer. Let me know if that’s not right though.
Yeah, that’s the kicker here. Kind of a “if it’s not broke, don’t fix it” situation. With all the adjustments you’d need to make in lightening RM, why even bother? Might as well go with what works, though the whole concept is still interesting. Appreciate your help. Learned something new and gave me a better look at gas pressurization over just calling the whole concept “dwell time.”
Dust devil
02-18-22, 22:55
Gas drive, reciprocating mass and spring rate are like three legs of a stool. Shorten or lengthen one leg and the stool starts to wobble and when you get enough wobble, the stool becomes unstable. The other two legs will also have to be adjusted to stop the wobbling.
3 legged stools never wobble.
Any way, the gas in an AR has to push the bolt forward against the case head to disengage the lugs, push the carrier back and rotate the bolt in the cam pin track. The point is the gas has to do a lot more than just push the mass of the carrier back. Why do you want to try to make everything more difficult for your 16" AR with rifle gas?
3 legged stools never wobble.
Any way, the gas in an AR has to push the bolt forward against the case head to disengage the lugs, push the carrier back and rotate the bolt in the cam pin track. The point is the gas has to do a lot more than just push the mass of the carrier back. Why do you want to try to make everything more difficult for your 16" AR with rifle gas?
The reason is/was primarily for fun. 16” on rifle gas is already crazy, so why not throw in a titanium carrier to spice things up a bit? The base premise for this setup was that if you had to open your port way up to account for X mass of a standard carrier, could you theoretically get away with a smaller port size by using a lighter mass carrier. Short answer is yes, but it’s dependent on feeding and adjusting your gas/cartridge. I.e. a whole headache. I still think it would be think it would possible to figure out your carrier momentum baseline for reliable feeding and working from there. Would be a bit of work and some trial and error. However, adding a can to the mix changes all of that to an extent.
3 legged stools never wobble.
Any way, the gas in an AR has to push the bolt forward against the case head to disengage the lugs…
Both statements are incorrect
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