help diagnosing poor groups

[yorketransport]

I'm working with a new barrel in a 270 wildcat slightly larger than a 27 Nosler and I'm getting some very odd groups so far while breaking in the barrel. The barrel is a Broughton 1-8.5" twist that had 45 shots though it before these groups were shot. I've had barrels that didn't start to shoot well until they had 50-75 rounds through them, but never quite like this.

These 3 groups were shot at 150 yards from a bipod with a rear bag with 3 different bullets, each seated .030" off the lands, and the same powder charge for each bullet. All cases are once fired in this chamber, from the same lot and prepped the same.

The black dot is 1.2" to give some scale.

145gr ELDx

140gr Nosler BT

140gr Sierra TGK

What's throwing me off is the fact that these groups were fired "round robin" style with one shot from each bullet fired with 1 minute between shots, and then the gun was allowed to cool for 15 minutes before the next string. If all of the groups were stinging either vertically or horizontally I'd start looking at bedding issues or my technique. If they were all grouping poorly but symmetrically I'd look at either the scope or my technique. To confuse myself even more, I put a proven barrel, with similar ballistics/recoil back on this action and shot a .5 MOA group at the same range, which is exactly how that barrel always performs.

I haven't ruled out the idea that this barrel just really hates this powder or bullet weight range and I have other bullets and powders to test still. I've just never seen 3 loads print 3 very different groups like this.

 

[Bc'z]

Why did you chose to start 30 off vs touch?

 

[yorketransport]

The .030" off was based on my past experience with these bullets. They've all shown to be very forgiving of seating depth in the past but it looks like that wasn't the case here!

 

[JFrank]

I would shoot a charge ladder with bullets seated from your CBTO then move to a seating test.
Any time I change a component I’m prompted to perform a ladder test.
Jim

 

[chkunz]

Not enough data with each bullet.

 

[yorketransport]

Is it reasonable to think that this is caused mostly by a very poor load? That wouldn't surprise me, although I've never run into a barrel which shot this poorly with any combination of powder or bullets. I'm just trying to rule out problems with the equipment or smith work right now. I couldn't think of any equipment problems that would cause groups like this.

 

[JFrank]

Well we already know that this is a different barrel, I would say ya can’t get much bigger of a change

 

[yorketransport]

Well we already know that this is a different barrel, I would say ya can’t get much bigger of a change

Fair enough, I was thinking more along the lines of the preexisting equipment like the action, stock, trigger and scope.

I’ve had bad barrels in the past and they just shot scatter plot groups with every load. I’ve never had one that would string one load vertically and another horizontally like this.

 

[Alex Wheeler]

Sporter barrels will shoot very small in tune, but typically can shoot pretty bad out of tune. If this is a light contour I would not be concerned until a proper load development procedure has been done including a few bullets, primers, and powders. Especially if this is a new to you cartridge. Harmonics occur is every direction so I would not worry about that.

 

[rwk]

Same subject i'm always left and high. I know its me same thing with 270wsm 30-06

 

[Ggmac]

N E D = not enough data , more shots fired , more loads tried .

 

[Ggmac]

Inspect your fired cases for any oddities. Check neck od of fired case for clearance , base square ?

 

[DaveTooley]

Why the 8.5 twist? I have experience with a fleet of rifles that have a twist rate that is fast enough that it effects accuracy.

 

[msinc]

I have to ask...just how bad does this thing recoil and exactly what scope, rings and bases are you using????? It might be helpful to know what the bullet weight is and how many grains of what powder you are burning, also, how heavy is the rifle???
I once had a big heavy scope on a very lightweight mountain rifle in a big caliber shooting heavy bullets. Bad idea...I initially used the rings and bases that the gunshop had in stock just to get me shooting and that was a mistake. I got groups very similar to yours. I went with a lighter scope, better, heavier rings and bases and a little different shooting technique and it straightened right up. Yours looks like it "is trying to shoot".

 

[Bc'z]

Just looking at groups
"what target is telling me"
140 TGK is asking for more powder.
Nosler ballistic tips looking for seating depth.
Each bullet is different in ogive and bearing surface.

 

[yorketransport]

Sporter barrels will shoot very small in tune, but typically can shoot pretty bad out of tune. If this is a light contour I would not be concerned until a proper load development procedure has been done including a few bullets, primers, and powders. Especially if this is a new to you cartridge. Harmonics occur is every direction so I would not worry about that.

This was my first thought, but I've never had any barrel shoot his poorly with any load! If it were a light sporter contour I wouldn't be as surprised but the barrel is a 28" heavy varmint contour.

Why the 8.5 twist? I have experience with a fleet of rifles that have a twist rate that is fast enough that it effects accuracy.

I intend to shoot the 170gr bullets (Berger, Matrix) and I'm at 500' elevation or less most of the time. A 9" twist would have worked but I found this 8.5" twist in stock, so that's what I went with. I was concerned that I might have too much twist for the 140 class bullets but I've shot all of these bullets in a 9" 270 RUM in the past with good results.

I have to ask...just how bad does this thing recoil and exactly what scope, rings and bases are you using????? It might be helpful to know what the bullet weight is and how many grains of what powder you are burning, also, how heavy is the rifle???
I once had a big heavy scope on a very lightweight mountain rifle in a big caliber shooting heavy bullets. Bad idea...I initially used the rings and bases that the gunshop had in stock just to get me shooting and that was a mistake. I got groups very similar to yours. I went with a lighter scope, better, heavier rings and bases and a little different shooting technique and it straightened right up. Yours looks like it "is trying to shoot".

The rifle with this particular barrel on there is 13.5# with a well proven Mk4 mounted in NF rings. The loads shot here use 92gr of Retumbo in a case with an internal capacity of 110gr of H20. After I saw these groups I put a different barrel with very similar recoil (6.5x338 RUM improved using a 160gr bullet and 95gr of US869) on the action and it shot .5 MOA as usual, leading me to believe that it isn't the optic or bedding. It also told me that it's unlikely that it was all caused by me just having an off day, which certainly happens!

All signs point to a terrible tune on the loads. While I didn't expect BR class accuracy from ammo just "thrown" together without doing a proper load workup, I've just never had any barrel shoot this poorly, especially a quality barrel blank. I've never even had a factory Remington barrel shoot like this!

 

[dsculley]

My rifle will open up more by changing seating depth than powder charge. I would suggest you select a bullet, do the Berger seating depth test (I have found this beneficial even with bullets that are not supposed to be sensitive to seating depth), then proceed your preferred method of testing charge weight.

 

[MislMan]

3 groups with 3 different bullets each seated 0.030 off the lands. You did measure the OAL to the lands for eack bullet combo correct? Looks like you simply took a wild arse guess (WAG) at the loads expecting to strike gold. Doesn’t look like you struck it.

 

[yorketransport]

3 groups with 3 different bullets each seated 0.030 off the lands. You did measure the OAL to the lands for eack bullet combo correct? Looks like you simply took a wild arse guess (WAG) at the loads expecting to strike gold. Doesn’t look like you struck it.

I guess I should have explained how I came up with the loads I fired resulting in those groups.

The powder charge chosen was based on an initial "ladder test" of sorts to determine the rough velocities I could get from this case and where I'd start seeing pressure signs. This was done with two different powders, IMR 7828 and Retumbo, stepping up in 1.0gr increments. These were shot at 100 yards while doing the initial barrel break in and sighting in the scope. Because they were shot while cleaning the barrel and getting velocities with a Magneto attached I paid very little attention to the groups/clusters on the target. The powder charge of 92.0gr of Retumbo that I used while shooting the groups in question was based on a load that gave good velocity and case fill while also showing a "flat spot" in the velocity increases while stepping up powder charges. Since these velocities were taken from a new, clean barrel and also firing loads that were 1.0 grains apart at 100 yards I wouldn't consider this a ladder test in any meaningful sense. With past wildcats this process has given me very satisfactory results and typically gets me within 1.0gr of my final powder charge.

The seating depth of .030" off the lands for each given bullet was chosen based on past experience working with these particular bullets. I've shot all 3 of the bullets used in the groups above in 270 Win, 270 WSM and 270/300 RUM barrels in the past. In each case, the bullets I used performed best when shot between .025"-.035" off the lands. Since these 3 bullets have also been very forgiving of seating depth in the past I just loaded them .030" off the lands, measured using a Hornady comparator to determine what that measurement would be for each individual bullet.

This process seems a bit unconventional to a lot of people but it's worked very well for me in the past when working with moderate to large capacity wildcats and other cases with limited load data. Unfortunately there isn't a lot of proven data out there for a .270 with a 110gr case capacity and Quickload is a great starting point, but not much more than that. I can see why you'd think it was just a WAG but the whole process is based on a system that's worked very well for me in the past and typically gets me in the .5-.75 MOA range in 30 shots or less. In an overbore case with a relatively limited barrel life, .5 MOA is perfectly adequate for what I use these guns for. They're just plinking guns that may be used for hunting on occasion and they really only need to be able to hold MOA to 15-1800 yards for me to be happy.

 

[yorketransport]

Here's an update after today's shooting. Today I shot at 380 yards doing a conventional ladder test and got significantly better results, although they were a bit unexpected. The wind today was very erratic and switching directions and speed, I did my best to pick similar conditions for each shot, but it was very unpredictable. With the exception of two odd flyers which I called and attribute to damaged tips after I knocked my ammo off the bench (it was that kind of day), everything is beginning to come together. The damaged tips were the high and low flyers on the left group. The black diamond is roughly 5" from point to point. The numbers next to each hole are the respective velocities.

The two loads were the 145 ELDx and 140gr TGK with charges starting at 90.0gr of Retumbo and working up to 94.0gr in .5gr increments. The first thing I noticed is that my velocities have jumped considerably from the initial pressure/velocity testing I did. In fact it's an almost uniform 100 fps compared to the same powder charges fired with the same bullets when there were 40 fewer shots through the barrel. I didn't make it all the way up to the 94.0gr charge with either bullet because I started to get a slightly heavy bolt lift at 93.0gr with the ELDx and 93.5gr with the TGK.

The results of this ladder test are much more in line with what I expected to see with the exception of the 100 fps velocity increase. I think I'll head back out and try the 145 ELDx at about 3550 fps and the 140 TGK at around 3700 fps again at 500 yards and see how they do.
I'll usually see a 40-50 fps increase after about 40 rounds through a barrel, but I've never seen 100 fps. This is only the 2nd Broughton barrel I've used, so maybe this is normal for them.

Whatever the case is, it appears I was prematurely concerned about the results I was seeing. This whole project has been full of complications and I actually had to change gunsmiths halfway through. As a result there were some concerns about possible problems resulting from the first smith to work on the barrels. When I started seeing the unusual (to me) groups while load testing I feared the worst. Smith #1 blamed smith #2, and smith #2 blamed smith #1 with each giving an explanation what the other had done wrong.

Regardless, thanks to everyone who's helped out so far.