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Posted: 11/28/2008 5:15:41 PM
Hello all!
This site is amazing. I rarely need to post with so many questions already answered. God bless the search function! I am a little confused on the dwell time vs. max pressure. I am considering a 16" CMMG carbine with a rifle-length system. CMMG M10 Full Length MedCon Rifle (scroll down) After reading this topic, I am now unsure if that is a wise choice. The charts Randall was so kind to produce show: 16" barrel with rifle gas = ~18k PSI and .078 dwell time 14.5" barrel with carbine gas = ~25k PSI and .206 dwell time Can someone explain which would be worse and why? Shorter dwell time vs higher pressure? I thought I had it figured out until I started to compare gas systems and barrel lengths. If .2 is the optimum dwell time, does that negate the higher pressures the carbine would be submitted to? I know that there are other options, but these were my two main selections. Carbine and rifle gas are more commonly available, and I am set on a 16" or less barrel. Longevity and reliability are the key concerns for the rifle. Am I suffering from Acute Anal Black Rifle Disease? 
Thanks for any help! S/F, Lee |
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Posted: 1/9/2009 11:33:18 PM
Would a cotter pin instead of a role pin holding the gas tube in the block allow enough movement in the tube to cause short stroke?
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Posted: 2/3/2009 12:44:08 PM
below is what my gun does how do i fix it? sometimes it will pick up the next case but it won't seat just right, and stops the carrier and you can see the case all crooked in the reciever. i haven't found a definative answer or how to adjust the timing? if someone could point me in the right direction that would be great.
#3 The extractor does NOT slip off the case, but keeps pulling. During this pulling, the bullet has JUST left the bore, pressures recede and the case shrinks down, allowing extraction. The rest of the cycle goes as normal, but you have strong pull marks on the case. Recoil will be higher than normal when the carrier is allowed to travel to the end of the buffer tube and bottom out swiftly against the end of the buffer tube. In normal operation, the buffer just kisses the end of the tube. |
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Posted: 11/2/2009 7:50:13 AM
[Last Edit: 11/2/2009 7:53:57 AM by KSODA]
Originally Posted By caneau:
Daniel, I am somewhat surprised by your post. First, Randall is not a moderator. Second, he gave one of the best write-ups about AR-15 functioning I have ever seen on this board. Hell, it's probably one of the five most useful things ever written on this board, much more valuble than slogging through page after page of EOTech vs. Aimpoint, Colt vs. Bushmaster, or 1/9 vs. 1/7 posts. It deserves a pin-up at the top of this forum. If you want to be a Mod, I advise you put that at the top of your agenda. Second, I have never seen Randall express any favoratism for one brand or another. Let me repeat that again. Randall has NEVER expressed favoratism to one brand or another, even his own products. If you want to be a mod, step up to the plate, help out on this forum, and do you part. I have no objections to you as mod in the absence of Tweak, but don't go after one of the most helpful people around. Otherwise, in a manner of speaking, LAY OFF. I don't know much about the politics or really why some concider a post like Randall's favortism of one brand or another, but if I may add an opinion as guy who just loves to shoot black rifles and loves to read all these posts; posts like the one iin question gave me...the novis a new understanding of the gas operation of the AR15. I wrote a post last night because yesterday my Bushmaster M4A3 stopped ejecting, but I have the Gas Tube Retrofit installed in my M4 which diverts the gasses away from the reciever/bolt assy, I found the gas ttube roll pin missing but was unsure whether the retrofit kit does away with that pin or not, so I wanted to understand more of the exact operation. I searched tthe message boards for a half hour to an hour before I came across Randall's post. While Ihave to re-read the post again, I don't recall reading about the Gas tube retrofit operation, but regardless, Randall's post was quite enlightening and answered many otherquestions I've had about the gas operation. Bushmaster offers a great anatomy of their M4, but not on the retrofit. http://www.bushmaster.com/anatomy_bushmaster.asp I guess what I'm trying to say is that as a bystander, I didn't see favortism in his post, but only great information which helped me understand a lot more than I did yesterday. I hope this helps. But I'd like to see that same post include the gas piston retrofit operation as it operates much differently by sending gasses through the front of the tube under the hand guard. |
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Posted: 2/21/2010 12:42:39 PM
Very nice write up. I hope this thread stays up which helped me understand the mechanics and physics involved with the AR system. Well done
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Posted: 2/16/2011 6:31:44 PM
a FEW THOUGHTS FROM A NEWBIE-
1. if the carrier is the newer colt open bottom the no longer present FA carrier "step" is relieved such that as the trigger/hammer sear is worn down even a hair the hammer can catch the fp not at rear but rather between fp rear and midpin enlarged ring resulting in a bent firing pin. Happened to me with a titanium pin and rapid semi auto fire in a newer colt 16" civvy model. This was explained to me by a guru whom I cannot name without permission. 2. Herters made a primer pocket diameter measuring tool for reloaders that works quite well to measure gas port diameter. As usual material is more easily removed than put back on. 3. To adjust these variables I started with a low profile stainless steel gas block. Drilled gas tube hole thru to front. Tapped block sides so smaller set screws can hold gas tube in place without penetrating. Finally tapped front hole to accept standard set screw. Then drilled set screws with varying size holes in center allowing gas porting out front via set screw hole. Basically a "bleeder system" a set of half dozen set screws now can serve as a regulation system. Started with wide open, no cycling- single shot and then gradually tightened ports until reliability assured with minimum of slamming bcg with excess pressure. This process seemed to work, at least for one guy with one set of parts, 11.5" barrel, fat stainless, .750" ID, heavy dual chamber muzzle break, old slickside chromed bcg, and a homebrew billet cut upper of 330 stainless steel. gradually enlarging a bleeder port seems easier than drilling/enlarging holes in barrel. A firm believer in the DI system where all moving metal is on same axis, has served us well. |
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Posted: 6/3/2011 10:30:31 PM
tried 5 different rifles. Tula ammo. All rifles FTE after 3-4 mags. Great technical stuff here, sometimes cheap cases do not work as well as brass. been around a long time, shoot every week.
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Posted: 10/23/2011 12:33:05 PM
Wow, I gotta say I learned soooooooo much from this post
Thank You Understanding the mechanics and how they are supposed to work really can help one when dealing with a problem Thanks again Marc |
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Posted: 1/12/2012 10:46:19 PM
this should be required reading
http://www.ar15.com/forums/topic.html?b=7&f=109&t=492845 |
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Posted: 2/18/2013 4:10:18 AM
Just found this very old post and read it with great interest. Not to stir up the bee's nest but something here does not seem right. I quote in blue/green the part that I have a problem with. -- #3 The extractor does NOT slip off the case, but keeps pulling. During this pulling, the bullet has JUST left the bore, pressures recede and the case shrinks down, allowing extraction. The rest of the cycle goes as normal, but you have strong pull marks on the case.
The part in green implies that the bolt extractor has begun pulling on the case rim while the bullet is travelling down the barrel. The problem I have with this is that the carrier does NOT begin any rearward movement at all until the bullet has already left the barrel. Since the carrier has not started rearward movement then how could the bolt/extractor start pulling while the bullet is still in the barrel? What am I missing here? |
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Dano523
Courses avec des cisaillements.
Moderator
Joined: Mar 2001  USA
 Posts: 16527 Feedback: 100% (4) Link To This Post |
Posted: 3/12/2013 10:16:52 AM
Think of it this way,
As soon as the bullet passes the barrel gas port, the gas action is pressurized, the carrier starts to be pressurized back, with the bolt starting to rotate in the barrel extension via the carrier cam. The spent case is still bound to the chamber wall due to bore pressure, so until the bolt can rotate around (being cam'd around as the carrier comes back) to allow the bolt lugs to unlock from the barrel extension lugs, extractor is just slipping around on the spent case rim. Now by the time the bolt does unlock from the barrel extension and the bolt can come back/ extractor pulling the spent cases out of the chamber, the residual pressure in the barrel bore that is pressure binding the spent case to the chamber walls has dropped to a level that the spent case can be pulled cleanly. So the extractor is always over the rim of the case, but until the bolt unlocks from the barrel extension, the extractor is not pulling on the spent case yet. Furthermore, since barrel residual pressure has to drop down enough that the extractor can pull the pressure bound spent case from the chamber, a bolt/rifle that is unlocking too fast, will short stroke as well. Not enough gas to the system, and the B/C does not have the correct energy to get the B/C all the way back. Too much gas,and although the B/C has energy, the spent case is still too pressure bound to be pulled from the chamber, that energy is lost trying to pull the too pressure bound case. Simply, the key thing about this topic is to see that although the working pressure of most rounds are in the 55K range, there is a drop in pressure after the peak dwell of the burn, which means that the gas action does not see 55K of working pressure. To take is one step farther since I refer back to this topic regarding gun powder burn rate, the slower the powder burns to achieve the working pressure of 55K, the higher of pressure that both the gas action will receiver, and the higher of the residual pressure of the spent case pressure bound to the chamber wall at the pull. |
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