Questions on ARs?

[KurtM]

Not buying that. If the bolt heats up, it would be a constant no matter the carrier, and 5 rounds is hardly enough to Heat a bolt to the point it would undergo thermal expansion. If in fact heat causes ARs to change group size all ARs would exhibit the same tendency, but they don't. Most all other AR-10s do not exhibited group size increases from dropping in a lightened carrier. Matter of fact it is so rare that J.P. rifles has never had a single case of it until this rifle.
If it were thermal expansion of the bolt and something was amiss in the lock up/alignment, it would exhibit stringing no matter what carrier used. Also, bolt lock up, can effect group size but not as much as observed in this case. Lapping in a bolt to the lugs is only worth 1/2 MOA at best and this is far more than that. Also most all thermal scattering is exactly that, groups open up, and it can be drastic, however it is never straight edge repeatable.

In this case I know it has nothing to do with heat, as with the light carrier the very first shot may be right on zero, or it may be up to 5" to the right. Next shot could be anywhere along that line. It is completely random as to where the next round goes along that line, but in a straight line it goes. If indeed it was heat, it would steadily walk across the target

 

[JEVapa]

I think it's a bolt speed issue. Changing the gas doesn't always slow it down enough. It sounds kinda like when we went to heavy barrels in our BLK I and had to go to H and H2 buffers. Getting bolt bounce and mistiming in auto and breaking lugs. Groups were not super tight sometimes as well. Sometimes they'd be deadnuts, sometimes string. One of the Crane weapons guys said that depending on the load consistency, the groups would change because the bolt hauling ass creates more or less muzzle flip and erratic exits. He might have been blowing smoke but it sounded good and the reliability kits and heavy buffers fixed everything.

 

[KurtM]

I agree that it has to have something to do with it, but the part that is absolutely confounding is it isn't erratic, ie. Sometimes high, sometimes low, right, left, etc. it is almost perfectly linear. Always right of center, and in a perfect line and at any one time it can be centered, then 2" right, and then the next shot centered, and then the next 5" right, then the next shot 1" right, then centered etc. It does it suppressed, or bare barrel. It does it hot, or cold. It is a conundrum!

I have seen all kinds of strange things. Had one barrel that was easily 1 MOA. Cold! Put a 30 rounder through it and only 1 or two would hit a 20"X 30" target at 100yds. Let it cool back down and back to 1 MOA. Had one that with 6 shots it would put 3 almost touching, centered and the other 3 almost touching 2" high and 3" left, but only with 77 grain bullets, lighter bullets just scattered! For sure harmonics and in each case we could figure out the cause. This one

 

[Kingfish]

I have never tried shooting exposed lead tip ammo through an AR platform. Will it affect feed reliability or cause any tip deformation leading to a decrease in accuracy?

 

[KurtM]

If your ramps are cut down a la M4 ramps, no they work just fine. Matter of fact we used to take a Dremel and "ramp" our barrels back before they came out with that mod.
My brother used the Hornady 55 grain soft points for Prairie dogs and it would easily go sub MOA. I used to use the Sierra 1310 55 grain hollow point, which was a huge hollow for its size before ballistic tips came along with no trouble on a home ramped barrel. Truth be known most non ramped barrels fed just fine as well, but it was added reliability for the occasional nose down failure to feed.

 

[danc46]

It has to do with bolt lock up from carbon build up, heat, or barrel not necessarily centered in upper. Some clowns epoxy the barrels into the upper to insure absolute consistent lock up.
Have you used head space gauges or taken fired cases from both BCG set up and used a comparator? Compared first fired from the BCG to a later one? If you find a difference you know it’s bolt lock up!
Youre getting a more consistent bolt lock up from the one that isn’t stringing!
Gas operated guns can be funny about bolt lock up. NM Garands and M14s have their gas valves locked down if done to AMTU standards.
Bolt lock up has a great deal to do with barrel harmonics!

 

[danc46]

I have never tried shooting exposed lead tip ammo through an AR platform. Will it affect feed reliability or cause any tip deformation leading to a decrease in accuracy?

I’ve shot exposed lead tips in M14s, Garands, ARs. The first in the magazine, last in the chamber have their tips beat to hell in the mag. That will hurt accuracy. I do like Ballistic tips for that reason.
The 64 gr bonded core Noslers work great accuracy wise and shooting deer in the ARs

 

[KurtM]

Do you know who J.P. rifles is? Builders of precision ARs and Bolt guns!

M1s don't have a gas valve to lock down or otherwise.
M-14s, which do have an on/off valve for grenade launching, are NOT locked down to keep the gas valve from moving.... Quite the contrary the front feral is unitized to the gas cylinder by drilling and taping into the gas valve spindle, NOT to keep the gas spindle in one place, but to keep the feral from bouncing around the on the gas cylinder and get consistent down tension when bedded.

Very little carbon gets into the extension of an AR, and the bolt is exposed to very little heat. Easy to test, do a mag dump and pull the bolt, you will find it barely feels warm. If the barrel wasn't centered in the upper, it would exhibit this phenomenon no matter the carrier, and it would easily be seen in bolt lug wear. By the way most "clowns" that bother to fit AR barrels to uppers, thermo fit them, not epoxy them.

As to bolt lock up, I direct you to the testing P.O. Ackley did on headspace and bolt lock up and see how little it does to group size. Now it does a lot to safety, ie blowing primers, blown cases, or having a bolt shear and come back, but on one barrel he tested it got more accurate as head space increased well over max.

 

[HoLeChit]

It has to do with bolt lock up from carbon build up, heat, or barrel not necessarily centered in upper. Some clowns epoxy the barrels into the upper to insure absolute consistent lock up.

Ewwww.... poors epoxy their barrels.... Thermofit is the way to be.

 

[KurtM]

Blue Loc-Tite was the choice of AR builders before we figured out Thermo fitting. John and my brother and I were actually talking about putting bearings on a Lotus Esprit axel when my brother said, wouldn't it be cool if we could do the same heat and cool process to ARs! Next week John came up with the how with an oversized extension, and I came back with, why not just wrap shim stock around stock extensions, that way you don't need a custom extension! This was around 2007.

 

[danc46]

Do you know who J.P. rifles is? Builders of precision ARs and Bolt guns!

M1s don't have a gas valve to lock down or otherwise.
M-14s, which do have an on/off valve for grenade launching, are NOT locked down to keep the gas valve from moving.... Quite the contrary the front feral is unitized to the gas cylinder by drilling and taping into the gas valve spindle, NOT to keep the gas spindle in one place, but to keep the feral from bouncing around the on the gas cylinder and get consistent down tension when bedded.

Very little carbon gets into the extension of an AR, and the bolt is exposed to very little heat. Easy to test, do a mag dump and pull the bolt, you will find it barely feels warm. If the barrel wasn't centered in the upper, it would exhibit this phenomenon no matter the carrier, and it would easily be seen in bolt lug wear. By the way most "clowns" that bother to fit AR barrels to uppers, thermo fit them, not epoxy them.

As to bolt lock up, I direct you to the testing P.O. Ackley did on headspace and bolt lock up and see how little it does to group size. Now it does a lot to safety, ie blowing primers, blown cases, or having a bolt shear and come back, but on one barrel he tested it got more accurate as head space increased well over max.

You haven’t shot High Piower with DCM (CMP) rifles obviously.. The Garands more often than not have the gas caps replaced with an adjustable gas valve. The M14s have the gas valves locked in the position the armorer found shot best. You can damn sure tell the difference in the sound of the bolt closure on one that has the valve turned down just better functiong. The unitized gas valve/cylinder/barrel band are never removed, just the cap and piston to use a hand turned drill bit to remove the carbon in the cylinder.
Resectfully I don’t give a rats ass who JP rifles are. I look to USMC and AMTU standards on the building of a NM rifle.
AS far as head space not affecting accuracy, bullshit. You shoot the throat out of a barrel you can adjust your sizing on reloads to restore accuracy.
NM barrels on the Garands and M14s are chambered with pull through reamer, the pressure applied by the bolt and recoil spring.
Get the head space gauges and or do the comparator thing.

 

[danc46]

Ewwww.... poors epoxy their barrels.... Thermofit is the way to be.

I’ve worked on two rifles by a well known and highly promoted rifle and pistol builder who used epoxy.

 

[HoLeChit]

I’ve worked on two rifles by a well known and highly promoted rifle and pistol builder who used epoxy.

JP doesn't use epoxy. As far as I know.

Who was the builder and what were the rifles for?

 

[KurtM]

Your depth of misunderstanding unitizing is impressive! Adjustable gas caps for M1s are not allowed in High Power, although they do exist to allow hotter to be ran to not bend the op rod. It has NOTHING to do with "tuning" the gas system.If you read the rules for High Power you will see they are not allowed!

I am rated a high expert in High Power, and I shot on several of the teams back in the day when we used M-14s. I spent lots of time with our armorers, and you are absolutely wrong about why the M-14 is unitized the way it is. The gas spindle on an M-14 is either on or off and there is no "positioning" them to "tune" the gas system.

Next, throat erosion has nothing to do with "sizing". Free lesson for you, case headspace is measured off the shoulder of the case to the base of the case and has absolutely nothing to do with the throat of the chamber for bottle neck cases! Your understanding of these things is at best vague!

J.P does not, nor has ever used epoxy on extensions. Thermo fit, absolutely, but no epoxy! Which makes me wonder where a builder would use epoxy on a pistol barrel

 

[conditionzero]

Is JP the only company selling thermo fit / undersized uppers?

 

[KurtM]

J. P. uppers are not undersized, but they do thermo fit all ARs except the J.P. 15 line. They have a nice explanation of thermo fit on their web site. Most manufacturers don't do it because of time and cost but most custom shops that built high end ARs do a thermo fit process. Now with that said, it doesn't really do a whole lot more than tightening the hell out of the barrel nut, but we have noticed that groups are a bit better, but what it DOES do is keep from getting a flier here and there from really banging the gun around like on a bipod or smacking you free float into a prop nice and hard. It is very worth while, and can easily be done at home. I saw about a 3/4 MOA improvement in my 6.8, but that is anecdotal at best. It also did a good job on my 20" AR-10 helping to get it around 1/2MOA.

 

[conditionzero]

J. P. uppers are not undersized, but they do thermo fit all ARs except the J.P. 15 line. They have a nice explanation of thermo fit on their web site. Most manufacturers don't do it because of time and cost but most custom shops that built high end ARs do a thermo fit process. Now with that said, it doesn't really do a whole lot more than tightening the hell out of the barrel nut, but we have noticed that groups are a bit better, but what it DOES do is keep from getting a flier here and there from really banging the gun around like on a bipod or smacking you free float into a prop nice and hard. It is very worth while, and can easily be done at home. I saw about a 3/4 MOA improvement in my 6.8, but that is anecdotal at best. It also did a good job on my 20" AR-10 helping to get it around 1/2MOA.

Hmmm this demonstration says that they are slightly undersized for this process.

 

[KurtM]

I stand corrected! They didn't used to be, but I guess John wanted to do that anyway, as he had talked about it at one point in time. Guess I should keep up on these things

Ahhh... after watching it, I now know WHY they went under sized!!! We used to wrap the extension with a piece of . 002 Stainless shim stock. Now he doesn't need shim stock. The old process was wrap extension, ( not overlapping and cut a v for the barrel pin) put the barrel extension in dry ice and alcohol, once nice and cold, heat the upper and slide it together, making sure to bottom the shoulder. Then let cool..... Like doing the axel we talked about. John always said he wanted to cut out a step or two, and I guess they finally did with a "tight upper".... I learned me something new!

 

[conditionzero]

I stand corrected! They didn't used to be, but I guess John wanted to do that anyway, as he had talked about it at one point in time. Guess I should keep up on these things

I might have to get one of these bad boys.

"The LTC-19 was deliberately manufactured with a precise undersized barrel extension bore, requiring use of our proprietary Thermo-Fit barrel installation process. Installed this way, the barrel attains a virtually unbreakable heat meld to the receiver, vastly increasing the accuracy and precision of the rifle."

 

[ClintC]

I was going to get the JP upper for my 6ARC but decided for a skeleton upper from F1. I wanted the thermo fit upper but liked the looks the F1 upper better.