A/R HEAD SCRATCHER

[searcher]

I have always used various mainsprings as a method of attaining proper cycling of light-bullet loads and for extra heavy bullet loads. Recently, I noticed that after changing mainsprings to better tune cycling on a particular rifle, I noticed the accuracy improved. I had always been under the impression the bullet was out of the tube before the bolt unlocked. If that were true, it would support my belief(?) that the mainspring has no influence on accuracy. So I changed the spring back and accuracy returned to where it was previously, more or less. So, my question to those more knowledgeable than I, do you believe the bullet will have exited the tube before the bolt unlocks? My springs say no. What do you think?

 

[Rocketvapor]

I have an AR with a wooden A2 stock shimmed with quarters for a carbine spring and buffer.
This sounds like an experiment once the weather warms up

 

[10XSHOOTER]

Play with your buffer weight!

 

[searcher]

I know that as soon as the bullet passes the gas port in the barrel, the unlocking of the bolt commences - just wondering if the bullet has left the bore before the bolt unlocks. I'm assuming it does, otherwise it would result in some brass expansion issues. I'd guess a heavier mainspring will slow a bolt from opening as quick as a lighter one (?), though that would also have seemingly no impact on accuracy if the bullet has left the bore.

Play with your buffer weight!

I have used different buffer weights on occasion to control bolt thrust when using certain powders like CFE223. I'm not much of a fan of the variable gas blocks.

 

[mgunderson]

I don't think we can talk in absolutes with the ar platform. The broad range of calibers, carrier masses, and length of gas systems says it is likely influential at times. The biggest danger is assuming it is always important or never important. Expect to find experiences from others on both sides.

My ar experience doesn't include enough guns, platforms, or calibers to draw firm conclusions.

 

[VA_XTC_Shooter]

I know that as soon as the bullet passes the gas port in the barrel, the unlocking of the bolt commences - just wondering if the bullet has left the bore before the bolt unlocks. I'm assuming it does, otherwise it would result in some brass expansion issues. I'd guess a heavier mainspring will slow a bolt from opening as quick as a lighter one (?), though that would also have seemingly no impact on accuracy if the bullet has left the bore.

I have used different buffer weights on occasion to control bolt thrust when using certain powders like CFE223. I'm not much of a fan of the variable gas blocks.

Unlocking and carrier movement are two different things. The carrier has to move rearward a decent bit to for the cam pin to engage the ramp in the carrier that facilitates the unlocking.

 

[Coyotefurharvester]

I have a bull barrel 17 rem AR I can turn the gas block off completely. It shoots slightly better groups as a single shot. Could be from no bolt movement, could be from better SD with gas off(a guess since my chrony rarely picks up a 17 cal 25 gr bullet at 3900+). Or maybe I just simply practice better shooting technique running single feed. And than one would have to test magazines when the bolt is running on gas. None of this I care about at all. So I'm not going to test anything that shortens barrel life, to many prairie dogs to use up bullets on.

 

[Doom]

Based on some observed issues I think we can make some conclusions. It's a known fact that the AR can be overpassed and the extraction process becomes a mess which seems to imply that the bullet is still in the chamber as the bolt continues rearward. Results in torn cases, jumping of extractors, etc. If this is the case then its likely that in a properly gassed system the bolt dwell should allow the bullet to exit the barrel before the bolt unlocks but the bolt carrier is likely moving before the bullet exits. That would seem to be the design of the BCG.

As @mgunderson noted, the platform now has several gas systems and barrel lengths and I would think that it's likely that it's possible to find systems that are mismatched. I have a feeling that the answer to this question lies in the hands of the Army Marksmanship Unit.

.

 

[Bill K]

I have the same results as Coyotefurharvester, with one of my AR's. Closed off the gas block and it shoots tighter groups. I feel it is due to less movement of the AR this way, without cycling the BCG, etc. More like a standard bolt rifle.
The movement of the BCG in cycle mode just causes more overall movement of the rifle during firing , so maybe your testing with lighter buffer, etc is along the same lines, just less movement no matter if the bullet has left the bore or not.
Just my thought.

 

[SKWERLZ]

I’ve heard the same with shutting off gas on a M14.

 

[Walt Krafft]

You should be changing the gas coming from the barrel with an adjustable gas block instead of changing the spring the way you are. A stronger spring is trying to overpower the gas system to keep the bolt carrier from moving back. An over gassed system can cause all kinds of issues, including the accuracy issue you are seeing.

 

[dellet]

There’s a lot going on when gas hits the carrier in the M16/AR15.

Before the gas starts it’s way down the port, the case head has slammed the bolt into the locking lugs with somewhere around 55-60,000 psi and is holding it there with what ever the chamber/bore pressure is, all the way to the muzzle.

Once the gas gets to the chamber in the carrier, there is a fight between the carrier and the bolt over who is going to move first and farthest. The bolt is supported by not only the pressure in the barrel, but also the brass. For the bolt to move forward, it needs to overcome that gas pressure, but also crush the brass after overcoming the friction of the case wall against the chamber.

The carrier pushed by the same pressure as the bolt, needs to not only overcome the pressures on the bolt face to twist the case in the chamber to move back and begin the cycling sequence, where the carrier also needs to overcome the buffer weight and spring tension.

The actual cycling of the carrier is accomplished by a very short burst of air pressure and the inertia of that blast. As soon as the carrier travels enough for the tube to come out of the key, all pressure on the carrier stops.

The bolt is held in place by the expanded brass gripping the chamber walls created by the pressure in the bore until the bullet exits the muzzle, allowing the pressure to drop. Then the carrier can jerk the spent case out.

Bolt speed and the time it takes to move far enough to unlock the bolt, vs how long it takes for the bullet to exit the muzzle is what tuning is all about. The biggest factor is gas pressure and volume. The most important control of that speed is how fast the gas is applied. This is controlled by the size of the port, distance to the carrier, and pressure. Keep in mind the gas on the way to the port is still expanding, so powder burn rate can be a factor.

Bottom line. There is a lot going on with pieces parts banging around and moving back and forth while the bullet is traveling down the barrel. Kind of like some one hitting the action on a bolt action 3-4 time with a hammer between the time you pull the trigger and the bullet exits the muzzle.

The cycling of an AR is pretty violent.

One thing else to consider. Shooting sub-sonic can change your thinking on tuning. Adding a suppressor creates more variables. Most people will agree that adding a suppressor will give a little boost in velocity. But consider this concerning an AR. The boost in velocity comes from pressure inside the suppressor after the bullet leaves the muzzle. It’s not that it adds to the velocity, it just lessens the velocity drop after leaving the muzzle. Without the suppressor, the bullet leaves the muzzle and pressure behind the muzzle drops to near zero almost immediately. With the suppressor there is a tail wind longer. The bullet decelerates less In the first foot of travel to the chronograph.

What happens when the action opens before the bullet leaves the Suppressor?

You lose the “boost” of the suppressor. All the back pressure creating the tail wind, blows out the ejection port. Simply changing spring weight, or anything else that will delay the bolt opening the chamber will affect velocity. It just doesn’t fall outside of acceptable variations until you slow things down.

 

[jds holler]

Dellet said --
There’s a lot going on when gas hits the carrier in the M16/AR15.

Dellet, that is a great explanation, and I'm gonna have to read it a few times -- but dang, you make it sound so violent that I kinda hate to put my rifle through it. jd

 

[6BRA]

Makes you wonder how full autos achieve high cyclic rates......
Once the bullet passes the gas port...the bolt begins it cycle back.
Even a binary trigger will spray empty cases....Im not sure how tight AR chambers are supposed to be for this very reason.

 

[searcher]

I have the same results as Coyotefurharvester, with one of my AR's. Closed off the gas block and it shoots tighter groups. I feel it is due to less movement of the AR this way, without cycling the BCG, etc. More like a standard bolt rifle.
The movement of the BCG in cycle mode just causes more overall movement of the rifle during firing , so maybe your testing with lighter buffer, etc is along the same lines, just less movement no matter if the bullet has left the bore or not.
Just my thought.

Yes - having better accuracy with the bolt locked up would support what I'm seeing, that the bolt is unlocking before the bullet exits.

 

[243winxb]

If unlocking to soon, will the head to datum be longer then the chamber?
The M1 Garand has been said to have a longer head to datum, after firing, compared to the chamber?

Hornady has said, action can open to soon using suppressors, with a certain powder?

Then there is gas port location & hole size.

 

[Knotwild]

There is a lot to think about and learn here. I have 4 AR's - two work and are plenty accurate enough for me, one isn't accurate enough to suit me (poor barrel choice when I built it), and one just stopped cycling (I haven't torn it down to look at it yet). I've certainly got some thinking to do I guess.

 

[markm87]

I never change the action spring or buffer weights on an AR from the standard Colt SS spring and rifle or H2 carbine buffer. I correct the gas setting with a Black River Tactical gas tube IF needed. I'm running everything from 55 gr to 85 gr with no adjustment in gas or buffer spring for bullet weight changes.

My only reasonable theory for a change in accuracy would be how the rifle is chambering rounds. Perhaps the lighter spring is less abusive to the rounds when feeding. OR... depending on your neck tension, perhaps the lighter spring is keeping the bullets set in the cases more consistently.

 

[dellet]

Some numbers to consider.

Near maximum cycling rate of an M16 is 900 rounds per minute, or 15 per second. Bolt carrier travel is 8” per cycle. That’s 120” or 10 feet per second. Slow down the cycling rate, slows down the velocity.
Some sources using high speed video put the velocity closer to 13 fps for initial speed before spring rate starts slowing down the rate of speed. I used 15 fps for the numbers Below.

Bolt carrier travels about .125” straight back before the cam pin starts to rotate the bolt.
.125” at 15 fps. takes .694 milliseconds.
A bullet that passes the gas port at 2200 fps. will travel 18” in .684 milliseconds. If it does not gain any more velocity and exit sooner.


If those numbers are reasonably correct (carrier speed), then a pistol gas system on a 24” barrel, might start to rotate the bolt before the bullet left the muzzle. The carrier and bolt will certainly be moving around in the receiver while bullet travels down the bore, but the amount of carrier travel required to actually unlock the bolt is well after the bullet leaves the muzzle.

Since no one measures one shot groups, it’s hard to tell how much the movement of the bolt carrier group, buffer and spring influence the difference in point of aim vs point of impact. But all the banging and clanging going on after the bullet leaves the muzzle certainly makes it more difficult to return the rifle to the exact point of aim than something that shoots a little smoother. That will influence group size.

 

[RegionRat]

FWIW
I took some snaps of Larry Vickers shooting a cutaway rig in slow mo.

You can slow the playback on YT to 0.25X and see for yourself, the bullet exits, then the bolt starts to move, then it vents, then it ejects.

The whole rig is recoiling before the bullet exits, which explains most of your accuracy issues with this rig and all other ones too. Guns start to recoil before a bullet exits. There is however a gas pulse going back down the tube on an AR that starts as soon as soon as the bullet gets close, then really spikes as the base goes past. We can see the difference in DI versus external piston rigs.

Study at 1:38