Cold Bore Shot?

[Whelenon]

What up with this?

A buddy and I were out fine tuning our Grendels, getting ready for the deer gun hunting season.

With my 18" Liberty/Saturn Grendel barrel I'm getting a cold bore shot about an 1.5" low which I have noticed before. After that it groups fine.

With my buddies 16" AA Grendel this does not happen. By the way I sold him the 16" barrel and kinda regretting it.

Any thoughts?

[Drifter]

Like you, I've seen some barrels that show the trait, while others do not. I suppose that it might have something to do with how the barrel was stress relieved. It might also relate to particular bullets, and / or the degree to which a particular barrel is prone to copper fouling. Excessively torqued muzzle devices, barrel nuts, and / or gas block screws could also be potential factors.

In any event, as long as it's predictable, you can easily adjust if that POI deviation matters on the cold-bore shot.

[mlmiller1]

Glad you noticed it before your trophy deer got gutshot or something.

[pds]

What up with this?

A buddy and I were out fine tuning our Grendels, getting ready for the deer gun hunting season.

With my 18" Liberty/Saturn Grendel barrel I'm getting a cold bore shot about an 1.5" low which I have noticed before. After that it groups fine.

With my buddies 16" AA Grendel this does not happen. By the way I sold him the 16" barrel and kinda regretting it.

Any thoughts?

The question you need to answer is it a cold bore issue, a clean bore issue or both.

pds

[Whelenon]

The question you need to answer is it a cold bore issue, a clean bore issue or both.

pds

Not a clean bore issue. I've also seen it with different bullets.

[LR1955]

What up with this?

A buddy and I were out fine tuning our Grendels, getting ready for the deer gun hunting season.

With my 18" Liberty/Saturn Grendel barrel I'm getting a cold bore shot about an 1.5" low which I have noticed before. After that it groups fine.

With my buddies 16" AA Grendel this does not happen. By the way I sold him the 16" barrel and kinda regretting it.

Any thoughts?

1.5" low at what range?

Here is something you can do if you have about twenty rounds to shoot.

Get a small indoor / outdoor thermometer that has a sensor attached to a wire.

Let your barrel cool down to the ambient temperature. Shoot one round, blow out the smoke from the chamber, and measure the increase in temperature of the barrel at the chamber. I bet it doesn't even go up five degrees F. You will find that you have to shoot a decent sustained fire string of ten to see a decent increase in temperature of maybe fifty degrees.

So, ask yourself if one shot that may raise the chamber temp two or three degrees will throw that first shot that far from a group.

Shoot a sustained string of ten in about a minute and measure again. Keep doing this until you get the chamber temp up about 100 or more degrees F. Then take a round of ammo and measure its temp as best as you can. Put it in the chamber and let it sit for five minutes. That ought to be enough time for the powder to heat up. Remove it and see how much hotter it got. Generally speaking, the hotter the powder temp, the more energetic will be its burn and so your velocity ought to be higher. You will find, though, that you really need to get the powder very, very, hot to see a difference of fifty fps. 60 or maybe 70 fps generally equates to about a minute.

I have watched thousands and thousands of 'cold bore' shots made at 600 yards from many guys. I have never seen a pattern set with any single rifle or even any single shooter.

I believe mirage coming off the barrel and downrange has more to do with shots not going to a zero than whether the rifle is clean or not and or the bore is cold or not. That said, I do believe a clean barrel will probably show the first shot or three to be slightly out of zero. Hard to tell, though.

I wouldn't get wrapped around the axle over it.

LR55

[wheelguner]

Although I have seen this in bolt guns (as an example look at the target I posted of my Swede 6.5x55 in another thread) it is more prevalent in self loading firearms. The first round is chambered by the op-rod or slide and not by the action of a fired round which is more vigorous. Try this, when chambering the first round pull the op-rod sharply to the rear and let it snap forward rather than closing the bolt with the bolt release. This fully compresses the recoil spring as would occur during normal cycling. I also make it a habit to palm smack the forward assist when charging the rifle. It has helped in the past for me and it's free. If you try this and it makes a difference or not, I would appreciate the feedback.

[Klem]

Probably because your mate is not cleaning it like you are.

I know you say it is not a cleaning issue but my experience is different. With thick SS barrels from reputable manufacturers fouling interfere's with MPI more than heat, unless you leave the next cartridge in a hot chamber to heat-up like an oven. The popular 'first cold bore' should really be called 'first clean bore' shot. Having shot for a number of years in F Class competition I can tell you that lubricity of the barrel influences MPI as much (if not more) as heat. As for heat, this comes from two actions; how the barrel warps when heated, but also how long you leave the cartridge in the chamber before pulling the trigger. Heat is compounded in an AR as it automatically feeds the next round in to the 'oven' where it acts as a heat-sink at the worse possible time, within a second of the last round fired. If the outside of your barrel feels warm to touch imagine how hot the powder is inside the next round sitting waiting to be fired. No doubt a few on this forum have seen machine guns get so hot it 'cooks-off' the next round and you get a runnaway gun.

In a bolt gun shooters tend to feed the cartridge in only when it is nearly time to shoot. Those mindful of hot chambers temperatures will deliberately leave the bolts open to facilitate air-flow through the barrel to cool it quicker. May as well do this in an AR if you are not in a situation requiring the gun to be at instant.

Back to fouling. In precision bolt-guns where variables such a cleaning and heating are more evident I shoot 1 MOA low when the barrel has been normally cleaned and then prepped for shooting, and 1.5MOA low when the barrel has been properly cleaned. Every 100rds it gets a full service with copper cleaning, compared to the normal fouling brushwork. Some shooters use the two or three sighters in a match as 'fouling' shots, deliberately paying minimal attention to where the shot goes knowing that these will print differently and for fear of 'chasing the spotter' (adjusting your windage and elevation based on the previous round). In an AR I often fire the first two rounds in quick succession into the butts to foul the barrel...plus it feels good for some reason...

Don't forget, whether it is fouling or heating causing your first shot to be low this will be compounded by your systems ability to group. For example, if your gun shoots 1.5MOA then expect every shot to land anywhere within a 1.5MOA circle, the MPI of which will be 1.5" low on the first shot. So, at 100yards you might find that sometimes the shot will be actually right on the aiming mark as the two variables negate each other, yet it is still being influenced by either fouling or cold barrel. And sometimes it will be a full 3" low as both variables compound each other.

I know of some shooters who prepare for a match by deliberately not cleaning the gun going into the match. It is not a widespread practise and if it were so successful it would be.

My point is, if it is the first couple of shots from a cold AR the heat will change the MPI the least of your total shooting session/string. But... at the start the lubricity of the barrel will have changed the most throughout your shot string. Fouling affects the start of your shooting session, whereas heat affects the end of your shooting session.


(I assume your 1.5" is at 100yds, so 1.4MOA)

[Whelenon]

Although I have seen this in bolt guns (as an example look at the target I posted of my Swede 6.5x55 in another thread) it is more prevalent in self loading firearms. The first round is chambered by the op-rod or slide and not by the action of a fired round which is more vigorous. Try this, when chambering the first round pull the op-rod sharply to the rear and let it snap forward rather than closing the bolt with the bolt release. This fully compresses the recoil spring as would occur during normal cycling. I also make it a habit to palm smack the forward assist when charging the rifle. It has helped in the past for me and it's free. If you try this and it makes a difference or not, I would appreciate the feedback.

I tried your suggestion, all rounds fed from the mag, sorry, no improvement. But at least it's consistent, at 50yds, first shot was an inch low.



Kelm,
As far as cleaning, I don't very often. Every 300-400 rounds and it will get a thorough cleaning.

I guess come deer hunting I'll have to aim a tick low on that neck shot.

[Sputnik]

[/QUOTE]I guess come deer hunting I'll have to aim a tick low on that neck shot.[/QUOTE]

I would try shooting a tick high on that first round.

[Whelenon]

I guess come deer hunting I'll have to aim a tick low on that neck shot.[/QUOTE]

I would try shooting a tick high on that first round.[/QUOTE]

My bad, absolutely right, I mean up.

[rasp65]

Here is a video from USAMU on cold bore shots http://bulletin.accurateshooter.com/...int-of-impact/

[zbay]

Let's see a picture of the boom stick! I had a 24 that tossed rounds a bit wild on the cold bore shots also.

[kamrr4437]

I never have any deviation from where i sighted mine in unless i have a different lot number ammo i will resight my weapon. If i change lot numbers it will usually be an inch high or low. Never any change in windage though. But once its sighted in for that lot number it doesnt move.

[noone]

Does the same thing happen when someone not familiar with the rifle, and it's throwing shots tries to shoot it? Some times, people can have self fulfilling prophecies..what they expect to happen is what they make happen. The rifle's owner already is aware of the issue. However, if someone who doesn't know about the rifle's issue of throwing cold bore shots has the same problem, then that eliminates the possibility that it is a shooter caused issue.

A simple solution just before hunting day would be to go to the range, fire a few shots, and leave a round that was fed by the magazine in the chamber for hunting day. That way, the round in the chamber should go where you want it to. Even though I always treat all guns as if they are loaded, I would still put a card on the rifle noting that it has a loaded chamber and to leave it alone.

Shooting a bit high is also a simple solution.