AR needs to "warm up"?

[PatMcD]

O.K., I'm a little stumped.
I traded into a Bushmaster CMP upper a few weeks ago (less than 100rds). Yesterday, at 600 yds, my first 12 shots (2 sighters, 10 record) were all over the place, scoring a lousy 89. The next 10 shots were a 7x clean! No wind, same conditions. Normally, I wouldn't try to read too much into it and would chaulk it up to chance, but the rifle did the exact same thing 2 weeks ago! First 12 shots crappy and the last 10 a high-X 100.

When it starts on that second string, it's probably the best shooting rifle I've ever shot, but before that, it really sucks.

Question; Have you ever heard of an AR that needs to warm up before it shoots well? If that's the case, I've got to reluctantly junk the barrel.

Perhaps my older eyes need to "warm up" before they see well?

 

[terraformer]

Not an AR but a thin barrel .270 Rem 700 I have does the same thing. The heat is changing the barrel's characteristics most likely warping it and you zero'd it in that state. Is it a pencil barrel?

 

[kristo]

If you shoot at those distances you should be familiar with terms such as "cold bore shot" and "thermal drift". These are known components of long distance shooting and the best barrels with the highest cold bore shot predictability and the lowest thermal drift are cryogenically treated barrels.

 

[PatMcD]

Not an AR but a thin barrel .270 Rem 700 I have does the same thing. The heat is changing the barrel's characteristics most likely warping it and you zero'd it in that state. Is it a pencil barrel?

This is BM's heavy barrel 1/8

 

[PatMcD]

If you shoot at those distances you should be familiar with terms such as "cold bore shot" and "thermal drift". These are known components of long distance shooting and the best barrels with the highest cold bore shot predictability and the lowest thermal drift are cryogenically treated barrels.

My "cold bore shot" on any other barrel used to be my 2 sighters .

I'm not familiar with thermal drift. Never had frozen barrel. Never had this problem before.

I think I'll throw a scope on it and try it at 200yds to see if it's me or the barrel.

 

[bpm990d]

Sounds like the barrel likes to shoot when it's fouled. Tell me was this a clean barrel, or was it already fouled?

B

 

[Admin]

Pat I've never heard of a barrel doing that.

 

[Rockrivr1]

Sounds like the barrel likes to shoot when it's fouled. Tell me was this a clean barrel, or was it already fouled?

B

+1 This could be part of your problem. Did you clean your bbl before you took it out to shoot? If not, the person who sold it to you may have cleaned it thoroughly before you took possession.

Some bbls like being dirty yes, but I'd bet there was a good film of residual cleaner or lube in the bbl that was throwing off your first shots. I've had this happen a few times with precision rifles. I was leaving to much lubrication in the bbl and it was messing with the first round of shots until it was dissapated from shooting.

I now run a clean dry patch through the bbl before I start shooting to make sure all the residual lube/cleaner is gone. Since doing that my beginning flyers have gone away.

 

[PatMcD]

Sounds like the barrel likes to shoot when it's fouled. Tell me was this a clean barrel, or was it already fouled?

B

Brian, Rockrivr1: the first time this barrel did this a few weeks ago, it was clean (by the previous owner). Since then, it is my standard "don't clean it" regimen. So when it did it yesterday, that barrel had 50+ rds through it. Even on a "clean" barrel, I've always had no problem getting back to a zero after only 5-10 shots or so.

On a side note, I wasn't overjoyed with how it shot at 200 and 300, either. Granted, I was trying to get some zeroes for it and was fiddling with the sights a bit, but still, it tended to string them a little.

Like I said before, I'm going to scope it on a rest a test it.


Derek, I've never heard of it in an AR, either.

 

[Rockrivr1]

Brian, Rockrivr1: the first time this barrel did this a few weeks ago, it was clean (by the previous owner). Since then, it is my standard "don't clean it" regimen. So when it did it yesterday, that barrel had 50+ rds through it. Even on a "clean" barrel, I've always had no problem getting back to a zero after only 5-10 shots or so.

On a side note, I wasn't overjoyed with how it shot at 200 and 300, either. Granted, I was trying to get some zeroes for it and was fiddling with the sights a bit, but still, it tended to string them a little.

Like I said before, I'm going to scope it on a rest a test it.


Derek, I've never heard of it in an AR, either.

Hummm. I'm hoping this isn't it, but look at the crown on the bbl. Does the rifling look to be worn down, ovaled, chipped or otherwise not round crisp and sharp? Worn crowns can cause significant issue with accuracy. This can happen when the bbl is cleaned with a metal cleaning rod from the crown end.

 

[bpm990d]

Well if it shot seven Xs it should shoot. Test it with the scope or have someone else shoot it. I've had barrels that I thought sucked ass, until I shot a 199 with a bunch. I still have a hard time determining if it's me or the rifle and I try not to forget Lanny's advice about bringing a performance up to meet my self image.

I'll throw these out in no particular order.

Try a different bullet/powder. If it does the same thing then something is off.
Take it apart and put it back together.
Have the barrel borescoped. I think Harry has one. If it's really rough or has lots of tool marks, I'd fire lap it.
Recrown
barrel frozen
Pitch the barrel in the river.​

B

 

[Jose]

the best barrels with the highest cold bore shot predictability and the lowest thermal drift are cryogenically treated barrels.

Negative. Kriegers are the king of barrels and they do not bother with such nonsense. Makers of cheap crap do advertise those magic treatments as they try to make up for poor metallurgy and inconsistent boring and rifling.

 

[PatMcD]

O.K., I tested it kind of quickly to get an idea of what's up. Here's the set-up (with my White Oak). Not the greatest of situations to test accuracy, but it should show something.

5 shot groups at 100yds
First group through the Bushmaster was with my 77gr mag-length rds. Everything else was my 80gr loads. None were really good, hovering around an inch. But it did seem like it was starting to tighten up a little with still a few fliers the more I shot.

I was getting a little pissed off and decided to throw the scope on my White Oak Armamant upper to see if it was me or the scope set-up that might be off. Three shots to adjust the scope and then a five shot group into just under 1/2". No problem with the scope set-up, I guess.

Now I'm thinking those good ten shot scores with the BM were just shit luck. I'm going to do a little more testing at 200yds next week. I guess technically, a 1" rifle is good enough to clean any target, but I feel a little better with something capable of better.
A word on the ammo: these were the same loads I have shot out of 3 different rifles and all shot very well. I really don't want to go back to the drawing board and work up new loads.

 

[M1911]

You're complaining about 1 MOA for Service Rifle competition?

 

[kristo]

Negative. Kriegers are the king of barrels and they do not bother with such nonsense. Makers of cheap crap do advertise those magic treatments as they try to make up for poor metallurgy and inconsistent boring and rifling.

No argument that Kriegers are the best and is the only barrel maker that is known to cryo treat the barrels TWICE in shop.

Krieger stainless steel competition rifle barrels usually appear on the top finisher's equipment lists in benchrest, high power, long range, and Palma competitions. Many top gunsmiths are recommending Krieger barrels because of their consistent quality and performance. This consistent performance is due to a combination of the processes Krieger employs - from 416 stainless steel made to their own specifications, the singe-point cut - rifling method used to rifle the barrels and the cryogenic treatment each barrel recieves before it is machined.

 

[terraformer]

No argument that Kriegers are the best and is the only barrel maker that is known to cryo treat the barrels TWICE.

Krieger stainless steel competition rifle barrels usually appear on the top finisher's equipment lists in benchrest, high power, long range, and Palma competitions. Many top gunsmiths are recommending Krieger barrels because of their consistent quality and performance. This consistent performance is due to a combination of the processes Krieger employs - from 416 stainless steel made to their own specifications, the singe-point cut - rifling method used to rifle the barrels and the cryogenic treatment each barrel recieves before it is machined.

So this cryo treatment. Is it done just prior to machining and the machining cuts it cold or is it cryo treated prior, brought back up to temp and then machined? I am having a problem envisioning the former as some thermal change will occur after the barrel warms and the last thing you want is to cut it and then have it change. So I am assuming the latter and the cryo is done to increase the tensile strength which decreases warping and thermal changes in the barrel during operation, but I was wondering if you knew what specifically that is what is going on.

 

[Rob]

I work in the 'high-tech' industry and have send out metals to be cryo treated. There is a reason this industry exists. For the non-believers, cryogenics is not snakoil.

In layman's terms, cryo treating involves heating and cooling cycles over a period of time to remove/reduce micro fractures inherent in metals and in the end makes the metal more homogeneous and thus stronger. Yes it works and yes it will generally make metals 'stronger' than they were before treating. Machining takes place after the cryo cycles are finished.

Most, if not all, NASCAR teams cryo every metallic engine part. There is also a large hand-tool industry for cryo treated drill bits, screwdrivers, etc.

Cryo barrel treatment is a great example of high tech practical to tactical.

 

[kristo]

Here is an interesting article. They quote John Krieger a lot.

 

[PatMcD]

You're complaining about 1 MOA for Service Rifle competition?

Yes.

I want to have no doubts that that "9" was me and not the rifle.

 

[bpm990d]

I bet you can get $200 for it on e-bay. A lot of guys sell their shot out tubes at the end of the season for pretty good money.

B

 

[ShadeWPI]

Just a thought - Prehaps it's the fit between the upper and the lower tightening up as the upper heats up?

Metal Expands as its heated and since the upper is heated more than the lower, this would tend to cause the upper to expand slightly more than the lower, which could be tightening the fit between the two slightly, eliminating a slight shot to shot variation in how the upper moves with respect to the lower.

 

[terraformer]

Just a thought - Prehaps it's the fit between the upper and the lower tightening up as the upper heats up?

Metal Expands as its heated and since the upper is heated more than the lower, this would tend to cause the upper to expand slightly more than the lower, which could be tightening the fit between the two slightly, eliminating a slight shot to shot variation in how the upper moves with respect to the lower.

The optics are mounted on the upper so any connection between the two is irrelevant. That is part of the design of the AR platform.

 

[ShadeWPI]

terraformer,

Yes, since the optics are mounted to the upper, the mating between the upper and lower wouldn't affect the point of aim verse the point of impact, but when we hold an AR, we hold the lower rigid and the upper reacts to recoil. If the upper and lower have a "loose" fit, the upper can move in response to recoil and could, cause the point of aim to move during the firing process and result in shot to shot variation or poor grouping. This would be similar to a poorly bedded bolt action rifle having poor accuracy, even if the optics are well mounted directly to the reciever.

 

[PatMcD]

terraformer,

Yes, since the optics are mounted to the upper, the mating between the upper and lower wouldn't affect the point of aim verse the point of impact, but when we hold an AR, we hold the lower rigid and the upper reacts to recoil. If the upper and lower have a "loose" fit, the upper can move in response to recoil and could, cause the point of aim to move during the firing process and result in shot to shot variation or poor grouping. This would be similar to a poorly bedded bolt action rifle having poor accuracy, even if the optics are well mounted directly to the reciever.

I've heard that theory before and not sure I buy it. I think the fit between the two halves would have to be really bad to effect it in any meaningful way.

 

[ShadeWPI]

Based on his posts, it sounds like he's talking about a 1MOA increase in his group size from a "cool" upper. That's only 1.5 thousands of an inch, about half the thickness of a sheet of paper 6" from the pivot point.