How exactly is a rainbow made? How exactly does a sun set? How exactly does a posi-trac rear-end on a Plymouth work?

[McNamara0851]

How exactly does the first round flyer on a cold fouled bore happen, and more importantly what causes it in an AR-10? After a brief search I recognized the Hide’s need for another cold bore thread. This particular example is an extreme case in my personal experience. I have done tests on multiple bolt guns and have never found anything this exaggerated. The rifle is an 8 twist 22 creedmoor chambered by Craddock using a 24” Bartlein barrel. JP VMOS BCG and HP bolt, JP H2 SCS, M5E1 receivers. During load work up I noticed a trend though out different powders and projectiles. The first round on a cold barrel would be ~.6 mil left, .1mil low, and most peculiar to me, it would be up to 100 FPS slower than average. The rifle functions flawlessly. It has over 300 rounds on the barrel. I recently scrubbed the bore clean in an attempt to cure this, with no change. Mag loaded, or set into the chamber and dropping the bolt makes no discernible difference. I have multiple loads with 3 powders and 3 projectiles that are sub 8 FPS SD and sub .5 moa. Brass is SRP alpha fired 2ce in this rifle. The rifle will be used solely for shooting coyotes in the dark, which makes this issue a problem. This is my first large frame gas gun. Is there a known issue in these autoloading witchcraft machines that I am unaware of? What tweaks can I make to remedy this problem?
Pictures for reference:
1- The Unit
2-6 consecutive groups at 100 that suggest the rifle doesn’t suck dick for coke
3-What happens at 350 yards (not exactly minute of coyote)
4-2 groups shot in succession at 100 yards 1 hour apart. Group 1 was magloaded, group two was magloaded with the first round set in the chamber with the bolt dropped on it.

Help me fix this shit heap before I sail it on gunbroker for twice what I have invested!

 

[XLR308]

Not on an AR-10 but on its little brothers I have noticed it.
I concluded and it may be in my head but have tested it numerous times is first round when you just drop the hammer on the first round with the bolt locked backed it does it.
Try using the charging handle to ride the bolt and first rou d into the chamber insted of slamming it in.
I know I probably don't need to say but will anyway to ensure the bolt is fully into battery and lugs rotated and locked into place.
By your description we may be talking about two different issues but worth a try.
If it works out I may be able to say it's not all in my head.

 

[McNamara0851]

Not on an AR-10 but on its little brothers I have noticed it.
I concluded and it may be in my head but have tested it numerous times is first round when you just drop the hammer on the first round with the bolt locked backed it does it.
Try using the charging handle to ride the bolt and first rou d into the chamber insted of slamming it in.
I know I probably don't need to say but will anyway to ensure the bolt is fully into battery and lugs rotated and locked into place.
By your description we may be talking about two different issues but worth a try.
If it works out I may be able to say it's not all in my head.

I figured the same, but I believe I have ruled that out. I’ve tried setting it in the chamber and hitting the bolt release, slamming it in, easing it in, stripping it off of an otherwise empty mag, and stripping it off a full mag with the bolt release, all similar outcomes. I am wondering if there is something in the gas system that is leaking off when it is cold, then sealing up after the first round.

 

[XLR308]

I figured the same, but I believe I have ruled that out. I’ve tried setting it in the chamber and hitting the bolt release, slamming it in, easing it in, stripping it off of an otherwise empty mag, and stripping it off a full mag with the bolt release, all similar outcomes. I am wondering if there is something in the gas system that is leaking off when it is cold, then sealing up after the first round.

Is there evidence of gas leakage after cleaning around the block and tube ?
Have you checked velocities for your first round to see if they are lower, if they are the same then something might just have stress on it with the rifle in its cold state or cough,cough cold shooter.

 

[Deleted member 10043]

Liked how posi-Trac in a Plymouth works

 

[lennyo3034]

The 100 fps slower is a helpful clue.

Is it possible that when the rifle is cycling, it slams the round into the chamber with such force that the bullet moves forward?

There is much more force when it’s cycling than when just dropping bolt because the carrier/buffer are bouncing off the tube.

How much neck tension do you have? You should have a lot.

Next time you’re at the range, fire a round and let it cycle. Then unchamber and measure the bullet seating depth to see if it moved.

 

[sam4886]

I agree that the issue is the speed difference of the first round. Finding the cause is probably going to be a pain in the dick though.

Think about what happens in the gun that could change pressures between shots 1 and the rest. We know the ammo itself is consistent, at least before something in the rifle acts on it, but I think if it were something like the bullets shifting like mentioned above, your subsequent shots wouldn't have the good numbers they do. No way the shifting would be that consistent.

What else is there? Gas/heat sealing up something? Maybe something going on with the extension/upper fitment? Are you letting the next rounds cook in a hot chamber?

Have you had someone else shoot it? What if you single load and wait 5 minutes between shots?

 

[flyfisherman246]

Try giving paul Craddock a call about it. He is super knowledgeable and helpful. If he doesn’t know, I don’t know if anyone will.

 

[bfoosh006]

How tight is all the various fitment of parts ?

Barrel snug in upper ?

Upper to lower a tight fit ?

 

[Chickentoast]

My '78 Volare had an open rear-end.

 

[Strykervet]

My '78 Volare had an open rear-end.

My '68 Dart GT had an 8-3/4 Sure Grip w/3.73's (and another set of gears with much longer legs). 8.75's can swap gears like an MRAD swaps barrels. Almost.

So on a Chrysler it's not called "posi" it's called a "Sure Grip" limited slip differential and it uses inertia to lock/unlock the rear end. Better than GM junk, best of both worlds.

Rainbows. So they form when light refracts through water droplets much like a prism.

Sun sets because of rotation of the Earth, one revolution equals roughly 24 hours --sun is stationary and does not move through space in relation to us. Tilt and distance from sun affect length of day.

Uh, shit, what was your other question?

 

[Phil McGrath]

Mopar's never use a Posi-trac... We used what was called a Sure-grip!

 

[dang472]

I hope you’ve gotten close to a solution for the problem. I should be testing my 22 Creed by next weekend. I finally got my barrel from Craddock and we’ll see how it goes. 22” Bart 8tw, hoping for good velocity with 75gr ELDms.

 

[McNamara0851]

We just got rolling into harvest when I posted this, I haven’t had a chance to look at it since. I had my local FFL assemble the upper because I don’t have the large frame tools. I saw mention of barrel torque and I’m wondering if it could be snugged down a touch more. I’ve also noticed some gas bleeding around the SA adj gas block that I’m going to address. Thanks for the replies.

 

[TheHorta]

Have you tried adjusting the dihedral vent pin? If you rotate it 1/4-turn CCW it will take up the slack and tighten your groups.

 

[4065HW]

Had cold bore flyers on bolt gun. The rest would all group. One day noticed all the burned on crud on the muzzle crown. I clean muzzle after every day shooting. 1st round flyers went away

 

[McNamara0851]

Have you tried adjusting the dihedral vent pin? If you rotate it 1/4-turn CCW it will take up the slack and tighten your groups.

Do what now?

 

[Long Range 338]

It just does!!!

 

[hlee]

Came here to learn about rainbows. Leaving disappointed.

See post 11...

 

[TheHorta]

My '68 Dart GT had an 8-3/4 Sure Grip w/3.73's (and another set of gears with much longer legs). 8.75's can swap gears like an MRAD swaps barrels. Almost.

So on a Chrysler it's not called "posi" it's called a "Sure Grip" limited slip differential and it uses inertia to lock/unlock the rear end. Better than GM junk, best of both worlds.

Rainbows. So they form when light refracts through water droplets much like a prism.

Sun sets because of rotation of the Earth, one revolution equals roughly 24 hours --sun is stationary and does not move through space in relation to us. Tilt and distance from sun affect length of day.

Uh, shit, what was your other question?

#HumbleBrag