A/R HEAD SCRATCHER

[searcher]

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.

This particular rifle shoots extremely accurately and I'm not having accuracy issues. I just noticed a slight change in accuracy when changing the springs to adjust my ejection and recoil pulse. and it got me to thinking about the relationship of bolt lockup and bullet exit. My question was whether the bullet leaves the bore before the bolt unlocks. Another noted that the bolt carrier is moving before it engages the cam pin - and that makes the most sense to me. There is bolt carrier inertia going on even if the bolt is still tightly locked up and whether the bullet is still in the bore - or not - and that is definitely going to affect things by changing springs (or gas adjustment as you might do). Rather than use gas blocks, I prefer to tune with the weights and springs. Much more reliable and easier to adjust in the field in my experience. If someone NEEDS a gas block - it's usually because they have a bad gas port or are running a short gas system.

 

[searcher]

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.

As the bolt carrier is in rearward motion BEFORE the bolt unlocks, it makes a lot of sense that a change in the mainspring pulse will influence accuracy - whether the bolt is open, locked or whether the bullet has left the muzzle.

A weaker spring will have allowed the bolt carrier to move rearward just a tad faster before the bolt unlocks - and that will be enough to explain this phenomenon. I know it was more than any lessened recoil from the weaker spring. It was something mechanical - and it was. Don't know why I didn't think of that!

 

[searcher]

Thanks for all your replies, folks!
I'm certain it is the movement of the bolt carrier - which is what I should have been focusing on - not the bolt. Any spring change is going to affect the position of the bolt carrier as it relates to bullet position in the bore.

 

[VA_XTC_Shooter]

The idea behind the Tubb Carrier Weight System delayed initial carrier movement and unlocking by making carrier heavier.
I can't find it, but someone explained the difference between the initial movement being delayed by more mass vs stiffer spring, but I can't find it. If I remember correctly, more mass was preferred for some reason.
Any engineer/physicists care to explain the difference?

 

[dellet]

The idea behind the Tubb Carrier Weight System delayed initial carrier movement and unlocking by making carrier heavier.
I can't find it, but someone explained the difference between the initial movement being delayed by more mass vs stiffer spring, but I can't find it. If I remember correctly, more mass was preferred for some reason.
Any engineer/physicists care to explain the difference?

I’ll take a non engineering stab at it and say that mass is easier adjusted than spring tension.

The Tubb system included a spring, because it is a complete system. Same with the captured spring buffer systems, you have different weights and springs an almost endless set of combinations to keep straight.

Tuning a gas operating system is a never ending exercise in chasing your tail. At some point you just stop and say that’s enough. Other wise every time you made a load change, you would make a buffer system change. There’s probably some OCD types that do.

It’s all a trade off. The best shooting gas operated rifle, will be the least reliable to cycle. The lightest system, bolt, carrier, buffer, spring tension, needs the least amount of gas to operate.
What are the instructions for an adjustable gas block?
Cut off the gas until the system fails, then open it back up. Just remember that depending on conditions, you need a certain amount of force to feed the next round. It’s possible to have extraction, without having enough force to chamber.

All of this component changing is done to manipulate the force created by the amount of gas supplied to the carrier. How about just cutting it off at the source?
That requires too much work. Order a custom barrel with a pilot hole for the gas port. Customize the port to fit your load and operating system. Then just don’t change your load.

Then there’s the elephant in the room very few people talk about. So much talk about what happens after ignition. What about before?
It’s a hammer fired rifle. People gave up on hammers in precision shooting more than 100 years ago.

 

[jds holler]

In spite of all this fly poop vs pepper pickin that's going on here, I think something needs to be said. Quite awhile back when some folks started mounting decent optics with decent mounts on AR's, a lot of us were pretty amazed to see the accuracy that the system delivered. Slap on a better barrel, which might cost $200 instead of $600, and it's even better.

I just got my first AR this year, -- I think the absolute cheapest base model you can find -- Anderson Utility Rifle, under 400 bucks. Slapped one of my spare scopes on it, and factory ammo gave 2 MOA. With very little load research, my very basic reloads give 1MOA. And if I discounted the pesky and common single flier, many of those groups would be .5 MOA. I don't think that most folks with an "off the shelf" bolt gun have better luck than that.

Now we're not seeing the platform winning bench-rest competitions -- yet -- but they are still pretty amazing for what they do deliver -- I think.
jd