Re: Differences shooting gas versus bolt guns?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jhuskey</div><div class="ubbcode-body">Just to throw in a side note that something has been documented happening that some of the above posters say "not".
Randall Rausch wrote one of the best details about AR operation over on AR-15.com you can find anywhere. Some of you need to read it.
The bolt CAN start opening before the bullet leaves the muzzle. This would be a rifle whose "timing" is off, ie a malfunction of operation. This is more common with over pressure loads.</div></div>
In a properly functionning AR (10 or 15) the bolt will not open before the bullet exists the bore and the pressure drops to zero. If it does open when there is over 50,000PSI of hot gases in the bore, this is generally known as a kabboom. If this occured for every shot fired, the aluminum receivers would be torn apart in short order, but I doubt the extractor would last more than a few cycles.
Permit me to introduce the concept of obturation. On firing, the bullet jumps into the rifling and fills the grooves thus and prevents the rapidly expanding hot gases from slipping by until the bullet exits the barrel and the hot, pressurized gases hit the comparatively cool, unpressurized atmosphere and create a muzzle blast. The shorter the barrel, the hotter and more pressurized the gases are and the larger the muzzle blast.
At the other end of the barrel, the cartridge, which is made of a material that expands, has swelled to a) release the bullet and b) obturate the chamber thus accomplishing two desirable things: 1- Containing all the hot pressurized gases so they are pushing the bullet out the bore. 2- Saving the bits behind the cartridge from feeling directly the effects of escaping hot pressurized gases. This includes the receiver and the shooter.
This also means that the cartridge is firmly pressed against the walls of the chamber at pressures exceeding 50,000PSI; it is hugging the walls and that is one of the reasons it is not going backwards to do damage to the bits behind it. For those of us who handload we do know to clean up sizing lube so as to not have this lube be present at ignition time and lower the coefficient of friction between the case and the wall and thereby dramatically increase the pressure on the bolt, (which may cause it to open before the bullet has exited the bore and thus causing a kabboom.)
Once the pressure drops due to the bullet exiting the barrel and the hot pressurized gases spending themselves into the atmosphere, the cartridge contracts a little and it becomes possible for the bolt to turn it and then pull it out, something that it could not do reliably milliseconds prior. The AR extractor is not the most powerful version of such a device and if the case does not come out easily, the extractor will either rip or more likely jump the rim and leave the case where it is.
As the case is ejected it carries with it a great deal of heat that thankfully does not get transfered to the chamber. (It can get transfered to your skin on occasion.) This is why AR cases are HOT and bolt rifle cases are cool. Guess where the heat goes in the latter rifle.
During the ejection cycle, the bolt is pushed to the back and the majority of the gases that were diverted from the barrel through the gas hole and then came back down the gas tube to send the bolt carrier on its rearward journey, these gases escape through the ejection port, the path of least resistance. In a bigger AR rifle with cartridges containing twice as much powder as the regular AR rifle, that will be quite a bit of gases and residue coming out that port.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I have spent the last few hours fruitlessly searching for the TECOM study that stated that for the M16 family to work best, there needed to be 4" between the gas port and muzzle. </div></div>
I think that's probably because it's not there. The original AR-15 had the gas hole located 12.001 inches on a 20 inch tube, leaving a smidgen under 8 inches from the hole to the bigger hole at the muzzle.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Barrels with longer distances between the muzzle and gas port are known to be a son-of-a-bitch to get properly timed when you add a suppressor and may cause early bolt opening because of increased gas pressure back to the bolt.</div></div>
By the time the bullet reaches the gas hole it is already going at a large fraction of its muzzle velocity. If we take an AR-15 with a 20 inch barrel and an MV of 3300FPS, at 12 inches the bullet is already going 2400 FPS and accelerating. The next 8 inches will be covered in less than 25 milliseconds. If you add a suppressor, that introduces different pressure curves because of the design of the baffles. BTW, I understand that suppressors get quite hot due to muzzle blast. I should think the receiver would get that hot also, if the bolt kept opening while there was 50,000PSI's worth of hot gasses in the bore. (This might be a way to keep the suppresor cooler.)
The original premise here was that there were three recoil events as the bolt opened, moved to the end of the buffer tube and then bounced forward collecting the next round and chambering it, all this occuring before the bullet left the barrel. These three recoils conspired to disturb the rifle and thereby cause the bullet impact to move differently on the last shot of a magazine. If this much activity was taking place in my rifle (which BTW has a 26 inch tube and a regular gas tube and I can reload my brass multiple times,) I would never be able to hit the X-ring at any distance. I doubt I would even be reliably on paper. We are talking about minutes of angle for accuracy and rifles in full recoil will move in multiple degrees.
Just for fun, the next time you are at the range, leave the bolt open and do not insert a magazine. Just take aim at the target holding the rifle as you normally would and then release the bolt and see how much the reticle will move around. I must be a very lucky shot because for me, it will move a lot, all over the black and maybe into the white.
I maintain that the bullet has left the barrel and is well on its way to the target before the bolt opens. Proper hold and follow through are critical because the human body has quirks and twitches which can impair accuracy. As I said before, about the only difference between this type of gasgun and a bolt rifle is the very long lock time. People tend to press the trigger and get ready for recoil or whatever comes next. If you focus more on aiming, trigger control, and then letting the rifle recoil before looking up, the results will be better. I absolutely love the picture that Lowlight posted. It does show the shooter to be totally focused on shooting. The bullet has probably hit the target already and he has not moved at all; his eye is looking through the scope still and that would be the first thing to move after the shot has been taken.
