cold bore shift

[stagpanther]

I got inspired to go out and shoot my ilion remlin 336 30-30 today--today's menu consisted of 150 gr speer hot cors driven by IMR 3031. This particular rifle has never seemed to like 150 gr bullets; but the faster I drove them the tighter the groups got. When I say groups--I mean out of 5 shots 4 would group pretty close together but there was always one which deviated somewhere around 3" from the other 4. The flier almost always occurred between cool downs between different charge groups. Thing is, I never got a total "cold bore" as if the barrel had cooled overnight, but rather maybe 5 minutes between each charge weight group. A true cold bore shot presents perhaps the only consistent baseline to start from, but that's not very practical (unless you're a sniper, maybe).

This is my top charge of 32 grs. of IMR 3031 at 117 yds.--at the end of shooting 3 other 5 shot groups.

 

[tangolima]

My cheap AR was shooting like that. In order to narrow down the cause, I recorded MV of each shot. Then I noticed such fliers almost always corresponds to outlying MV, too fast or too slow. That got me started trickling powder charge and weight sorting brass headstamp and weight. It has made marked improvement. Even then such fliers and outlying MV still happen occasionally. My latest is to fine sort the brass according to MV; I file one notch on the head if it shoots under average speed, 2 notches if is over average. When I shoot I group ammos according to the notches. That definitely has helps shrink the groups.

With other load development tricks, including my poor man's barrel tuner, I can shoot under 1moa on more regular basis. Components are expensive. I have to make each round count more.

-TL

PS. Cold bore shift is real. It could take as many as 6 rounds to settle with heavy barrel. But such shift has predictable trend (e.g. poi high to low) and is small (1 moa ish).

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[stagpanther]

Sort brass according to velocity? New one for me! Definitely cold bore shift is real--but I'm not quite sure how that can be applied beyond the first shot when shooting large numbers of cartridges.

 

[tangolima]

Sort brass according to velocity? New one for me! Definitely cold bore shift is real--but I'm not quite sure how that can be applied beyond the first shot when shooting large numbers of cartridges.

New to me too. I went out of tricks, so I was desperate. Remember the debate about the correlation between weight and volume? That works around it. I still headstamp and weight sort. MV sort is the final polish.

I think cold bore shift is caused by different metal parts, barrel shank and receiver shoulder for example, not reaching thermal equilibrium. The stress forces between them are changing as the temperature stabilizes. It will take more than one shots to reach that point. My RPR with heavy barrel takes about 6 shots of moderate fire. However once it reaches equilibrium and you keep up similar rate of fire, it shouldn't happen. If it does, it is most certainly not cold bore shift. In my cheap AR, it is due to MV variations and careful sorting of components and trickling powder charge seem to reduce / eliminate it.

-TL

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[44 AMP]

This is my top charge of 32 grs. of IMR 3031

First point, where is the data showing 32gr of 3031 with a 150 bullet??
That's more than 2gr over anything I have seen...

Next point, I think you folks have it backwards, its not a COLD bore shift, its a WARM /Hot bore shift. And look where this is most often seen in its most extreme form, light weight hunting rifles. Particularly big game rifles. Guns not intended or built to deliver one hole group accuracy, but guns made to put the first shot where its aimed and the next two or three close enough for big game purposes.

Even gun that don't have barrel bands and tube magazines hanging off the barrel do it. POI moves (walks) as the barrel heats up. Its just what rifles do.

While bull barrel varmint guns do have stiffer barrels, their biggest advantage is the mass of metal acting as a heat sink allowing a longer string of fire before the barrel heats up enough to move the POI away from the cold bore POA.

Basic metallurgy, when you heat metal, it expands. Shooting a Marlin lever gun with everything fastened to the barrel NOT being heated the way the barrel is, your point of impact WILL shift as the barrel heats up.

 

[stagpanther]

As for the charge weight with 3031--the lower charges were distinctly underpowered and only started showing signs of consistency once I got up around 29 grs. QL returns a psi of 38,649 (rated pmax MAP @ 42,000 psi) for 32 grs of IMR 3031 behind the Speer FN 2011 seated to 2.550". It also said that filling was at 102% of spare useable case capacity at 32 grs--but for whatever reason it was not, it was clearly below the bullet base no matter what brand brass I used. Excluding the "cooled bore flier" the group above would otherwise be about 1 MOA (I've considered the possibility that maybe i just do the same stupid pull at the the start of each group, but then even that would be pretty hard to do with consistency)

As for your comment that it really is about warm bore shift as the barrel warms up--and not cold bore shift--that is pretty much my conclusion as well; but nonetheless it is essential to know what that first shot offset is because that's really the only consistent baseline measurement you can have to calculate where the follow-up shot offsets are going to land. While shooting it seemed there was a certain "warm spot" where the groupings tended to stay tighter (this is different from cold bore shift, but related). Problem is that "warm spot" would be nearly impossible to guage with consistency. Maybe a temperature guage strip down the length of the barrel?

 

[ocharry]

i agree that the stuff hanging off the barrel dont help anything...harmonics and such

but it could have something to do with barrel fouled v clean

i have a contender in 223 that i leave dirty until it says clean me....and when i do clean it, like clock work the 1st shot will be straight vertical 4" high..2nd shot will be 3" high..3rd shot, 2" high..4rth shot, 1" high...and the 5th shot will be back to zero and it will group there until it gets to dirty and then i start the fouling all over again..usually somewhere around 40 shots...i just let 4 fly and dont try to keep track of them..i have done the test enough to know where they are going

i have measure this several times and it is very predictable...exactly the same everytime

this is kinda what tangolima is seeing with his RPR...but my rifle doesnt seem to change cold to hot..maybe it is because it is a single shot and i dont really get the temp to change that much ...shot to shot......but if i clean it...i know exactly what it is gona do

just my .02

ocharry

 

[FrankenMauser]

Interesting to see another Marlin do this.
The custom 444 that I built from parts and a 336 receiver always throws the first round from a freshly, fully loaded magazine a bit high and slightly left. (Been a while, so I don't remember exactly, but I think it's +1.25" and L-0.5" @ 100 yd.)
Then the rest stack beautifully in the "same hole" - generally giving me ~3/4" groups.
The rifle has the typical M444 14.5" magazine tube, with the usual 5-round capacity.
So I end up with a great 4-shot group, with the additional 'flyer'. But a very, very predictable 'flyer'.

However, if I continuously feed the magazine a new round, after chambering a cartridge but before each shot, they keep stacking in the same place. It is only the first round from a freshly loaded magazine that will give me the high-left impact.

On the flip side, if I load the magazine, fire one shot, fully unload and reload the magazine, and repeat the sequence, I also get very nice groups. I get "one ragged hole" where the first round 'flyer' lives.

I've seen semi-autos "throw" impacts when the cartridge is manually chambered. And I have seen bolt actions "throw" impacts when the cartridge is manually fed, or chambered weirdly. But I haven't really spent much time analyzing this 444, and why it does what it does; because the 'first round flyer' is close enough to be of minimal consequence at the ranges where the rifle is intended to be used.

 

[tangolima]

i agree that the stuff hanging off the barrel dont help anything...harmonics and such



but it could have something to do with barrel fouled v clean

Fouling a clean barrel is something additional to cold bore shift. The MV is also settling. Some my guns would take more than 10 shots to foul. I don't clean my guns unless I have to. I simply can't afford burning 20% more ammo for fouling shots (I shoot about 50 rounds during each range trip).

My RPR cold bore shifts (no prior cleaning) by about 1.5" left and 1.5" high. It moves to zero in about 6 shots moderate fire. Mostly gradually, but sometimes it ditters between 2 subgroups, perhaps depending on how fast I fire.

Cold bore shift vs warm bore shift. Semantics. The mechanism is the same. If the gun is supposed to shoot warm, it is cold bore shift; warm bore shift otherwise. I mostly shoot warm. Occasionally I do cold bore practice. But it is time consuming even I have multiple guns to rotate. I just lump everything under cold bore shift.

-TL


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[tangolima]

Frankenmauser has a good point. Tube magazine is a factor. Single loading will tell.

-TL

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[44 AMP]

Ok, you got your load off a computer program, which you trust for the pressure, but it got the case capacity wrong....hmmmm

My old Lyman data shows a max at 29.5gr, and a pressure at 39,400 cup.

I loaded 30gr back in the 80s, and while not unsafe, it wasn't suitable, my 70s vintage Marlin didn't like it.

but it could have something to do with barrel fouled v clean

Fouled vs clean is a different matter, though it has similar results, and you cannot get barrel heating POI change without fouling.

 

[stagpanther]

Ok, you got your load off a computer program, which you trust for the pressure, but it got the case capacity wrong....hmmmm

 

[zeke]

Shooting some levers for accuracy can get real frustrating. Took to resting fore end in my off hand on top of bag, instead of resting the forearm directly on the sand bag.

 

[stagpanther]

Shooting some levers for accuracy can get real frustrating. Took to resting fore end in my off hand on top of bag, instead of resting the forearm directly on the sand bag.

Interesting idea--I too have noticed that certain firearms seem to be easier in finding a "stability spot" when holding directly as opposed to direct rest (handguns, for example, in a non ransom rest)--might be because it mitigates other pressures when firing?

 

[44 AMP]

Unless the game is actually bench rest shooting, shooting from a bench can give misleading impressions compared to shooting from "field positions" including supported ones.

What matters most to me, is not what the gun can do, shot off a bench, but what the gun can do, in my hands. SO, when I use a bench, bags, a rest of some kind, the gun is in my hands.

The rest supports ME, and I support the gun. This shows me what I can do with the gun, and gives me a reasonable idea what I ought to expect my best to be, when there is no bench or other rest available.

A Sub-MOA rifle off the bench isn't a sub-moa rifle in the field, unless I can shoot it that well, in the field.

With handguns, its even more important to shoot them from your hands, and I would recommend making sure there is enough clearance between your rest and your hands so you don't get pinched by recoil.

I support my forearms on a bag when shooting handguns from a bench. Not my hands, and not the gun itself.

This is what works for me. I do me, you do you, and if what you do works for you, then, do it....

 

[tangolima]

I have tried both. The difference depends on whether you grab hold of the forearm to control muzzle rise or not.

If I don't hold the the forearm, the off hand is just a cushion between the rest and the gun, there is little or no difference. And I basically lose the ability to fine tune POA by squeezing the rear bag.

If I grab hold of the forearm, it could be like preloading bipod, if I do it correctly and consistently.

I like having the rear bag, so I choose letting go of the forearm.

-TL

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[zeke]

Interesting idea--I too have noticed that certain firearms seem to be easier in finding a "stability spot" when holding directly as opposed to direct rest (handguns, for example, in a non ransom rest)--might be because it mitigates other pressures when firing?

Am also gripping the fore end of the levers, but not death gripping em.

Yes, am doing similar for handguns that i sight in, then shoot free hand to check. For handguns, my wrists/rear edge of hand are supported by bag with leading edge of frame against another bag. Revolvers need a sacrificial leather to keep from destroying too many bags. Again, checked by free hand.

 

[FrankenMauser]

Up to about .30-30, maybe a little beyond, you can get away with not holding the fore-end on a lever gun, at least for load development. As mentioned above, my support hand usually ends up squeezing a rear bag when I'm working with a .30-30, the .307, or the .44 Mag 1894 from a bench.
So long as the rest is padded in some way, that is. If not, it may cause damage when the rifle comes back down after recoil. I've seen many barrel band style Marlins shear or break the rear barrel band screw (and crack a few stocks) by smashing down on a hard rest when shot from a bench or an unpadded tripod. (Like the H-style and T-style rests commonly made from steel, 2x4 lumber, and/or concrete, and available at many ranges.)

But the bigger boomers require more control. The aforementioned .444 Marlin is not something you shoot one-handed. Even when rested, a good grip on the fore-end is required. Which, for me, means my hand is rested, not the rifle.

Whether there is a hand on the fore-end, or not, one must be careful to preload the same - as tangolima mentioned.
I have most of my Marlins set up so that the barrel takes all pressure that is put on the fore-stock. The magazine is essentially "free floated" through the stock and only contacts the barrel at the end of the tube (whether barrel band or tenon style attachment). That partial isolation seems to help settle lever guns down by reducing the number of points where barrel movement during firing is forced to interact with the mag tube. But, it also means that preload by a hand, or on a rest, has more influence on the barrel.

A Sub-MOA rifle off the bench isn't a sub-moa rifle in the field, unless I can shoot it that well, in the field.

That is the primary reason why I started shooting NRL22, with a close second being getting my son a lot of trigger time in situations that somewhat simulate hasty field positions (plus stress, score, a timer, and safety considerations in play)
Terrible shooting positions on unstable and awkward props make the shooter find a way to "build" a stable shooting position with their body (and some large support bags, for some shooters). And then you complicate it with tiny, difficult targets.

Standard competition shooting positions, on level ground, are rarely used in the field - at least in my part of the country.

NRL22 will show your weaknesses in a startlingly amplified manner. It is a beautiful thing.

We're not shooting centerfire, and it is not a perfect analog for actual hunting situations. But .22 LR keeps it cheap enough to be affordable, makes it even more challenging (in my opinion), and any awkward precision shooting on reactive targets is better practice for the field than trying to translate from lazy, easy shooting from a bench or prone on level ground.

Some people argue that NRL22 (or NRL-Hunter) is no longer representing "proper" field shots, because off-hand shooting is no longer built into the stages. But I don't care and it doesn't matter to me. I have only made three standing shots on big game in my life. And they were close enough to be repeatable any day, with pretty much any rifle and many handguns - 90 yards (deer), 75 yards (elk), and 25 yards (elk). (Edit: I remembered a fourth. I stopped an antelope with a heart shot, that was mortally wounded by another hunter, but running full speed. ~120 yards, crossing right to left.)
The rest of the time, I found a way to get as stable as possible to make a "perfect" shot, from trees, logs, or other features of the terrain. I don't take shots on big game when I don't have 98%+ confidence in a guaranteed one-shot kill.
I take great pride in being known for the shots that I passed on, as much as the ones that I did take; as well as being known for generally coming home with one round fired per animal tagged. I want it "perfect" - quick, clean, and exactly as I intended.

Sorry about that going long.
If NRL22 discussion is to continue, we can peel it off into its own thread.

 

[Shadow9mm]

Imho, based on my limited experience with a 44mag lever gun, based on the fore-end being attached to the barrel, there is no way to get around the cold bore shift. There will always be some kind of pressure on the barrel which will change once the gun heats up.

 

[stagpanther]

After cleaning the bore yesterday on my 336 I hastily grabbed it this morning hoping to do some more testing but realized I might encounter some consistency issues. Can you spot the problems?