I have a question for you machinists.

Is there a way to physcically see if a barrel was turned to fast or too much metal removed with each pass?

The reason I ask is that I have a few barrels that shoot well for the first few shots, then go to heck from there on, until the barrels get cold again.

If so, will crygenic treating fix (or lessen) the problem?

How thin are these?  I can't imagine a barrel losing dramatic accuracy after only a couple rounds.

Quoted: I have a question for you machinists. Is there a way to physcically see if a barrel was turned to fast or too much metal removed with each pass? The reason I ask is that I have a few barrels that shoot well for the first few shots, then go to heck from there on, until the barrels get cold again. If so, will crygenic treating fix (or lessen) the problem?

I am not a professional machinist, nor do I have a lot of knowledge about barrels, but I have used a lathe a bit.  The big ways to tell that you are removing too much material or removing material too fast are by the surface finish of the part.  Bad machining practices often result in extra heat delivered to the part instead of to the tool, and that can affect the heat treatment on a piece of stock (not sure how big of a deal this is with gun barrels).

What guns are these on, who made the barrels, etc?  Bad bedding jobs or very tight fitting stocks can cause horrible accuracy after a few shots.

I am a machinest (CNC lathe programmer, setup and operator).
Turning the stock too fast or cutting too deep will show up more in tool breakage or burning that in the apprearance of the barrel.
Rifle barrels are so thick that the above two operations will destroy the tool before it harms the barrel.  Carbide tools are sensative to shock (interrupted cuts) which will chip them out, turning too fast will burn them and you will loose the edge or cutting too deep will also break the tool.  A carbide tool should be good for hundreds of barrels or, with the steel and stainless I turn, good for thousands of operations.
Your problem sounds like poor quality barrels (wrong grade metal for the use), improper heat treat of finished barrels or bullet problems.  You might have the wrong twist to your rifling for the weight bullet loaded and this would cause an unstable bullet.
Tell us about your rifling and bullets.

Bullets are 69 and 77 grain Sierra's: home grown, and in WOA barrel are sub-MOA.  Even when using Sierra 52 grain bullets, groups are still 2-3 MOA, then open up to 4-5 MOA once the barrel gets hot.

One barrel is a lightweight profile 1:9 chrome moly AR barrel; the other barrel is a factory blue Mini-14 barrel.

Sounds like I have crappy barrels.

From now on, I am only using Armalite, BM, or WOA (for match stuff) barrels.

I have been a tool and die maker / tool designer and gunsmith for 24 yrs and have turned alot  of barrels. I have always used light  cuts with slow feed rates to turn barrels. I dont think i ever had a barrel go ''bad'' after re-profiling using this method. If a barrel is turned using heavy cuts and rapid feed rates it will  have alot of internal stresses put into it, and if not properly stress relieved it can show up on paper as the barrel heats up. A couple of thousanths of bow in the barrel can mean a couple of inches down range.It's  possible your barrel has this condition.  cant say for sure without further investigation. If you have ruled out any other problems with the rifle  you might want to try cryo treatment before you replace the barrel.    let me know if i can help you any further.   PATRIOT ARMORY 07 FFL

Quoted: I have been a tool and die maker / tool designer and gunsmith for 24 yrs and have turned alot  of barrels. I have always used light  cuts with slow feed rates to turn barrels. I dont think i ever had a barrel go ''bad'' after re-profiling using this method. If a barrel is turned using heavy cuts and rapid feed rates it will  have alot of internal stresses put into it, and if not properly stress relieved it can show up on paper as the barrel heats up. A couple of thousanths of bow in the barrel can mean a couple of inches down range.It's  possible your barrel has this condition.  cant say for sure without further investigation. If you have ruled out any other problems with the rifle  you might want to try cryo treatment before you replace the barrel.    let me know if i can help you any further.   PATRIOT ARMORY 07 FFL

Good write up.  Machining introduces stresses.   Aggressive machining can introduce excessive stresses.  These should be relieved in some fashion prior to use.  Stresses in barrels can greatly increase the natural barrel movement due to heat up and cool down as they can tend to "bend" more at a stress point.  Not 100% sure why, but it does happen.

Well I'm not sure if I should get into this but I'll give it a go anyways.  

Machining operations do not introduce stress but do introduce strain.  There is two types of strain, elastic and plastic.  The vast majority of strain being introduced is of the elastic type which means when the metal is compressed from the pressure of the cutting tool is springs back to it's original shape that is elastic stress.  Now if the pressure is enough and you will get plastic strain and that is where the metal doesn't bounce back to it's original shape.  Stress is actually the resisting force to strain, it's an internal force which in the case of elastic strain pushes the metal back to it's original shape, but if you have introduced plastic strain then you part will have compressive stress (there is also tensile stress).

Interestingly enough the intentional introduction of elastic strain is a form of stress relieving called shot peening.

So to keep your parts stress free use sharp cutters, strong setups and don't ever let your part chatter and take reasonable cuts and if anything you will relieve stress not introduce it.  But if you get nuts on your chip load, have a dull tool, hello stress!

Not to get into a pissing contest...but  in your first sentence you said'' machining does not introduce stress but does introduce strain''. then you go on to say'' to keep your parts stress free use sharp cutters etc.''  '' But if you get nuts on your chip load, have a dull tool, hello stress! ''Did i miss something? whatever you want to call it , stress, strain  it all has the same effect on the barrel.   PATRIOT ARMORY 07 FFL

Quoted: Not to get into a pissing contest...but  in your first sentence you said'' machining does not introduce stress but does introduce strain''. then you go on to say'' to keep your parts stress free use sharp cutters etc.''  '' But if you get nuts on your chip load, have a dull tool, hello stress! ''Did i miss something? whatever you want to call it , stress, strain  it all has the same effect on the barrel.   PATRIOT ARMORY 07 FFL

You missed the two types of strain, elastic and plastic.  If you get too agressive when you are cutting the part you start to introduce plastic strain and the result will be compressive stress, but if you are only introducing elastic strain there is no resulting stress.  It's just a more indepth look at what stress actually is and how not all machining operations result in stress.

Stiles,

Excellent explanation. I was just reading a machinists book about the types of stress and strain that influence metals, or anything else for that matter.  Very interesting from a metallurgical standpoint.

Reading the text answered so many questions I had about metal.  Much of what I already knew became crystal clear as the reasons for the behavior of solids was reveled.

Not everyone has an interest in the “why”.

The GI in the field doesn’t need to know “why” the bullet goes down range.

I find the information fascinating and I think the understanding adds to me abilities as a gunsmith.

Good show.