Weird flyers

[Plattelandseun]

Okay, just came back from the range.
This is how it went... But first off all I also figured it might me case prep and neck tension, so I went through my normal (as in for the last 9 years of reloading) procedure and loaded up 15 rounds. I also cleaned the silencer but not much gunk came out. I did observe a hell of a lot of rust on the baffles after cleaning though.
So let me tell you guys about the groups today.
Group 1 - with silencer attached
Shot 1 - Not even on A4 target paper
Shot 2 - On paper at least but 2.34" low and 2.10" right
Shot 3,4 & 5 - Grouped 1.34" low and 0.61" right. These 3 were a 0.84 MOA group

So I waited about 5 min...
Group 2 - still with silencer attached
Shot 1 - 0.90" low and 0.43" right
Shot 2 - 2.05" low and 0.56" right
Shot 3 - Not on paper
Shot 4 - Cuts shot 1 to group 0.18 MOA
Shot 5 - Not on paper

5 min later I take on the next group. Probably not even cooldown, but was a bit pressed for time...
Group 3 - silencer removed
Okay so I haven't shot this rifle without the silencer since attaching it back in 2010, but I remember from then that I had about a 5" POI shift down with the silencer attached, so I figured it would be high without it...
Shot 1 - 4.85" high and 2.31" left
Shot 2 - 2.79" high and 0.86" right
Shot 3 - 5.57" high and 0.70" right
Shot 4 - 2.25" high and 1.47" right
Shot 5 - 3.66" high and 1.52" right

So shot 2,4 & 5 gave a 1.53 MOA group, at least all 5 were on paper for a change.

Now I want to ask the following: I did my load development with the silencer attached, and I know that the silencer changes the barrel's harmonics. Is it possible that the load is not right for the rifle without the silencer? Therefore the bad grouping without the silencer? I am asking this because I actually expected it would group tighter without the silencer...

Also to answer some of your questions, I thoroughly clean my rifle barrel after every 100 rounds, when I load up a new batch of rounds.
Let me try and explain my rifle to you, I have a custom stock setup, the barrel is surrounded by a barrel guard same as you would find on a AR. There is about a 0.5" clearance 360 degrees around the barrel. The only place were my stock actually touches the barrel is 1" in front of the recoil lug, this is a 360 degree touch, it is how the guard attaches. I do not believe it to be the problem since it is with this stock I did the original load development. The best 3 shot group back then was 1.32 MOA and the worst was 2.37 MOA @ 100 yards. No flyers or POI shift then.

Opinions will be appreciated...
Thank you

[Plattelandseun]

I also think I should add that I bought this rifle out of a estate back in the day, I have no idea how many years the previous owner had the weapon nor the number of rounds he shot. I've read online that some 600 zkk's has been manufactured as far back as 1968 and some as recent as 1992. I have but about 1000 rounds through it over the years I've own it. That is why I am wondering about the barrel being warn out...

[Andrew Leigh]

The silencer does NOT alter the barrel harmonics so rule that out.

On the assumption that you can shoot as well as you say 1.5MOA and we remove that component.

To have such dramatic variations between shots simply cannot be brought about by erratic loading, neck tension etc.

Check mechanical things;

- Stock cracked.
- Action screws not tight / over tight.
- Bedding OK? Nothing caught under the bedding.
- Recoil lug (where bedded) is free on the bottom face, side faces and front face.
- Scope not tight.
- Scope is "torqued", mounts are not in alignment bending the scope tube when tightened.
- Scope reticule is loose and moving with each shot.
- Front rest too hard / suspect

- Are you holding the barrel when shooting?
- 0.5mm (i assume you mean mm and not inch) barrel relief is not sufficient on this calibre in my opinion, 0.80mm to 1.00mm would be more appropriate. Your stock could have warped. How did you measure? Folded paper increasing layers and slipping up the barrel channel?

- What is your bullet and mass, load (powder and gr.) and speed?

[Plattelandseun]

No I actually meant inch not mm
150gr SST from Hornady
S361 59.8gr
2840fps

[Plattelandseun]

If the scope was the problem, would POI return to previous shots or just stay erratic?

[Andrew Leigh]

Focus boet .

0.5" gap is 12.7mm radial gap don't think so.

If 0.005" or 5 thou them the gap is 0.127mm most definitely not enough.

Please confirm.

On the scope issue the scope would be all over the place, returning at times to the same setting and then at times not.

[Plattelandseun]

Your turn to focus boet.
So you made me go to my workshop and get my caliper...
9.1mm to be exact!
Did you maybe misread the part where I tried to explain how my barrel is setup? Ever heard of an AICS AX chassis? Basically the same thing. Therefore absolutely nothing can touch the barrel, except for 1" in front of the recoil lug.

On the scope issue, it's a brand new scope I picked up at Cabela's on a recent trip to Canada.
It's a Covenant FFP.

[Andrew Leigh]

Yip missed that.

you have problems, on that chassis and you cannot clover leaf.

You got another scope you can pop on it? It the thing that has change between good and bad groups the scope?

[Pirate]

What scope mounts are used? Are they CZ/BRNO specific (i.e do they mount directly to the dovetails on the action, and does the rear mount locate with a recoil lug in the cutout on the left hand side of the rear bridge?)

The difference with the silencer removed may be a clue... what does the silencer's crown look like? Are you sure the bullet is not touching the silencer somewhere?

If you rifle has a 10-digit serial number it's an "older" ZKK, and then the first two digits indicate the year of manufacture. If it has a shorter serial number then it's "newer", probably made CZ and then the digits don't indicate the year of manufacture.

If it was me, I'd also put the rifle in a real ZKK-600 stock first to see how it shoots with the support it was meant to have. My 46year old factory-standard -601 shoots sub-MOA with anything from 150gr to 220gr bullets.

[Plattelandseun]

It use to have those CZ scope mounts on it, but currently the action was tapped to add a picatinny which runs into the rail over the barrel to link everything up. It has rings which goes directly on the rail. I had a Nikko Stirling on it (with the same rings), but it's windage knob does not move the crosshair anymore. So I bought the new scope. I won't be able to zero it, but with a big enough target I would probably be able to at least get a group on paper...

I'll load up a few more rounds to go and test the action on the old factory stock, as well as try to get a group with another scope.

And I'll mail Cabela's since it has a lifetime warranty...