Bolt rifle challenging me, need some input

[bikemutt]

I've been working with a Howa 1500 mini-action chambered for 6.5 Grendel. Here's what I've done so far:

I sent the rifled action to Long Rifles, Inc. where they threaded the barrel.

It's mounted in an MDT LSS chassis with a fully adjustable MDT skeleton stock.

The shooter gets to squeeze on a Timney trigger set to 1.5 lbs.

It wears a Sightron III scope sitting in Burris 30mm rings on Leupold split bases.

The muzzle ends in an APA Little Bastard 6.5mm brake.

I'm handloading Hornady brass with Hornady 120 grain Amax projectiles under 28.9 grains of Varget ignited by CCI BR-4 primers. Chrony says these are running at 2460 fps, 20 fps slower than Hornady Match factory runs.

Anyway, after shooting a few hundred rounds with this rifle something has made itself very clear to me; group size is closely related to shoulder contact. If I can stay almost solely focused on keeping consistent shoulder contact, the rifle will shoot sub-MOA. If I don't, I'm looking at 2-MOA. Accuracy deviations are almost entirely vertical.

I went to the range to try out a few more things yesterday and came away a bit perplexed. I removed the brake and noted the rifle shot 4 MOA low from POA to POI. Then I screwed on my YHM 30 cal suppressor; the rifle shot 6 MOA high POA to POI. I put the brake back on and it shot to POA.

I've never owned a rifle that shifted POI to this degree depending on what's bolted up to the muzzle, especially not a suppressor. An inch here and there yes, 6 inches at 100 yards , no. Similarly, I own a few rifles that need to be handled "a certain way" for them to shoot nice, this one seems especially finicky to me, after all, I'm flesh and bone, not a lead sled.

I really like this rifle for a host of reasons but I'm starting to feel like I'm running out of ideas to have it be a steady-Eddie sort of shooter for me.

Figured it couldn't hurt to ask the team here at THR for some more ideas, or maybe some pointers as to what I may be doing wrong.

Thanks.

 

[Art Eatman]

I'm not surprised at the shifts in POI from the various configurations. It has to do with the harmonics of barrel vibration. (Which is about all I know about the vibrations.) I've no idea if the amount of shift is unduly large.

 

[z7]

How heavy is the rifle?
Is the shot surprising you occasionally? If so add some weight to the pull so you can get Used to it
Is everything torque down and tight enough?

Make sure of a few things:
Consistent pressure on the rifle, Pull It gently into the shoulder pocket, straight back with as much pressure as you would shake a woman's hand
Speaking of shoulder pocket, same place same way every time
Trigger pull, straight to the rear,
This is a starting point. Inconsistent poi is all about you if the ammo and barrel are good.

 

[denton]

Shifts of that magnitude are to be expected when changing a weight out on the end of the barrel.

When you pull the trigger, the shock of the pin dropping and ignition of the powder travel down the barrel about 10X as fast as they would in air. So the barrel is vibrating when the bullet exits.

The vibration causes two problems: 1) The barrel is pointing in different directions when the bullets exit, and 2) The barrel imparts a transverse velocity to the bullet.

As the tip of the barrel swings in an arc, there is a moment when it pauses to change direction. If bullets exit near that point, small variations in barrel transit time will make little difference. If they exit far from that point, small variations in transit time will make much larger differences in POI. The situation is further complicated because the barrel will vibrate in more than one mode at the same time.

Changing the length of the barrel or the mass, particularly near the muzzle, will make major differences in the resonant frequency of the barrel and the amplitude of the arc.

 

[LoonWulf]

Check that the muzzle threads are true and strait, and your bullets arnt lightly clipping something on the way out.
Is this a lite or heavy barreled gun?
did it do this before being worked on?
Have you tried other loads, and if so do they do the same thing.

My own grendel shifts about 2" left depending on what muzzle device im using. brake, clean, or aluminum shroud. My .458 dosent seem to care, and my other threaded guns i dont remember.

 

[Captcurt]

I'm not surprised at the shifts in POI from the various configurations. It has to do with the harmonics of barrel vibration. (Which is about all I know about the vibrations.) I've no idea if the amount of shift is unduly large.

Got to go with Art on this one. With all other things equal harmonics is probably the culprit. A deviation that size is unusual but not impossible with a light or standard weight barrel. I wouldn't think that a heavy barrel would be affected that much, but I have been wrong before. Just ask my wife.

 

[bikemutt]

The barrel on this one is 20" long and tapers to 3/4" at the muzzle.

She weighs in at about 11-1/2 lbs without ammo, this includes the scope, brake and magazine.

One thing I'm considering is backing off the charge a little. The starting load in the Hornady manual is 27.6 grains of Varget which runs about 200 fps slower. I don't have a great deal of data points at that charge to refer back to but, that's the beauty of handloading, I can always start over. I really just want to use this rifle to punch paper at 100-300 yards. The reason I threaded the barrel and put a brake on it is because I felt the rifle was more on the lively side of polite for the bench.

I have reviewed the scope rings for tightness, I probably should double-check the mounts too even though I used blue threadlocker. And there's the action bolt torque, can't hurt to check it again.

 

[CoalTrain49]

Art and Denton are spot on.

I have a .223 Howa Mini action. Howa makes 3 barrel configurations. light, standard and heavy. The Mini is the light configuration. My mini is 1 moa until it starts to heat up and that happens pretty fast. I like everything about the rifle except that. My guess is because your barrel is so light the harmonics are changed with added weight on the end. Actually weight on the end isn't necessarily a bad thing but it has to be adjustable to find the sweet spot, like Browning's BOSS system.

http://www.browning.com/support/frequently-asked-questions/boss-system.html

To overcome the harmonics and heat I purchased a heavy barrel 1500 about a month ago. Seems to be working out pretty well. I haven't noticed any stringing due to heat like my Mini and groups are better.

I watch these AR shooters go thru 50 rounds in a few minutes and I'm wondering how hot those barrels must be. Precision must be non existent.

 

[CoalTrain49]

The barrel on this one is 20" long and tapers to 3/4" at the muzzle.

She weighs in at about 11-1/2 lbs without ammo, this includes the scope, brake and magazine.

One thing I'm considering is backing off the charge a little. The starting load in the Hornady manual is 27.6 grains of Varget which runs about 200 fps slower. I don't have a great deal of data points at that charge to refer back to but, that's the beauty of handloading, I can always start over. I really just want to use this rifle to punch paper at 100-300 yards. The reason I threaded the barrel and put a brake on it is because I felt the rifle was more on the lively side of polite for the bench.

I have reviewed the scope rings for tightness, I probably should double-check the mounts too even though I used blue threadlocker. And there's the action bolt torque, can't hurt to check it again.

That sounds like a standard barrel. I'm sure mine is a light barrel. They must have gone with the std. barrel for the Grendel but I thought they were all 22". My heavy barrel is 20" and my Mini is 22". Go figure.

 

[bikemutt]

That sounds like a standard barrel. I'm sure mine is a light barrel. They must have gone with the std. barrel for the Grendel but I thought they were all 22". My heavy barrel is 20" and my Mini is 22". Go figure.

Mine is the 20" heavy barrel, #6 contour. Long Rifles, Inc. would not have agreed to thread it if there was anything less than .725" shoulder diameter at the muzzle.

Here's a link to the mini-action catalog page: https://www.legacysports.com/viewcatalog/lsi-web-specs.pdf#page=3

 

[CoalTrain49]

Mine is the 20" heavy barrel, #6 contour. Long Rifles, Inc. would not have agreed to thread it if there was anything less than .725" shoulder diameter at the muzzle.

Here's a link to the mini-action catalog page: https://www.legacysports.com/viewcatalog/lsi-web-specs.pdf#page=3

Got it.

I have no clue what's going on.

Seems odd.

But I like your style.

 

[denton]

One other thing, which you've probably thought of, but maybe worth mentioning: Stuff mounted on the end of the barrel has to be absolutely snug and tight. The least bit of looseness will affect POI.