Weird problem shooting...

[Scratchthejeepguy]

I think I have a goofy problem that I’d like some advice on… since I've been load testing, I’ve been shooting one round at a time, single feeding them, with an empty magazine. I pull one cartridge from the box, set it in the open chamber, close the bolt, aim and fire, then open the bolt, store the empty, and grab another cartridge from the box, insert into chamber, etc…

I know I should get used to cycling the bolt, muscle memory and all… and will start doing that again, once I find a load I like, (which I think I have) but I noticed this problem during my last shooting session. I had 36 rounds to shoot, and was planning to shoot all 5 shot groups.
I shot my cold bore, and then shot five groups, five shots each, each shot loading a single cartridge into the chamber, pulled from the box. As you can see, the first 4 groups were under .5 MOA with the 5th group right at .7 MOA.

Then for the next five rounds, I loaded 5 rounds into the AI mag first, and then shot them cycling the bolt each time. The group opened up to 1.15 MOA…? Seeing the change in group size, and knowing the only variable that changed was loading from the mag, I switched back to single feeding, and my group shrunk back down to.949 MOA.

Was it a Fluke? Chalk it up to low experience... Any ideas?

Here’s my setup. I shoot off a bag for the front and a sand sock for the rear. (I put the sock under the stock though, not the pistol grip like in this pic)
It's a Remington 700 SPS Tactical, .308, in an XLR Carbon Chassis, factory barrel, bedded and lapped Vortex optics, Jewell trigger, shooting 168gr A-Max's with 44.0 Varget.

Here’s the first target, the top left was a cold bore shot

Here’s the next target. The top right one is the one I’m concerned with. After I started loading them one by one again, I shot the bottom right group.

By the way... I don't know if you remember me asking questions about what kind of groups should I be getting with this rifle, but I am entirely happy with the all these groups, (except that one...) I think I found a good recipie.
They look acceptable for this rifle and my low experience, to you guys?

 

[kraigWY]

Without watching you shoot, based on what you posted, I think that when you loaded the mag with 5 rounds as opposed to single loading, you were moving around and not getting set again for the next round after working the bolt.

It's like in high power where you shoot both rapid fire and slow fire in the same position. Lets say you're shooting 10 rounds each. 10 slow, 10 rapid. In slow fire you end up shooting 10 one shot strings with a good position (after adjustment) for each shot. In rapid fire people tend to shoot 1 10 shot string. Not having a position and NPA after recovery.

Setting Rapid has always be my best position. Someone asked me how I shot rapid fire? My Reply was "one shot at a time".

I would recommend working on your position so you fall back to your natural point of aim after recoil and loading the next round.

 

[Dan Newberry]

Good points from Kraig...

I would also suggest that if you're working with neck tension too light, the bullets in the magazine may be shifting under recoil. This is a problem with the bushing type dies from Redding, if you don't get the right size bushing... neck tension too light, and bullets shift under recoil.

If you're using standard sizing dies, disregard that possibility.

Edit to add... it looks like your POI shifted right, and somewhat down from earlier groups. And there's the follow up tight group when you went back to single loading, is it low and right of previous POI also? That may actually point to a loose scope base, rings, or action screws. Check those as well.

Dan

 

[Greg Langelius *]

Another point about neck tension is that it changes as the neck gets work hardened; so unless the brass is kept segregated into lots, and sometimes even if it is, neck tension tends to become less consistent. This can have a vast consequence in the accuracy of a given batch of brass from loading to loading, regardless of how effective one is about keeping the other factors consistent. Because ammo in magazines is subjected to bullet setback, one may find they cannot adequately compensate for this factor in ammo that is fed by magazines. I have a kooky kinda method for gaging/managing neck tension, but I doubt it would work with repeaters because its criteria are centered around keeping neck tension minimal.

Greg

 

[Scratchthejeepguy]

Thanks so far, for the advice. I am using standard dies, not crimping, and I anneal after every firing. I am trying to keep everything as identical as possible but I can see where the recoil could be affecting setback.

I'll also keep working and try to be more aware of my shooting positions, and double check the torque on all my screws.

 

[Skigolfmike]

You could also be slamming the bolt forward too hard and knocking the bullet and nect out of concentricity. You want to be smooth (yes, you can still be fast) and let the cartridge slide into the chamber. It's probably less of a problem single feeding since, by the nature of it, you don't manipulate the bolt as quickly.

 

[Scratchthejeepguy]

I don't "think" I'm slamming the bolt forward too hard. I'm pretty sure I was closing it the same as when single feeding, but I could be wrong. I'll double check next time. Thanks.

 

[Dan Newberry]

Do you have any idea why your POI (point of impact) shifted from center to low, right?

 

[Scratchthejeepguy]

Do you have any idea why your POI (point of impact) shifted from center to low, right?

No, but I did double check all my screws and the torque specs were all good, nothing loose.

Maybe "I" had a screw loose....

 

[smokebomb]

How fast were you shooting these strings? By my count your shots didn't start to open til your 22nd round... Wasthe barrel getting hot?

Did you break position and readjust the rifle for the mag-fed group?

Not discreditting neck tension or a mag-fed anomaly, but 1 5 shot group out of 36 rounds fired doesn't seem to be a strong enough basis yet for certainty. Especially since your last 5 shot group single fed was literally over TWICE the size of your average groups prior to shot 22.

There is also shooter fatigue to consider. Or fouling, or natural POA, should/cheek/grip/trigger pressure, mental focus...etc

Now if you have repeated results like this in further testing, we've got something deeper to delve into with your dbm system.

Your first 4 groups, damn fine by the way! Good write up and nice cold bore shot!

 

[AKONA]

Before you work on the loads that are doing well in single feed make sure that this is actually the issue or at least part of the issue.
So bring the calipers and bullet gauge with you and do everything the same loading the full mag, chambering at the same speed and everything.
Don't fire, extract the round slowly and run the COAL with the gauge.
Shifting POI one has to consider heat build up, weather etc... Go take a break and repeat again from a cold system.
And keep in mind that while a factory SPS are amazing shooters out of the box they do not guarantee the kind of repeatability others provide.
That is why the best gunsmiths are so busy taking those factory SPS and truing bolts, actions, and cutting $600 barrels.
Always keep in mind the human factor is there too. Personally I would go down prone with a nice bypod and a couple of sandbags and take some of
those variables out of the equation.

 

[Scratchthejeepguy]

I think my next course of action should be, now that I think I'm happy with the load, I'll load up some more ammo, then shoot them all in the magazine. I can't see how loading from the mag could have caused the problem. I can see fatigue, or barrel temp, although I was probably shooting once very 15 seconds or so. Also I agree on one group being not a good test. So I'll try some more.

 

[smokebomb]

You shot 36 rounds total in approx ~10 minutes? Around 15 seconds between shots, how long between groups?
If you were shooting 36 rounds in 10 or even 15-20 minutes, I would be very suspicious that heat in your factory barrel played a role in your last 3 groups.

 

[Scratchthejeepguy]

I do remember now that the second target, the one with the 20 shots and the questionable group, were shot with probably a good half hour to 45 minute wait from the first target, since my brother came down and we talked for a while. Which means that top left group was shot with a cold fouled bore. It was also almost my best group and things started going downhill after that....

I need more experience to interpret all this stuff I think!
Ugh.

 

[Greg Langelius *]

When I do load development for match loads, one of my crucial criteria is to test the loads under the same conditions they are intended for. Barrel heating must be anticipated when doing final load testing, because barrel/chamber heating changes the load's performance characteristics.

Greg

 

[Deadshot2]

A couple of other variables to consider that are not position or technique related.

When you single load, any pressure against the side of the bolt from the follower is consistent. When you shoot from a loaded magazine the initial pressure against the side of the bolt (shot #1) is greater than the last shot that was fed from the magazine. A variable that can have different effects depending on tolerance in the bolt to action as well as tolerance within the chamber. While the "cone" of the shoulder centers the front of the case in he chamber it can move slightly at the head due to bolt movement. Fire formed cases that were merely neck sized will do this less.

Next is the possible movement of the bullet or tip damage from the feed ramp. Too loose on the neck tension and the bullet gets set back. To rough a feed ramp or misaligned feed lips on the mag and the bullet tip can be damaged. One way to check some of this is to chamber and extract several rounds, measuring before/after dimensions. Also "Ink Up" some bullets on loaded rounds and do the same only look for any irregular markings on the marked surface. Ideally it's nice and even with no signs of tip damage or indications that it's struck the top of the chamber with any force.

Just a couple of possibilities besides the obvious.