Bolt theory question........

[Mgordon]

Would you agree or disagree that the bolt in an AR is a floating type much like a Savage in relationship to the bolt face???

Off the top of my head it seems like the gas rings center the bolt but it mainly "floats" in the BCG and the cartridge head should square it up correct???

Mark

 

[doc76251]

Re: Bolt theory question........

Disagree:

Due to the cam pin bearing on the carrier at lock up as well as the gas rings you mentioned. The Savage just hangs on the pin.

.02 for the pot

Cheers,

Doc

 

[Mgordon]

Re: Bolt theory question........

So it would be at the mercy of the cam pin orientation in the BCG?

Meaning that how ever the cam pin is resting against the BCG and through the bolt that would move the bolt accordingly?

Also, do you think the lugs in the barrel extension play a larger effect?

 

[Andrew Blubaugh]

Re: Bolt theory question........

There is another point of contact on the bolt shaft just forward of the cam hole.

 

[Blaster4]

Re: Bolt theory question........

doesn't the bolt carrier key engage the gas tube as well...?

 

[SScott]

Re: Bolt theory question........

The gas tube is engaged first, but it has some movement.
I have about 100 rounds down my DPMS LR, and I noticed the shiny spot rubbed on the bottom of the gas tube, it was because it was bent slightly down too much. With a wooden dowel and a hammer I centered it up. I don't think it has enough strength to move things out of alignment. It is what gets moved into alignment.
SScott

 

[Hellbender]

Re: Bolt theory question........

When chambered, the ejector places a sideways, forward load on the head of the cartridge and presses the cartridge forward to wherever the shoulder hits (the opposite side from the ejector).

So the tightness of the chamber and the headspace dimension will affect the orientation of the head in relation to the bolt face when chambered. (Unless you are jamming the lands with the bullet)

When the cartridge fires, everything will expand pretty equally, the primer moves rearward first and makes first contact with the bolt face, then the case comes back, and as soon as the ejector is compressed, the head of the shell pushes rearward on the bolt face, and things will move rearward until bolt lugs hit.

When the cartridge fires, all that matters, (as far as bolt head contact and alignment)is the locking lugs on the bolt, and the locking lugs on the bbl extension. Lap them together properly for good contact and all will be well.

Many have only 3 lugs touching when new.

 

[Mgordon]

Re: Bolt theory question........

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hellbender</div><div class="ubbcode-body">When chambered, the ejector places a sideways, forward load on the head of the cartridge and presses the cartridge forward to wherever the shoulder hits (the opposite side from the ejector).

So the tightness of the chamber and the headspace dimension will affect the orientation of the head in relation to the bolt face when chambered. (Unless you are jamming the lands with the bullet)

When the cartridge fires, everything will expand pretty equally, the primer moves rearward first and makes first contact with the bolt face, then the case comes back, and as soon as the ejector is compressed, the head of the shell pushes rearward on the bolt face, and things will move rearward until bolt lugs hit.

When the cartridge fires, all that matters, (as far as bolt head contact and alignment)is the locking lugs on the bolt, and the locking lugs on the bbl extension. Lap them together properly for good contact and all will be well.

Many have only 3 lugs touching when new. </div></div>

Interesting,

Can you possibly share any pictures or explain how you did it?

Mark

 

[LimaBravo]

Re: Bolt theory question........

Maybe I misread the post above but the case doesn't move rearward after detonation. There is expansion but it's small and sideways.The case is flush against the boltface from the time it seats in the chamber. The ejector is compressed at that time providing a straight forward pressure,the extractor is grasping the rim of the case providing the pull outwards and the lugs lock up after the BCG takes the bullet into the chamber.

The first movement is when gas forces the bolt carrier to the rear.The lugs unlock as the bolt rotates caused by the cam pin following the groove until it can't go any further forward.Cam pin actually stays relatively stationary (following the groove) as the BCG travels to the rear.The case is pulled out of the chamber as the BCG travels rearward and the extractor and ejector provide constant pressure.The only thing that keeps the expended shell casing in the upper receiver is the chamber and then the upper itself.When the casing matches up to the ejection port it's thrown clear.Of course this happens so fast that the casing flies out and strikes the deflector on it's way to that neat pile of brass on the ground.

The bolt itself doesn't float at all.The relationship between bolt and bolt carrier is a zero slip fit.The gas rings compress when the bolt is placed in the carrier.

Check your rings by inserting the bolt into the carrier.Grasp the lugs and check to see if it will support the weight of the carrier.It should because there is no room in there with the rings properly compressed.This is done without the cam pin, firing pin or firing pin retaining pin installed.If the bolt carrier falls off (into the palm of your other hand hopefully)replace your gas rings.

The gas system relies on timing for everything to work properly.If the case moved rearward to make contact then compress the ejector it would completely foul the timing.There is no allowance for this movement in any of the AR15 literature I've seen.

 

[Hellbender]

Re: Bolt theory question........

LimaBravo...

The rearward case movement I am discussing is very small, only the cartridge headspace distance, maybe .003-.008" (Depending on many factors, I bump my shoulders .003-.005" on fired AR cases). It isn't anything you could see or feel. The same thing I am discussing happens in a bolt gun also, if it has a spring loaded ejector in the face of the bolt. I am not discussing the actual action of the gun cycling. Sorry for the confusion.

Mark....

FIRST, this is not for a regular combat carbine, this will only make a difference on a precision rifle with good ammo, a gun you want everything "perfect" to get that last .100" better group consistency......

I'll explain how to lap the lugs now, and I'll have one (DPMS LR) I'll be doing in a week or two that I'll take some pics and post.

Order some silicon carbide (NOT Alum. Oxide, it embeds) lapping compound from Brownells, 400 grit and 600 grit will do.

Remove your ejector and extractor from the bolt, reassemble BCG (without ejector and extractor)

Degrease lugs on bolt and bbl extension (I use brake cleaner).

Reassemble the rest of the gun, with recoil spring and buffer, you will need it all together so you can work the BCG in and out with the charging handle.

Take a cartridge (fired from the gun, if possible) and split it in half (sideways, not lengthwise).

Find a spring that fits inside the cartridge case and cut it to fit, leave the spring a little long and cut a little at a time until the BCG will just close on the split case with you dropping the BCG about 2" or so.

Use a black Sharpie or Dycom blue and paint the rear of the bolt lugs.

Close the BCG once carefully and check your contact on the lugs, most are not good, unless they have been shot a LOT.

Put some 400 grit on the back of all the lugs, and start slightly opening and closing the BCG, do it about 30 times, clean everything and check contact again with your Sharpie.

Repeat until you get approx. 75% contact and then switch to 600 grit until you hit about 85-90%. Don't try to get to 100% as you could increase headspace too much if you go too far.

I have also used a good nylon coated cleaning rod down the muzzle to put the rearward pressure on the fired case. Put it against the wall and push while working the BCG.

Reassemble everything and you are done!

 

[Greg Langelius *]

Re: Bolt theory question........

When I had my AR, my Eldest Brother was impressed with the way the bolt locks up. He felt that the main conceptual weakness of conventional bolt designs was the way the two bolt engagement points only stabilized the bolt face primarily on a single plane. He felt that three (or more) lugs with relatively equal radial disperson resolved this issue and had a high opinon of Stoner's apprach to this with the AR design. His opinion was that other points of contact between the carrier and receiver would have only a negligible effect on bolt face stability by comparison. I see some value to his logic.

Greg

 

[Hellbender]

Re: Bolt theory question........

Greg, I agree with your brother, but on many AR's it's only 3 lugs (out of 7) on one side of the bolt!! It's a testament to Stoner's genius that they work so well, and are as accurate as they are. I have owned many, and they would all shoot under an MOA (with good ammo), even an old 1-12" twist SP1 I bought new in 1981 for $249.

Usually 4-600 rounds through one will give at least 4-5 lugs hitting 60% or so.

The Weatherby MK 5's with the 9 lug system usually have only 3-4 lugs touching when you check them. Lot's of backup, though!

 

[LimaBravo]

Re: Bolt theory question........

Hellbender,
I got it.I reread my post and didn't mean it to sound so strong.The confusion was all on my part.