Gunsmithing Smiths/Rifle Builders: Let's Talk about Cold Bore Deviation

Hasgun Willtravel

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I decided to put this here, rather than in the marksmanship section, because this is not a marksmanship issue. This is an issue with a repeatable, predictable, and ultimately correctable cold bore deviation on my AIAE MKIII. This is the first rifle that I have had that has this issue. Every cold bore shot is exactly 2.125" low, every time. The second shot goes to POA/POI. I can dial .6mil on the scope, take the .6 off, and hit the same POI, every time. I shoot with another member that has a .2-.3mil deviation on his AE MKI. He has lived with it, and adjusted for it, for the life of his rifle, with fine results.

My OCD is getting the best of me on this issue, so my first question is, why does this happen? Assume that this is not shooter error, because in my case, as well as my shooting partner it is not. What happens to that rifle/barrel/mating surfaces, after 1 shot is fired?..., you can walk away for lunch for an hour or two, come back and have no deviation, but let it sit overnight, and in the am, the CB deviation is back?

My second question is something is moving or settling repeatably, causing the same results every time, and I have measured it, and again, can correct for it....however I am completely stumped. I am very curious about the mechanics of this "phenomenon".

I would like a smiths' perspective on this issue, because there has to be a mechanical flaw somewhere in the system.

Note:

1. I have not had the AE out of the chassis.
2. I have not bore scoped anything
3. Different loads with different bullets 168/175 show the same effects
 

jakelly

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    Lowlight was discussing it some in a thread I can't recall. He mentioned needing to tear it all the way down, but it can be fixed. Not a bore issue either.
     

    Brutas

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    Please clarify Between Clean/Cold and fouled/cold. Clean Cold POI Shifts can be a result of over cleaning.
     

    lowlight

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    I have worked with Mile High and I have helped with customer support when it comes to the AE and the Cold Bore stuff.

    What I have found is most of the time, either an Action Screw Torque adjustment or Barrel Torque adjustment reduces that shift down to very small, in some cases I got it down to nothing.

    Generally speaking you start with the action screws, add 5 inch pounds, try it, then 10, try it. If that doesn't do it, the barrel needs to be retightened, which has helped.

    In a few cases, both need to be altered from factory.

    You don't see with with the AWs or AXs and the funny part, same barrels, the difference, is the removable chassis, and that recoil lug. So unless you just got unlucky with a problem child barrel, the chances are it's within the action.
     

    lowlight

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    No, I have the books from the armorers class, but I don't recall off the top of my head. I would have to hit the vault and find the values.

    There is probably a post on here with them.

    I basically hand everything to the Smith at Mile High (Adam) and let him do the hard part...
     

    TxShooter63

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    Guys I am fairly new to gunsmithing but have done alot of research on this and here is what I know to be fact(most of the time):
    It is common and ordinary to have "Clean Bore" shift. This typically results in lower velocities but, depending on whether you are tuned off the bottom or top
    of the barrel modulation, your slower Clean Bore shot maybe higher or lower than your fouled bore even though the velocity is as much as 100fps lower. I had a barrel where this was the case. Clean bore was right at 100fps slower and 1" or so high at 100yds.
    Cold Bore: If your rifle is fouled, you should not see a POI shift of any significance between the cold bore and the next 4 shots. If you are, this is almost always caused by excess tolerance somewhere in the rifle. I learned this when my gunsmith undercut the barrel tenon threads on one of my rifles ever so slightly. The first shot was about 1" off from the next 4 which were a .120"-.250" single hole. I wrapped the tenon threads with teflon tape so that the action screwed on snug and smooth and solved the problem 100%. Sub quarter minute groups cold to hot.

    This same thing can be caused by Frank's example of the action not mating with the chassis for any of a number of reasons. Skim bedding will often cure this and may be needed even on something like an AI depending on your action. Folks, the Remington 700 action maybe worshipped for another 100 years but some of them have horrible tolerances from the factory so keep that in mind if your throwing something like a 700AAC-SD into an AI chassis and getting a true cold/fouled bore shift.
    While it would have been much more trouble, my guess is that in Frank's case he could have left the torque the same and skim bedded and solved the problem the long way. I don't know of too many people who given those choices wouldn't have gone with increasing the torque! Bedding of any kind has the potential to be a mess. As long as the action bolts are rated for what your torquing them to, problem solved with no real downside.

    Muzzle brakes are another area where I learned another lesson. My groups opened up when the muzzle brake I was using was installed. Again the culprit was undercut threads. (Those of you who have never threaded a barrel go easy, threading is an art form and undercutting is very easy to do if your new to it.) Having a precision ground shoulder and proper thread tolerance will keep your muzzle brake from throwing the cold bore shot. My experience has been that muzzle brake issues cause a much smaller issue, say normal 1/4" groups open to 1/2" where as reciever issues tend to be much more pronounced.

    These are just my experiences and again I am new to building rifles but sometimes when you have a rifle with a problem, you become an expert on that problem by the time you solve it. I feel comfortble in saying that any true cold/fouled bore to warm bore shift is almost always due to an excess of tolerance somewhere or an incorrect tolerance created by improper torquing or untrue surfaces mating. Hope my personal challenges help someone down the road to quickly solve a problem that I spent about 150 rounds tuning my load to try and solve a tolerance problem with my tenon threads.
     
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    Hasgun Willtravel

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    Assuming the threads were undercut on the barrel, can that be touched up, or do you cut, re-thread, and re-chamber? I am a know nothing novice at machining, but have been learning about it. This, obviously, will not be something that I am doing on my own. Are undercut threads something that is obvious with the naked eye?

    Thanks for the info Paul.
     

    TxShooter63

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    You should normally not have "play" in the receiver once it is screwed most of the way on. If you have the receiver installed to where the recoil lug is with 1/4" to 1/8" of the barrel shoulder and you can still wiggle the receiver and feel slop, then you may have undercut threads. Other than my teflon tape trick, I don't know of a way to cure that. I only figured out one solution to one problem. If your having true cold bore shift, there is a tolerance issue somewhere. Hell, it could be your barrel improperly chambered and as the chamber heats and cools you could be seeing POI. My undercut tenon threads is just one possible cause. The barrel could have run out in it that changes as it heats and cools. Again, I am really new to building rifles but what I would do is start at the back and work forward. Check the receiver fit to the tenon threads, try increasing or decreasing your action torques specs slightly, put a very thin layer of grease on your chassis and bolt the receiver in and torque then remove and see if the grease is consistent width down both sides of the receiver, check the chamber with a go/no gauge, check the recoil lug for straightness. Chuck the barrel up off of the receiver and see how much "bend" it has in it. (I have a buddy with a factory .308 Remington with a barrel that you can see with the naked eye isn't straight and the damn thing shoots sub 1/2 moa so this isn't really going to tell you alot if you find out the barrel isn't straight.) Try removing the muzzle brake if you have one and see if you notice a difference. Work the problem one piece at a time until you find something that appears to be out of spec. I am sure someone with more experience than me can add some ideas to mine.
     

    Hasgun Willtravel

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    I have worked with Mile High and I have helped with customer support when it comes to the AE and the Cold Bore stuff.

    What I have found is most of the time, either an Action Screw Torque adjustment or Barrel Torque adjustment reduces that shift down to very small, in some cases I got it down to nothing.

    Generally speaking you start with the action screws, add 5 inch pounds, try it, then 10, try it. If that doesn't do it, the barrel needs to be retightened, which has helped.

    In a few cases, both need to be altered from factory.

    You don't see with with the AWs or AXs and the funny part, same barrels, the difference, is the removable chassis, and that recoil lug. So unless you just got unlucky with a problem child barrel, the chances are it's within the action.

    Frank,

    Upon taking your advice, I took the rifle out of the chassis last night, and simply torqued it down to about "umph" ft/lbs. I snuck out after work to chrono a few rounds, for the load I settled on, dialed in the .6 mil and fired, took the .6 off, fired again and knew I was in business. The first shot was exactly 2.125" higher than the following 5 shots. I can not confirm this absolutely, because I only get one CB shot per day, however, this is the first time that the cb shot went exactly where it was supposed to.

    The target page to the right is todays group, with the CB shot 2" high, because I dialed for it, and the subsequent "warm shooter" group to the right of that. Most of the groups on the right side of these targets look like that, with the "cold shooter" groups on the left, so with me at least, the cold shooter/warm shooter theory is valid, lol.

    a4ce23dd-6143-4e54-a91d-251edbceac15.jpg
     
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