Dwell Time: does a longer dwell time have ANY effect on velocity? If I HAD to make a guess-Id guess it would increase it, if only slighty. Am I right or wrong?
Just something Ive been thinking on.
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Dwell Time: does a longer dwell time have ANY effect on velocity? If I HAD to make a guess-Id guess it would increase it, if only slighty. Am I right or wrong?
Just something Ive been thinking on.
As I understand it, dwell time is directly linked to the amount of barrel between the gas port and the muzzle and that gas system pressure returns to atmospheric quickly thereafter (with a slight amount of blow down time) once the bullet has uncorked the barrel. This dimension, along with bore diameter and gas port size, is what determines the dwell time of the system.
You can get real precise and start to calculate the volume of the gas manifold/block and gas tube, but you get the idea.
I would imagine that, if there were any effect on muzzle velocity, it would be difficult to measure above the statistical noise of shot to shot variation between different barrels and would be more likely linked to large differences in gas port dimension... but I emphasize that this would be nearly impossible to measure with accuracy IMHO. But, in my observation of various clips of high speed video, the bullet has left the barrel before any effect could be placed on it by the gas system (and before the movement of any components has begun). At least in terms of a properly functioning DI AR.
So, for barrels of equal lengths, there will be no difference.
Very good answer sir, I thank you!!
Only at the extreme would a difference be noticeable.
Calculation along with trial and error such as KAC spending the time/resources to find a "self-regulating gas sytem" as oppossed to an Armalite "switch block" gas regulator/block for suppressor/no suppressor use.
OR the "boring out" of 10.5" SBRs/Colts.
Straight Shooter, don't get "wrapped around the axle" unless you are super-fine tuning your AR system/combo.
Dwell time equates to the reaction time the action/bolt/BCG of a weapon has time to properly go through a weapon's cycle of action and repeat reliably.
Eugene Stoner seems to have come up with an AWESOME solution and anyone in charge of those using it keeps trying to come up with a different solution although based on user/logistic requirements.
You want short? Go Short.
You want long? Go Long.
You want average? Go Middie.
And if want velocity, approach from that angle.
I could imagine if you had a 16" with a pistol gas port location and a 16" with a rifle gas port location that you would possibly find a miniscule speed increase with the rifle gas simply because the gas is allowed to push nearly the full length of the barrel before any is bled off through the gas port. But I agree with what others have already posted, it probably would not be a measurable amount, if for no other reason than standard deviations in velocity of even very high quality match ammo.
I seriously doubt you could measure any difference in MV based on gas system length, and I would not be surprised if there IS no difference.
Simple test? Shoot several rounds through a chrono with your AR. Then shoot several more of the SAME EXACT LOAD through your chrono with a bolt-action .223 with the SAME TWIST AND BARREL LENGTH as your AR. Note any difference in average muzzle velocity between groups, taking into account the standard deviation for the load. I would bet there is no statistically significant difference, or that any difference would be overshadowed by other factors, such as variances in seating depth, machining tolerances, actual barrel twist, etc.
Has anyone actually done this type of test? The results would be interesting.
That's an idea. Except with an adjustable gas block fitted barrel. You wouldn't want to flood the gas tube with gas... this would bleed too much off of the barrel to make a good differentiation.
My block is Gen 1 Syrac that wont' shut down all the way, and it's mounted on an 11.5 anyway. I'd guess that an 11.5 would be too short to make a huge difference.
I have no experience with adjustable gas blocks. Can they be COMPLETELY closed down at the barrel gas port? If so, you shouldn't need the wood dowel- in fact, I'd be leery of using it at all since the bolt and carrier stresses due to gas pressure would be enormous (I would think.)
Yes. The current two big fish in adjustable blocks (Syrac and SLR) can be shut completely off.
The problem is that two randomly chosen barrels of the same length may produce velocities 100-200fps apart with the same ammunition, due to differences in chamber dimensions, exact bore diameter, variations in bore diameter as the bullet travels down the barrel, rifling type, metal alloy and hardness, surface smoothness, etc. You can get significant variations (though probably not 100fps) even in two identically spec'd barrels from the same maker. The best way to test the effect of the gas system would be to chrono test loads with the gas system functioning, then weld the gas port shut to test it with no gas leakage at all. Considering an AR15 barrel can be had for $200 or less, maybe someone would be willing to do this?
That wouldn't work because the bullet is out of the barrel before the gas has a chance to get the carrier moving.
All else being equal, a vented (gas ported) barrel would produce lower velocities because a part of the energy is lost when the gas escapes. A vent that's closer to the muzzle would lose less energy than a vent of the same diameter that's close to the chamber. The higher pressure and longer "dwell time" of the bullet means the gas has more time to vent at a higher rate of flow before the bullet uncorks the muzzle. The amount of velocity normally lost is probably small enough that it would probably get lost in the noise of all the variables, but it can be calculated
Another quick way to test. Use a standard LMT bolt carrier group, shoot 20 over chrono. Replace with Enhanced LMT BCG which increases dwell time for more reliable extraction in certain setups. Chrono 20 more. Quick way to test if you have both already. Although turning off gas system would be the best!
Again, that won't work because the bullet will have exited the muzzle before the gas can reach the expansion chamber
Your would need a barrel with a gas port and block at carbine length and a gas port and gas block at rifle length. Shut the port off on rifle block and check MV, then shut off the carbine and open Rifle and measure MV would be the only way to truly test it. On the same barrel because chamber sizes, bulges, etc contribute, so you would have to test on the exact same barrel like the test where they kept cutting off a half in or in at a time to measure differences on MV
Yes.
My personal theory, with absolutely no data to support it, is that significant velocity is lost from the gas system, and more velocity is lost with shorter length gas systems because the barrel is bleeding gas for more of the bullet's time in the barrel. Reading lots of chrono data over the years (all internet, so it must be true!) it has appeared to me that a given commercial load usually chrono's fastest in bolt actions, next up in piston type semiautos, and last in AR15 style rifles. If you think about it, the bolt actions are not bleeding any gas*, the pistons are bleeding some but it's usually limited by the piston chamber volume and any relief ports or loose clearances (like in an AK) in there, and the AR setup basically has constant bleed once the bullet passes the port, limited only by the port diameter since the gas tube is fairly large in internal volume.
*the problem with this, even assuming that internet chrono data is accurate and relevant, is that all these different rifles have different barrels in terms of chamber dimensions, bore dimensions, rifling, material, bore uniformity, etc. If you take a tight bolt gun barrel, chrono it, then ream the chamber to a 5.56 NATO dimension and chrono it again with the exact same ammo (match grade from the same batch or precise handloads), I expect you will see a measurable loss of velocity.
Edit: thought occurred to me that someone could take a .223 bolt action, chrono test a couple loads as a reference point, then ream the chamber to 5.56 NATO spec, chrono those same loads again, then drill a hole in the barrel comparable to a gas port hole (leaving gas to vent to the atmosphere as practically equivalent to a gas tube) and chrono it again. Anyone with time and interest to do this?